39 PRESERVATION 

BRIEFS 

Holding the Line: Controlling Unwanted 

Moisture in Historic Buildings 

Sharon C. Park, AlA 

U.S. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

Uncontrolled moisture is the most prevalent cause of 

deterioration in older and historic buildings. It leads to 

erosion, corrosion, rot, and ultimately the destruction of 

materials, finishes, and eventually structural components. 

Ever-present in our environment, moisture can be controlled 

to provide the differing levels of moisture necessary for 

human comfort as well as the longevity of historic building 

materials, furnishings, and museum collections. The 

challenge to building owners and preservation professionals 

alike is to understand the patterns of moisture movement in 

order to better manage it- not to eliminate it. There is 

never a single answer to a moisture problem. Diagnosis and 

treatment will always differ depending on where the 

building is located, climatic and soil conditions, ground 

water effects, and local traditions in building construction. 

Remedial Actions within an Historic 

Preservation Context 

In this Brief, advice about controlling the sources of 

unwanted moisture is provided within a preservation 

context based on philosophical principles contained in the 

Secretary of the Interior's Standards for the Treatment of Historic 

Properties. Following the Standards means significant 

materials and features that contribute to the historic 

character of the building should be preserved, not damaged 

during remedial treatment (see fig.l). It also means that 

physical treatments should be reversible, whenever possible. 

The majority of treatments for moisture management in this 

Brief stress preservation maintenance for materials, effective 

drainage of troublesome ground moisture, and improved 

interior ventilation. 

The Brief encourages a systematic approach for evaluating 

moisture problems which, in some cases, can be undertaken 

by a building owner. Because the source of moisture can be 

elusive, it may be necessary to consult with historic 

preservation professionals prior to starting work that would 

affect historic materials. Architects, engineers, conservators, 

preservation contractors, and staff of State Historic 

Preservation Offices (SHPOs) can provide such advice. 

Regardless of who does the work, however, these are the 

principles that should guide treatment decisions: 

• A void remedial treatments without prior careful 

diagnosis. 

• Undertake treatments that protect the historical 

significance of the resource. 

• Address issues of ground-related moisture and rain runoff thoroughly. 

• Manage existing moisture conditions before introducing 

humidified I dehumidified mechanical systems. 

• Implement a program of ongoing monitoring and 

maintenance once moisture is controlled or managed. 

• Be aware of significant landscape and archeological 

resources in areas to be excavated. 

Finally, mitigating the effects of catastrophic moisture, such 

as floods, requires a different approach and will not be 

addressed fully in this Brief. 

Fig. 1. Moisture problems, if not properly corrected, will increase damage to 

historic buildings. This waterproof coating trapped moisture from the leaking 

roof, causing portions of the masonry parapet to fail. Photo: NPS Files. 

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How and Where to Look for 

Damaging Moisture 

Finding, treating, and managing the sources of damaging 

moisture requires a systematic approach that takes time, 

patience, and a thorough examination of all aspects of the 

problem-including a series of variable conditions (See this 

page). Moisture problems may be a direct result of one of 

these factors or may be attributable to a combination of 

interdependent variables. 

Factors Contributing to Moisture Problems 

A variety of simultaneously existing conditions 

contribute to moisture problems in old buildings. For 

recurring moisture problems, it may be necessary for the 

owner or preservation professional to address many, if 

not all, of the following variables: 

• Types of building materials and construction systems 

• Type and condition of roof and site drainage systems 

and their rates of discharge 

• Type of soil, moisture content, and surface I subsurface 

water flow adjacent to building 

• Building usage and moisture generated by occupancy 

• Condition and absorption rates of materials 

• Type, operation, and condition of heating, ventilating, 

cooling, humidification/ dehumidification, and 

plumbing systems 

• Daily and seasonal changes in sun, prevailing winds, 

rain, temperature, and relative humidity (inside and 

outside), as well as seasonal or tidal variations in 

groundwater levels 

• Unusual site conditions or irregularities of construction 

• Conditions in affected wall cavities, temperature and 

relative humidity, and dewpoints 

• Amount of air infiltration present in a building 

• Adjacent landscape and planting materials 

Fig. 2. Historic buildings plagued by dampn~ss problems will benefit 

from systematic documentation to set a baselzne agamst whzch mozsture 

changes can be measured. Exte:ior ar~as with hisher moisture levels may 

have algae growth or discoloratwn stams. Drawmg: fohn H. Stubbs. 

Diagnosing and treating the cause of moisture problems 

requires looking at both the localized decay, as well as 

understanding the performance of the entire building and 

site. Moisture is notorious for traveling far from the source, 

and moisture movement within concealed areas of the 

building construction make accurate diagnosis of the source 

and path difficult. Obvious deficiencies, such as broken 

pipes, clogged gutters, or cracked walls that contribute to 

moisture damage, should always be corrected promptly. 

For more complicated problems, it may take several months 

or up to four seasons of monitoring and evaluation to 

complete a full diagnosis. Rushing to a solution without 

adequate documentation can often result in the unnecessary 

removal of historic materials-and worse-the creation of 

long-term problems associated with an increase, rather than 

a decrease, in the unwanted moisture. 

Looking for Signs 

Identifying the type of moisture damage and discovering its 

source or sources usually involves the human senses of 

sight, smell, hearing, touch, and taste combined with 

intuition. Some of the more common signs of visible as 

well as hidden moisture damage (see fig. 2, 3) include: 

• Presence of standing water, mold, fungus, or mildew 

• Wet stains, eroding surfaces, or efflorescence (salt 

deposits) on interior and exterior surfaces 

• Flaking paint and plaster, peeling wallpaper, or moisture 

blisters on finished surfaces 

• Dank, musty smells in areas of high humidity or poorly 

ventilated spaces 

• Rust and corrosion stains on metal elements, such as 

anchorage systems and protruding roof nails in the attic 

• Cupped, warped, cracked, or rotted wood 

• Spalled, cracked masonry or eroded mortar joints 

• Faulty roofs and gutters including missing roofing slates, 

tiles, or shingles and poor condition of flashing or gutters 

• Condensation on window and wall surfaces 

• Ice dams in gutters, on roofs, or moisture in attics 

Fig. 3. The deterioration of this wooden cover was a sign that water was leaking from the fan coil unit behind. Photo: author. 
Uncovering and Analyzing Moisture 

Problems 

Moisture comes from a variety of external sources. Most 

problems begin as a result of the weather in the form of rain 

or snow, from high ambient relative humidity, or from high 

water tables. But some of the most troublesome moisture 

damage in older buildings may be from internal sources, 

such as leaking plumbing pipes, components of heating, 

cooling, and climate control systems, as well as sources 

related to use or occupancy of the building. In some cases, 

moisture damage may be the result of poorly designed 

original details, such as projecting outriggers in rustic 

structures that are vulnerable to rotting, and may require 

special treatment. 

The five most common sources of unwanted moisture 

include: 

• Above grade exterior moisture entering the building 

• Below grade ground moisture entering the building 

• Leaking plumbing pipes and mechanical equipment 

• Interior moisture from household use and climate control 

systems 

• Water used in maintenance and construction materials. 

Above grade exterior moisture generally results from 

weather related moisture entering through deteriorating 

materials as a result of deferred maintenance, structural 

settlement cracks, or damage from high winds or storms 

(see fig. 4). Such sources as faulty roofs, cracks in walls, 

and open joints around window and door openings can be 

corrected through either repair or limited replacement. Due 

to their age, historic buildings are notoriously "drafty," 

allowing rain, wind, and damp air to enter through missing 

mortar joints; around cracks in windows, doors, and wood 

siding; and into uninsulated attics. In some cases, 

excessively absorbent materials, such as soft sandstone, 

become saturated from rain or gutter overflows, and can 

allow moisture to dampen interior surfaces. Vines or other 

vegetative materials allowed to grow directly on building 

Fig. 4. Deferred maintenance often leads to blocked gutters and 

downspouts. Thzs cracked gutter sy ~z allowed moisture to penetrate the upper exterwr wall, erode mortar JOznts, and rot fascia boards. Photo: NPS files. 

materials without trellis or other framework can cause 

damage from roots eroding mortar joints and foundations 

as well as dampness being held against surfaces. In most 

cases, keeping vegetation off buildings, repairing damaged 

materials, replacing flashings, rehanging gutters, repairing 

downspouts, repainting mortar, caulking perimeter joints 

around windows and doors, and repainting surfaces can 

alleviate most sources of unwanted exterior moisture from 

entering a building above grade. 

Below grade ground moisture is a major source of 

unwanted moisture for historic and older buildings. Proper 

handling of surface rain run-off is one of the most important 

measures of controlling unwanted ground moisture. Rain water 

is often referred to as "bulk moisture" in areas that receive 

significant annual rainfalls or infrequent, but heavy, 

precipitation. For example, a heavy rain of 2" per hour can 

produce 200 gallons of water from downspout discharge 

alone for a house during a one hour period. When soil is 

saturated at the base of the building, the moisture will wet 

footings and crawl spaces or find its way through cracks in 

foundation walls and enter into basements (see fig. 5). 

Moisture in saturated basement or foundation walls-also 

exacerbated by high water tables-will generally rise up 

within a wall and eventually cause deterioration of the 

masonry and adjacent wooden structural elements. 

Builders traditionally left a working area, known as a 

builder's trench, around the exterior of a foundation wall. 

These trenches have been known to increase moisture 

problems if the infill soil is less than fully compacted or 

includes rubble backfill, which, in some cases, may act as a 

reservoir holding damp materials against masonry walls. 

Broken subsurface pipes or downspout drainage can leak 

into the builder's trench and dampen walls some distance 

from the source. Any subsurface penetration of the 

foundation wall for sewer, water, or other piping also can 

act as a direct conduit of ground moisture unless these 

holes are well sealed. A frequently unsuspected, but 

serious, modern source of ground moisture is a landscape 

irrigation system set too close to the building. Incorrect 

placement of sprinkler heads can add a tremendous amount 

of moisture at the foundation level and on wall surfaces. 

Fig. 5. Excavating this foundation revealed that the downspout pipe had corroded at the " u-trap" and was leaking moisture into the soil. Openings 

around the horizontal water supply line and cracks in the wall allowed 

moisture to penetrate the basement in multiple locations. Photo: author. 

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The ground, and subsequently the building, will stay much 

drier by 1) re-directing rain water away from the 

foundation through sloping grades, 2) capturing and 

disposing downspout water well away from the building, 3) 

developing a controlled ground gutter or effective drainage 

for buildings historically without gutters and downspouts, 

and 4) reducing splash-back of moisture onto foundation 

walls. The excavation of foundations and the use of 

dampproof coatings and footing drains should only be used 

after the measures of reducing ground moisture listed 

above have been implemented. 

Leaking plumbing pipes and mechanical equipment can 

cause immediate or long-term damage to historic building 

interiors. Routine maintenance, repair, or, if necessary, 

replacement of older plumbing and mechanical equipment 

are common solutions. Older water and sewer pipes are 

subject to corrosion over time. Slow leaks at plumbing 

joints hidden within walls and ceilings can ultimately rot 

floor boards, stain ceiling plaster, and lead to decay of 

structural members. Frozen pipes that crack can damage 

interior finishes (see fig. 6 ). In addition to leaking 

plumbing pipes, old radiators in some historic buildings 

have been replaced with water-supplied fan coil units 

which tend to leak. These heating and cooling units, as well 

as central air equipment, have overflow and condensation 

pans that require cyclical maintenance to avoid mold and 

mildew growth and corrosion blockage of drainage 

channels. Uninsulated forced-air sheet metal ductwork and 

cold water pipes in walls and ceilings often allow 

condensation to form on the cold metal, which then drips 

and causes bubbling plaster and peeling paint. Careful 

design and vigilant maintenance, as well as repair and 

insulating pipes or ductwork, will generally rid the 

building of these common sources of moisture. 

Fig. 6. Uninsulated plumbing pipes close to the exterior wall froze and 

cracked, wetting this ornamental plaster ceiling before the water supply 

line could be shut off. As a result, limited portions of the ceiling needed 

reattaching. Photo: author. 

Interior moisture from building use and modern 

humidified heating and cooling systems can create serious 

problems. In northern U.S. climates, heated buildings will 

have winter-time relative humidity levels ranging from 

10%-35% Relative Humidity (RH). A house with four 

occupants generates between 10 and 16 pounds of water a 

day (approximately 1-2 gallons) from human residents. 

Moisture from food preparation, showering, or laundry use 

will produce condensation on windows in winter climates. 

When one area or floor of a building is air-conditioned and 

another area is not, there is the chance for condensation to 

occur between the two areas. Most periodic condensation 

does not create a long-term problem. 

Humidified climate control systems are generally a major 

problem in museums housed within historic buildings. 

They produce between 35%-55% RH on average which, as 

a vapor, will seek to dissipate and equalize with adjacent 

spaces (see fig. 7). Moisture can form on single-glazed 

windows in winter with exterior temperatures below 30 F 

and interior temperatures at 70 F with as little as 35% RH. 

Frequent condensation on interior window surfaces is an 

indication that moisture is migrating into exterior walls, 

which can cause long-term damage to historic materials. 

Materials and wall systems around climate controlled areas 

may need to be made of moisture resistant finishes in order 

to handle the additional moisture in the air. Moist interior 

conditions in hot and humid climates will generate mold 

and fungal growth. Unvented mechanical equipment, such 

as gas stoves, driers, and kerosene heaters, generate large 

quantities of moisture. It is important to provide adequate 

ventilation and find a balance between interior 

temperature, relative humidity, and airflow to avoid 

interior moisture that can damage historic buildings. 

Fig. 7. Condensation dripping from the large overhead courtyard 

skylight was damaging the masonry in this museum. A new skylight 

with thermal glazing was installed, replacing the deteriorated singleglazed unit. A new climate control system monitors interior temperature 

and humidity. Photo:© Isabella Stewart Gardner Museum, Boston. 

Moisture from maintenance and construction materials 

can cause damage to adjacent historic materials. Careless 

use of liquids to wash floors can lead to water seepage 

through cracks and dislodge adhesives or cup and curl 

materials. High-pressure power washing of exterior walls 

and roofing materials can force water into construction 

joints where it can dislodge mortar, lift roofing tiles, and 

saturate frame walls and masonry. Replastered or newly 
plastered interior walls or the construction of new additions 

attached to historic buildings may hold moisture for 

months; new plaster, mortar, or concrete should be fully 

cured before they are painted or finished. The use of 

materials in projects that have been damaged by moisture 

prior to installation or have too high a moisture content 

may cause concealed damage (see fig. 8). 

Fig. 8. Damaging moisture conditions can occur during construction. 

Peeling paint on this newly rehabilitated frame wall was attributed to 

wall insulatio11 that had become wet during the project and was not 

discovered. Photo: NPS Files. 

Transport or Movement of Moisture 

Knowing the five most common sources of moisture that 

cause damage to building materials is the first step in 

diagnosing moisture problems. But it is also important to 

understand the basic mechanisms that affect moisture 

movement in buildings. Moisture transport, or movement, 

occurs in two states: liquid and vapor. It is directly related 

to pressure differentials. For example, water in a gaseous or 

vapor state, as warm moist air, will move from its high 

pressure area to a lower pressure area where the air is 

cooler and drier. Liquid water will move as a result of 

differences in hydrostatic pressure or wind pressure. It is 

the pressure differentials that drive the rate of moisture migration 

in either state. Because the building materials themselves 

resist this moisture movement, the rate of movement will 

depend on two factors: the permeability of the materials 

when affected by vapor and the absorption rates of 

materials in contact with liquid. 

The mechanics, or physics, of moisture movement is 

complex, but if the driving force is difference in pressure, 

then an approach to reducing moisture movement and its 

damage is to reduce the difference in pressure, not to 

increase it. That is why the treatments discussed in this 

Brief will look at managing moisture by draining bulk moisture 

and ventilating vapor moisture before setting up new barriers 

with impermeable coatings or over-pressurized new climate 

control systems that threaten aging building materials and 

archaic construction systems. 

Three forms of moisture transport are particularly 

important to understand in regards to historic buildingsinfiltration, capillary action, and vapor diffusion -remembering, at the same time, that the subject is infinitely 

complex and, thus, one of continuing scientific study (see 

fig. 9). Buildings were traditionally designed to deal with 

the movement of air. For example, cupolas and roof 

lanterns allowed hot air to rise and provided a natural draft 

to pull air through buildings. Cavity walls in both frame 

and masonry buildings were constructed to allow moisture 

to dissipate in the air space between external and internal 

walls. Radiators were placed in front of windows to keep 

cold surfaces warm, thereby reducing condensation on 

these surfaces. Many of these features, however, have been 

altered over time in an effort to modernize appearances, 

improve energy efficiency, or accommodate changes in use. 

The change in use will also affect moisture movement, 

particularly in commercial and industrial buildings with 

modern mechanical systems. Therefore, the way a building 

handles air and moisture today may be different from that 

intended by the original builder or architect, and poorly 

conceived changes may be partially responsible for chronic 

moisture conditions. 

Moisture moves into and through materials as both a visible 

liquid (capillary action) and as a gaseous vapor (infiltration 

and vapor diffusion). Moisture from leaks, saturation, 

rising damp, and condensation can lead to the deterioration 

of materials and cause an unhealthy environment. 

Moisture in its solid form, ice, can also cause damage from 

frozen, cracked water pipes, or split gutter seams or spalled 

masonry from freeze-thaw action. Moisture from melting 

ice dams, leaks, and condensation often can travel great 

distances down walls and along construction surfaces, 

pipes, or conduits. The amount of moisture and how it 

deteriorates materials is dependent upon complex forces 

and variables that must be considered for each situation. 

Fig. 9. The dynamic forces that move air and moisture through a building are important to understand particularlly when selecting a treatment to correct a moisture problem. Air infiltration, capillary 

action, and vapor diffusion all affect the wetting and drying of materials. 

Drawing; NPS Files. 

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Determining the way moisture is handled by the building is 

further complicated because each building and site is 

unique. Water damage from blocked gutters and 

downspouts can saturate materials on the outside, and high 

levels of interior moisture can saturate interior materials. 

Difficult cases may call for technical evaluation by 

consultants specializing in moisture monitoring and 

diagnostic evaluation. In other words, it may take a team to 

effectively evaluate a situation and determine a proper 

approach to controlling moisture damage in old buildings. 

Infiltration is created by wind, temperature gradients (hot 

air rising), ventilation fan action, and the stack or chimney 

effect that draws air up into tall vertical spaces. Infiltration 

as a dynamic force does not actually move liquid water, but 

is the vehicle by which dampness, as a component of air, 

finds its way into building materials. Older buildings have 

a natural air exchange, generally from 1 to 4 changes per 

hour, which, in turn, may help control moisture by diluting 

moisture within a building. The tighter the building 

construction, however, the lower will be the infiltration rate 

and the natural circulation of air. In the process of 

infiltration, however, moisture that has entered the building 

and saturated materials can be drawn in and out of 

materials, thereby adding to the dampness in the air (see 

fig. 10). Inadequate air circulation where there is excessive 

moisture (i.e., in a damp basement), accelerates the 

deterioration of historic materials. To reduce the unwanted 

moisture that accompanies infiltration, it is best to 

incorporate maintenance and repair treatments to close 

joints and weatherstrip windows, while providing 

controlled air exchanges elsewhere. The worst approach is 

to seal the building so completely, while limiting fresh air 

intake, that the building cannot breathe. 

Fig. 10. Infiltration of damp air can occur around loose-fitting or deteriorated window sash and through cracks or open joints in building 

exteriors. Photo: Ann Brooks Prueher. 

Capillary action occurs when moisture in saturated porous 

building materials, such as masonry, wicks up or travels 

vertically as it evaporates to the surface. In capillary 

attraction, liquid in the material is attracted to the solid 

surface of the pore structure causing it to rise vertically; 

thus, it is often called "rising damp," particularly when 

found in conjunction with ground moisture. It should not, 

however, be confused with moisture that laterally 

penetrates a foundation wall through cracks and settles in 

the basement. Not easily controlled, most rising damp 

comes from high water tables or a constant source under 

the footing. In cases of damp masonry walls with capillary 

action, there is usually a whitish stain or horizontal tide 

mark of efflorescence that seasonally fluctuates about 1- 3 

feet above grade where the excess moisture evaporates 

from the wall (see fig. 11). This tide mark is full of salt 

crystals, that have been drawn from the ground and 

building materials along with the water, making the 

masonry even more sensitive to additional moisture 

absorption from the surrounding air. Capillary migration 

of moisture may occur in any material with a pore structure 

where there is a constant or recurring source of moisture. 

Fig. 11. Capillary rise of moisture in masonry is often accompanied with a horizon tal tide-mark line several feet above the grade, as seen here. 

Removing or redirecting as much ground moisture as possible usually 

helps reduce moisture within a wall. Photo: NPS Files. 

The best approach for dealing with capillary rise in building 

materials is to reduce the amount of water in contact with 

historic materials. If that is not possible due to chronically 

high water tables, it may be necessary to introduce a 

horizontal damp-proof barrier, such as slate course or a 

lead or plastic sheet, to stop the vertical rise of moisture. 

Moisture should not be sealed into the wall with a 

waterproof coating, such as cement parging or vinyl wall 

coverings, applied to the inside of damp walls. This will 

only increase the pressure differential as a vertical barrier 

and force the capillary action, and its destruction of 

materials, higher up the wall. 

Vapor diffusion is the natural movement of pressurized 

moisture vapor through porous materials. It is most readily 

apparent as humidified interior air moves out through 

walls to a cooler exterior. In a hot and humid climate, the 

reverse will happen as moist hot air moves into cooler, 

dryer, air-conditioned, interiors. The movement of the 

moisture vapor is not a serious problem until the dewpoint 

temperature is reached and the vapor changes into liquid 

moisture known as condensation. This can occur within a 

wall or on interior surfaces. Vapor diffusion will be more of 
a problem for a frame structure with several layers of infill 

materials within the frame cavity than a dense masonry 

structure. Condensation as a result of vapor migration 

usually takes place on a surface or film, such as paint, 

where there is a change in permeability. 

The installation of climate control systems in historic 

buildings (mostly museums) that have not been properly 

designed or regulated and that force pressurized damp air 

to diffuse into perimeter walls is an ongoing concern. These 

newer systems take constant monitoring and back-up 

warning systems to avoid moisture damage. 

Long-term and undetected condensation or high moisture 

content can cause serious structural damage as well as an 

unhealthy environment, heavy with mold and mildew 

spores. Reducing the interior I exterior pressure differential 

and the difference between interior and exterior 

temperature and relative humidity helps control unwanted 

vapor diffusion. This can sometimes be achieved by 

reducing interior relative humidity. In some instances, 

using vapor barriers, such as heavy plastic sheeting laid 

over damp crawl spaces, can have remarkable success in 

stopping vapor diffusion from damp ground into buildings. 

Yet, knowledgeable experts in the field differ regarding the 

appropriateness of vapor barriers and when and where to 

use them, as well as the best way to handle natural 

diffusion in insulated walls. 

Adding insulation to historic buildings, particularly in 

walls of wooden frame structures, has been a standard 

modern weatherization treatment, but it can have a 

disastrous effect on historic buildings. The process of 

installing the insulation destroys historic siding or plaster, 

and it is very difficult to establish a tight vapor barrier. 

While insulation has the benefit of increasing the efficiency 

of heating and cooling by containing temperature 

controlled air, it does not eliminate surfaces on which 

damaging moisture can condense. For insulated residential 

frame structures, the most obvious sign of a moisture 

Electrical 

Outlet 

WARM 

MOIST 

INTERIOR 

Codensation 

On Cool 

Surface 

Fig. 12. Vapor diffusion can result in damp air migrating into absorbent 

materials and condensing on colder surfaces, thereby wetting insulation, 

damaging electrical conduits, and causing deterioration of the wooden 

framing. Drawing: NPS Files. 

diffusion problem is peeling paint on wooden siding, even 

after careful surface preparation and repainting. Vapor 

impermeable barriers such as plastic sheeting, or more 

accurately, vapor retarders, in cold and moderate climates 

generally help slow vapor diffusion where it is not wanted. 

In regions where humidified climate control systems are 

installed into insulated frame buildings, it is important to 

stop interstitial, or in-wall, dewpoint condensation. This is 

very difficult because humidified air can penetrate breaches 

in the vapor barrier, particularly around electrical outlets 

(see fig. 12). Improperly or incompletely installed retrofit 

vapor barriers will cause extensive damage to the building, 

just in the installation process, and will allow trapped 

condensation to wet the insulation and sheathing boards, 

corrode metal elements such as wiring cables and metal 

anchors, and blister paint finishes. Providing a tight wall 

vapor barrier, as well as a ventilated cavity behind wooden 

clapboards or siding appears to help insulated frame walls, if 

the interior relative humidity can be adjusted or monitored 

to avoid condensation. Correct placement of vapor 

retarders within building construction will vary by region, 

building construction, and type of climate control system. 

Surveying and Diagnosing Moisture Damage: 

Key Questions to Ask 

It is important for the building to be surveyed first and the 

evidence and location of suspected moisture damage 

systematically recorded before undertaking any major work 

to correct the problem. This will give a baseline from 

which relative changes in condition can be noted. 

When materials become wet, there are specific physical 

changes that can be detected and noted in a record book or 

on survey sheets. Every time there is a heavy rain, snow 

storm, water in the basement, or mechanical systems 

failure, the owner or consultant should note and record the 

way moisture is moving, its appearance, and what variables 

might contribute to the cause. Standing outside to observe a 

building in the rain may answer many questions and help trace 

the movement of water into the building. Evidence of 

deteriorating materials that cover more serious moisture 

damage should also be noted, even if it is not immediately 

clear what is causing the damage. ( For example, water 

stains on the ceiling may be from leaking pipes, blocked fan 

coil drainage pans above, or from moisture which has 

penetrated around a poorly sloped window sill above.) 

Don't jump to conclusions, but use a systematic approach to 

help establish an educated theory- or hypothesis- of 

what is causing the moisture problem or what areas need 

further investigation. 

Surveying moisture damage must be systematic so that relative 

changes can be noted. Tools for investigating can be as simple 

as a notebook, sketch plans, binoculars, camera, aluminum 

foil, smoke pencil, and flashlight. The systematic approach 

involves looking at buildings from the top down and from 

the outside to the inside. Photographs, floor plans, site plan, 

and exterior elevations - even roughly sketched- should 

be used to indicate all evidence of damp or damaged 

materials, with notations for musty or poorly ventilated 

areas. Information might be needed on the absorption and 

permeability characteristics of the building materials and 

soils. Exterior drainage patterns should be noted and these 

base plans referred to on a regular basis in different seasons 

and in differing types of weather (see fig. 13). 

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Glossary: 

Air flow! infiltration: The movement that carries moist air into 

and through materials. Air flow depends on the difference 

between indoor and outdoor pressures, wind speed and 

direction as well as the permeability of materials. 

Bulk water: The large quantity of moisture from roof and 

ground run-off that can enter into a building either above 

grade or below grade. 

Capillary action: The force that moves moisture through the 

pore structure of materials. Generally referred to as rising 

damp, moisture at or below the foundation level will rise 

vertically in a wall to a height at which the rate of 

evaporation balances the rate at which it can be drawn up by 

capillary forces. 

Condensation: The physical process by which water vapor is 

transformed into a liquid when the relative humidity of the 

air reaches 100% and the excess water vapor forms, 

generally as droplets, on the colder adjacent surface. 

Convection: Heat transfer through the atmosphere by a 

difference in force or air pressure is one type of air transport. 

Sometimes referred to as the "stack effect," hotter less dense 

air will rise, colder dense air will fall creating movement of 

air within a building. 

Dewpoint: The temperature at which water vapor condenses 

when the air is cooled at a constant pressure and constant 

moisture content. 

Diffusion: The movement of water vapor through a material. 

Diffusion depends on vapor pressure, temperature, relative 

humidity, and the permeability of a material. 

Evaporation: The transformation of liquid into a vapor, 

generally as a result of rise of temperature, is the opposite of 

condensation. Moisture in damp soil, such as in a crawl 

space, can evaporate into the air, raise the relative humidity 

in that space, and enter the building as a vapor. 

Ground moisture: The saturated moisture in the ground as a 

result of surface run-off and naturally occuring water tables. 

Ground moisture can penetrate through cracks and holes in 

foundation walls or can migrate up from moisture under the 

foundation base. 

Monitoring instrumentation: These devices are generally used 

for long term diagnostic analysis of a problem, or to measure 

the preformance of a treatment, or to measure changes of 

conditions or environment. In-wall probes or sensors are 

often attached to data-loggers which can be down-loaded 

into computers. 

Permeability: A characteristic of porosity of a material 

generally listed as the rate of diffusion of a pressurized gas 

through a material. The pore structure of some materials 

allows them to absorb or adsorb more moisture than other 

materials. Limestones are generally more permeable than 

granites. 

Relative humidity (RH): Dampness in the air is measured as 

the percent of water vapor in the air at a specific 

temperature relative to the amount of water vapor that can 

be held in a vapor form at that specific temperature. 

Survey instrumentation: technical instrumentation that is used 

on-site to provide quick readings of specific physical 

conditions. Generally these are hand-held survey 

instruments, such as moisture, temperature and relative 

humidity readers, dewpoint sensors, and fiber optic 

horoscopes. 

DATE: June 2 1992 

WEATHER:.__,Fu.aw..ir __ 

TEMP: ...,70'- 0 __ 

TIME: 10:00AM 

Fig. 13. Using sketch plans and elevation drawings to record the 

moisture damage along with the date, time, and weather conditions will 

show how moisture is affecting buildings over time. Drawing: Courtesy, 

Quinn Evans Architects. 

It is best to start with one method of periodic 

documentation and to use this same method each time. 

Because moisture is affected by gravity, many surveys start 

with the roof and guttering systems and work down 

through the exterior walls. Any obvious areas of water 

penetration, damaged surfaces, or staining should be noted. 

Any recurring damp or stain patterns, both exterior and 

interior, should also be noted with a commentary on the 

temperature, weather, and any other facts that may be 

relevant (driving rains, saturated soil, high interior 

humidity, recent washing of the building, presence of a 

lawn watering system, etc.). 

The interior should be recorded as well, beginning with the 

attic and working down to the basement and crawl space. 

It may be necessary to remove damaged materials 

selectively in order to trace the path of moisture or to 

pinpoint a source, such as a leaking pipe in the ceiling. 

The use of a basic resistance moisture meter, available in 

many hardware stores, can identify moisture contents of 

materials and show, over time, if wall surfaces are drying 

or becoming damper (see fig. 14). A smoke pencil can chart 

air infiltration around windows or draft patterns in interior 

spaces. For a quick test to determine if a damp basement is 

caused by saturated walls or is a result of condensation, 

tape a piece of foil onto a masonry surface and check it 

after a day or two; if moisture has developed behind the 

foil, then it is corning from the masonry. If condensation is 

on the surface of the foil, then moisture is from the air. 

Comparing current conditions with previous conditions, 

historic drawings, photographs, or known alterations may 

also assist in the final diagnosis. A chronological record, 

showing improvement or deterioration, should be backed 

up with photographs or notations as to the changing size, 

condition, or features of the deterioration and how these 

changes have been affected by variables of temperature and 

rainfall. If a condition can be related in time to a particular 

event, such as efflorescence developing on a chimney after 

the building is no longer heated, it may be possible to 

isolate a cause, develop a hypothesis, and then test the 

hypothesis (by adding some temporary heat), before 

applying a remedial treatment. 
Fig. 14. Using instruments in this damp-check kit can help determine the relative change in wet conditions over time. This involves readings of 

air temperature, computing dewpoint temperatures, and tracking the 

moisture content of materials to indicate if they are drying properly. 

Photo: Dell Corporation. 

If the owner or consultant has access to moisture survey 

and monitoring equipment such as resistance moisture 

meters, dewpoint indicators, salt detectors, infrared 

thermography systems, psychrometer, fiber-optic 

horoscopes, and miniaturized video cameras, additional 

quantified data can be incorporated into the survey (see fig. 

15). If it is necessary to track the wetting and drying of 

walls over a period of time, deep probes set into walls and 

in the soil with connector cables to computerized data 

loggers or the use of long-term recording of 

hygrothermographs may require a trained specialist. 

Miniaturized fiber-optic video cameras can record the 

condition of subsurface drain lines without excavation (see 

fig. 16). It should be noted, however, that instrumentation, 

while extremely useful, cannot take the place of careful personal 

observation and analysis. Relying on instrumentation alone 

rarely will give the owner the information needed to fully 

diagnose a moisture problem. 

Fig. 15. Psychrometric charts quantify the amount of relative humidity a building can tolerate before dewpoint condensation occurs. This is 

important when the range of temperature and humidity are critical to both collections management and historic building preservation. Chart: Landmark Facilities Group. 

Fig. 16. Contractors specializing in 

building diagnostics often have video cameras or fiber optic equipment that 

allow the viewing of 

inaccessible areas. This is particularly 

helpful in chimney 

flues or subsurface drains, as shown 

here. In the past, these areas would 

need to be excavated 

for visual 

inspection. Photo: author. 

To avoid jumping to a quick-potentially erroneousconclusion, a series of questions should be asked first. This 

will help establish a theory or hypothesis that can be tested 

to increase the chances that a remedial treatment will 

control or manage existing moisture. 

How is water draining around building and site? What is 

the effectiveness of gutters and downspouts? Are the 

slopes or grading around foundations adequate? What are 

the locations of subsurface features such as wells, cisterns, 

or drainage fields? Are there subsurface drainage pipes (or 

drainage boots) attached to the downspouts and are they in 

good working condition? Does the soil retain moisture or 

allow it to drain freely? Where is the water table? Are there 

window wells holding rain water? What is the flow rate of 

area drains around the site (can be tested with a hose for 

several minutes)? Is the storm piping out to the street 

sufficient for heavy rains, or does water chronically back up 

on the site? Has adjacent new construction affected site 

drainage or water table levels? 

How does water/moisture appear to be entering the 

building? Have all five primary sources of moisture been 

evaluated? What is the condition of construction materials 

and are there any obvious areas of deterioration? Did this 

building have a builder's trench around the foundation that 

could be holding water against the exterior walls? Are the 

interior bearing walls as well as the exterior walls showing 

evidence of rising damp? Is there evidence of hydrostatic 

pressure under the basement floor such as water 

percolating up through cracks? Has there been moisture 

damage from an ice dam in the last several months? Is 

damage localized, on one side of the building only, or over 

a large area? 

What are the principal moisture dynamics? Is the moisture 

condition from liquid or vapor sources? Is the attic 

moisture a result of vapor diffusion as damp air comes up 

through the cavity walls from the crawl space or is it from a 

leaking roof? Is the exterior wall moisture from rising 

damp with a tide mark or are there uneven spots of 

dampness from foundation splash back, or other ground 

9 
10 

moisture conditions? Is there adequate air exchange in the 

building, particularly in damp areas, such as the basement? 

Has the height of the water table been established by 

inserting a long pipe into the ground in order to record the 

water levels? 

How is the interior climate handling moisture? Are there 

areas in the building that do not appear to be ventilating 

well and where mold is growing? Are there historic 

features that once helped the building control air and 

moisture that can be reactivated, such as operable skylights 

or windows? Could dewpoint condensation be occurring 

behind surfaces, since there is often condensation on the 

windows? Does the building feel unusually damp or smell 

in an unusual way that suggest the need for further study? 

Is there evidence of termites, carpenter ants, or other pests 

attracted to moist conditions? Is a dehumidifier keeping 

the air dry or is it, in fact, creating a cycle where it is 

actually drawing moisture through the foundation wall? 

Does the moisture problem appear to be intermittent, 

chronic, or tied to specific events? Are damp conditions 

occuring within two hours of a heavy rain or is there a 

delayed reaction? Does rust on most nail heads in the attic 

indicate a condensation problem? What are the wet 

patterns that appear on a building wall during and after a 

rain storm? Is it localized or in large areas? Can these rain 

patterns be tied to gutter over-flows, faulty flashing, or 

saturation of absorbent materials? Is a repaired area 

holding up well over time or is there evidence that 

moisture is returning? Do moisture meter readings of wall 

cavities indicate they are wet, suggesting leaks or 

condensation in the wall? 

Once a hypothesis of the source or sources of the moisture 

has been developed from observation and recording of 

data, it is often useful to prove or disprove this hypothesis 

with interim treatments, and, if necessary, the additional 

use of instrumentation to verify conditions. For damp 

basements, test solutions can help determine the cause. For 

example, surface moisture in low spots should be 

redirected away from the foundation wall with regrading 

to determine if basement dampness improves. If there is 

still a problem, determine if subsurface downspout 

collection pipes or cast iron boots are not functioning 

properly. The above grade downspouts can be 

disconnected and attached to long, flexible extender pipes 

and redirected away from the foundation (see fig. 17). If, 

after a heavy rain or a simulation using a hose, there is no 

improvement, look for additional ground moisture sources 

such as high water tables, hidden cisterns, or leaking water 

service lines as a cause of moisture in the basement. New 

data will lead to a new hypothesis that should be tested and 

verified. The process of elimination can be frustrating, but is 

required if a systematic method of diagnosis is to be successful. 

Selecting an Appropriate Level of Treatment 

The treatments in chart format at the back of this 

publication are divided into levels based on the degree of 

moisture problems. Level I covers preservation 

maintenance; Level II focuses on repair using historically 

compatible materials and essentially mitigating damaging 

moisture conditions; and Level ill discusses replacement 

and alteration of materials that permit continued use in a 

chronically moist environment. It is important to begin 

Fig. 17. In testing a theory for the cause of basement wetness, the owner used long black extender pipes to direct roof run-off away from the 

foundation. This test established that the owner did not need expensive 

waterproofing of the foundation, but a better drainage system. Photo: 

Baird M. Smith. 

with Level I and work through to a manageable treatment 

as part of the control of moisture problems. Buildings in 

serious decay will require treatments in Level II, and 

difficult or unusual site conditions may require more 

aggressive treatments in Level ill. Caution should always 

be exercised when selecting a treatment. The treatments 

listed are a guide and not intended to be recommendations 

for specific projects as the key is always proper diagnosis. 

Start with the repair of any obvious deficiencies using 

sound preservation maintenance. If moisture cannot be 

managed by maintenance alone, it is important to reduce it 

by mitigating problems before deteriorated historic 

materials are replaced (see fig. 18). Treatments should not 

remove materials that can be preserved; should not involve 

extensive excavation unless there is a documented need; 

and should not include coating buildings with waterproof 

sealers that can exacerbate an existing problem. Some 

alteration to historic materials, structural systems, 

mechanical systems, windows, or finishes may be needed 

when excessive site moisture cannot be controlled by 

drainage systems, or in areas prone to floods. These 

changes, however, should, be sensitive to preserving those 

materials, features, and finishes that convey the historic 

character of the building and site. 

Ongoing Care 

Once the building has been repaired and the larger 

moisture issues addressed, it is important to keep a record 

of additional evidence of moisture problems and to protect 

the historic or old building through proper cyclical maintenance 

(see fig. 19) In some cases, particularly in museum 

environments, it is critical to monitor areas vulnerable to 

moisture damage. In a number of historic buildings, inwall moisture monitors are used to ensure that the moisture 

purposely generated to keep relative humidity at ranges 

appropriate to a museum collection does not migrate into 

walls and cause deterioration. The potential problem with 

all systems is the failure of controls, valves, and panels over 

time. Back-up systems, warning devices, properly trained 

staff and an emergency plan will help control damage if 

there is a system failure. 
4" Top 

Perforated 

Pipe Around -++H-+'*" 

Perimeter 

Walls 

Fig. 18. This detail drawing shows a sub-surface perimeter drain in 

conjunction with a historic brick ground gutter system to help control 

roof run-off moisture from entering the historic foundation. Detail: 

Courtesy, Gunston Hall Plantation. Photo: Elizabeth Sasser. 

Fig. 19. Maintaining gutters and downspouts in good operable 

condition, repairing exteriors to keep water out, redirecting damaging 

moisture away from foundations and con trolling interior moisture and 

condensation are all important when holding the line on moisture 

deterioration. Photo: Nebraska State Historical Society. 

Ongoing maintenance and vigilance to situations that could 

potentially cause moisture damage must become a routine 

part of the everyday life of a building. The owner or staff 

responsible for the upkeep of the building should inspect 

the property weekly and note any leaks, mustiness, or 

blocked drains. Again, observing the building during a rain 

will test whether ground and gutter drainage are working 

well. 

For some buildings a back-up power system may be 

necessary to keep sump pumps working during storms 

when electrical power may be lost. For mechanical 

equipment rooms, condensation pans, basement floors, and 

laundry areas where early detection of water is important, 

there are alarms that sound when their sensors come into 

contact with moisture. 

Conclusion 

Moisture in old and historic buildings, though difficult to 

evaluate, can be systematically studied and the appropriate 

protective measures taken. Much of the documentation and 

evaluation is based on common sense combined with an 

understanding of historic building materials, construction 

technology, and the basics of moisture and air movement. 

Variables can be evaluated step by step and situations 

creating direct or secondary moisture damage can generally 

be corrected. The majority of moisture problems can be 

mitigated with maintenance, repair, control of ground and 

roof moisture, and improved ventilation. For more complex 

situations, however, a thorough diagnosis and an 

understanding of how the building handles moisture at 

present, can lead to a treatment that solves the problem 

without damaging the historic resource. 

It is usually advantageous to eliminate one potential source 

of moisture at a time. Simultaneous treatments may set up 

a new dynamic in the building with its own set of moisture 

problems. Implementing changes sequentially will allow 

the owner or preservation professional to track the success 

of each treatment. 

Moisture problems can be intimidating to a building owner 

who has diligently tried to control them. Keeping a record 

of evidence of moisture damage, results of diagnostic tests, 

and remedial treatments, is beneficial to a building's longterm care. The more complete a survey and evaluation, the 

greater the success in controlling unwanted moisture now 

and in the future. 

Holding the line on unwanted moisture in buildings will be 

successful if 1) there is constant concern for signs of 

problems and 2) there is ongoing physical care provided by 

those who understand the building, site, mechanical 

systems, and the previous efforts to deal with moisture. 

For properties with major or difficult-to-diagnose 

problems, a team approach is often most effective. The 

owner working with properly trained staff, contractors and 

consultants can monitor, select, and implement treatments 

within a preservation context in order to manage moisture 

and to protect the historic resource. 

11 
12 

MOISTURE: LEVEL I PRESERVATION MAINTENANCE 

Exterior: Apply C1jclical maintenance procedures to eliminate rain and moisture 

infiltration. 

Roofing/ guttering: Make weather-tight and operational; inspect and 

clean gutters as necessary depending on number of nearby trees, but at 

least twice a year; inspect roofing at least once a year, preferably spring; 

replace missing or damaged roofing shingles, slates, or tiles; repair 

flashing; repair or replace cracked downspouts. 

Walls: Repair damaged surface materials; repoint masonry with 

appropriately formula ted mortar; prime and repaint wooden, metal, or 

masonry elements or surfaces; remove efflorescence from masonry with 

non-metallic bristle brushes. 

Window and door openings: Eliminate cracks or open joints; caulk or 

repoint around openings or steps; repair or reset weatherstripping; check 

flashing; repaint, as necessary. 

Ground: Apply regular maintenance procedures to eliminate standing water and vegetative 

threats to building/site. 

Grade: Eliminate low spots around building foundations; clean out 

existing downspout boots twice a year or add extension to leaders to carry 

moisture away from foundation; do a hose test to verify that surface 

drains are functioning; reduce moisture used to clean steps and walks; 

eliminate the use of chlorides to melt ice which can increase freeze/ thaw 

spalling of masonry; check operation of irrigation systems, hose bib leaks, 

and clearance of air conditioning condensate drain outlets. 

Crawl space: Check crawl space for animal infestation, termites, ponding 

moisture, or high moisture content; check foundation grilles for adequate 

ventilation; seasonally close grilles when appropriate- in winter, if not 

needed, or in summer if hot humid air is diffusing into air conditioned 

space. 

Foliage: Keep foliage and vines off buildings; trim overhanging trees to 

keep debris from gutters and limbs from rubbing against building; remove 

moisture retaining elements, such as firewood, from foundations. 

Basements and foundations: Increase ventilation and maintain surfaces to avoid moisture. 

Equipment: Check dehumidifiers, sump pump, vent fans, and water 

detection or alarm systems for proper maintenance as required; check 

battery back-up twice a year. 

Piping/ductwork: Check for condensation on pipes and insulate I seal 

joints, if necessary. 

Interior: Maintain equipment to reduce leaks and interior moisture. 

Plumbing pipes: Add insulation to plumbing or radiator pipes located in 

areas subject to freezing, such as along outside walls, in attics, or in 

unheated basements. 

Mechanical equipment: Check condensation pans and drain lines to keep 

clear; insulate and seal joints in exposed metal ductwork to avoid drawing 

in moist air. 

Cleaning: Routinely dust and clean surfaces to reduce the amount of 

water or moist chemicals used to clean building; caulk around tile floor 

and wall connections; and maintain floor grouts in good condition. 

Ventilation: Reduce household-produced moisture, if a problem, by 

increasing ventilation; vent clothes driers to the outside; install and 

always use exhaust fans in restrooms, bathrooms, showers, and kitchens, 

when in use. 

A. Inspecting the overall building on at 

least an annual basis will identify areas needing maintenance. A bucket lift is 

helpful for large buildings. Photo: author. 

B. Repair exterior surfaces, paint, and recaulk as needed. Photo: Williamsport Preservation 

Training Center (WPTC), NPS. 

C. Cleaning out gutters and downspouts 

should be done at least twice a year. 

Photo: WPTC, NPS. 

D. Protect the building from damage by 

maintaining equipment and using alarms, like 

this floor water sensor. Photo: Dell Corporation. 
MOISTURE: LEVEL II REPAIR AND CORRECTIVE ACTION 

Exterior: Repair features that have been damaged. Replace an extensively deteriorated 

feature with a new feature that matches in design, color, texture, and where possible, 

materials. 

Roofing: Repair roofing, parapets and overhangs that have allowed 

moisture to enter; add ice and water shield membrane to lower 3-4 feet or 

roofing in cold climates to limit damage from ice dams; increase attic 

ventilation, if heat and humidity build-up is a problem. Make gutters 

slope@ 1 I 8" to the foot. Use professional handbooks to size gutters and 

reposition, if necessary and appropriate to historic architecture. Add 

ventilated chimney caps to unused chimneys that collect rain water. 

Walls: Repair spalled masonry, terra cotta, etc. by selectively installing 

new masonry units to match; replace rotted clapboards too close to grade 

and adjust grade or clapboards to achieve adequate clearance; protect or 

cover open window wells. 

Ground: Correct serious ground water problems; capture and dispose of downspout water 

away from foundation; and control vapor diffusion of crawlspace moisture. 

Grade: Re-establish positive sloping of grade; try to obtain 6" of fall in the 

first 10' surrounding building foundation; for buildings without gutter 

systems, regrade and install a positive subsurface collection system with 

gravel, or waterproof sheeting and perimeter drains; adjust pitch or slope 

of eave line grade drains or French drains to reduce splash back onto 

foundation walls; add subsurface drainage boots or extension pipes to 

take existing downspout water away from building foundation to the 

greatest extent feasible. 

Crawl space: Add polyethylene vapor barrier (heavy construction grade 

or Mylar) to exposed dirt in crawlspace if monitoring indicates it is 

needed and there is no rising damp; add ventilation grilles for additional 

cross ventilation, if determined advisable. 

Foundations and Basements: Correct existing high moisture levels, if other means of 

controlling ground moisture are inadequate. 

Mechanical devices: Add interior perimeter drains and sump pump; add 

dehumidifiers for seasonal control of humidity in confined, unventilated 

space (but don't create a problem with pulling dampness out of walls); 

add ventilator fans to improve air flow, but don't use both the 

dehumidifier and ventilator fan at the same time. 

Walls: Remove commentates coatings, if holding rising damp in walls; coat 

walls with vapor permeable lime based rendering plaster, if damp walls 

need a sacrificial coating to protect mortar from erosion; add termite 

shields, if evidence of termites and dampness cannot be controlled. 

Framing: Reinforce existing floor framing weakened by moisture by 

adding lolly column support and reinforcing joist ends with sistered or 

parallel supports. Add a vapor impermeable shield, preferably nonferrous metal, under wood joists coming into contact with moist 

masonry. 

Interior: Eliminate areas where moisture is leaking or causing a problem. 

Plumbing: Replace older pipes and fixtures subject to leaking or 

overflowing; insulate water pipes subject to condensation. 

Ventilation: Add exhaust fans and whole house fans to increase air flow 

through buildings, if areas are damp or need more ventilation to control 

mold and mildew. 

Climate: Adjust temperature and relative humidity to manage interior 

humidity; Correct areas of improperly balanced pressure for HV AC 

systems that may be causing a moisture problem. 

A. Mitigate poor drainage with gravel, filter cloth, or the use of subsurface drainage mats under 

finished paving. Photo: Larry D. Dermody. 

B. Repair roofs and add ice and water shields at eaves and under valleys in cold climates. Photo: 

Larry D. Dermody. 

C. Develop new drainage systems for roof run-off 

that remove moisture from the bas.e of the 

building. Photo: WPTC, NPS. 

D. Install ventilating fans when additional air 

circulation will improve damp conditions in 

buildings or reduce cooling loads. Photo: Ernest 

A. Conrad, P. E. 

13 
14 

MOISTURE: LEVEL III REPLACEMENT I ALTERATIONS -

A. This Lead sheet was installed at the base of the 

replacement column to stop rising damp. Photo: 

Bryan Blundell. 

B. Wood sills set on grade were replaced with 

concrete pier foundation and new wooden sill 

plates. Changes were not visible on the exterior 

(see C). Photo: WPTC, NPS. 

C. The new ground gutter gravel base helps 

drainage around the concrete foundation (see B 

above) which is not visible behind the replaced 

wooden wall shingles. Photo: WPTC, NPS. 

Exterior: Undertake exterior rehabilitation work that follows professional repair practices 

-i.e., replace a deteriorated feature with a new feature to match the existing in design, 

color, texture, and when possible, materials. In some limited situations, non-historic 

materials may be necessary in unusually wet areas. 

Roofs: Add ventilator fans to exhaust roofs but avoid large projecting 

features whose designs might negatively affect the appearance of the 

historic roof. When replacing roofs, correct conditions that have caused 

moisture problems, but keep the overall appearance of the roof; for 

example, ventilate under wooden shingles, or detail standing seams to 

avoid buckling and cracking. Be attentive to provide extra protection for 

internal or built-in gutters by using the best quality materials, flasl'llng, 

and vapor impermeable connection details. 

Walls: If insulation and vapor barriers are added to frame walls, 

consider maintaining a ventilation channel behind the exterior cladding 

to avoid peeling and blistering paint occurrences. 

Windows: Consider removable exterior storm windows, but allow 

operation of windows for periodic ventilation of cavity between exterior 

storm and historic sash. For stained glass windows using protective 

glazing, use only ventilated storms to avoid condensation as well as heat 

build-up. 

------------------------------ Ground: Control excessive ground moisture. This may require extensive excavations, 

new drainage systems, and the use of substitute materials. These may include concrete or 

new sustainable recycled materials for wood in damp areas when they do not impact the 

historic appearance of the building. 

Grade: Excavate and install water collection systems to assist with 

positive run-off of low lying or difficult areas of moisture drainage; use 

drainage mats under finished grade to improve run-off control; consider 

the use of column plinth blocks or bases that are ventilated or 

constructed of non-absorbent substitute materials in chronically damp 

areas. Replace improperly sloped walks; repair non-functioning catch 

basins and site drains; repair settled areas around steps and other 

features at grade. 

D. In a flood plain, rotted joists were replaced 

with a concrete slab and sleepers designed to 

drain water. Spaced flooring allowed drainage 

and room for damp wood to swell without 

buckling. Harper's Ferry Center, NPS. 

E. Mechanical systems on the Lower Level were placed on platforms above the flood 

line. Harper's Ferry Center, NPS. 
- - FOR CHRONICALLY DAMP CONDITIONS 

Foundations: Improve performance of foundation walls with damp-proof treatments 

to stop infiltration or damp course layers to stop rising damp. Some substitute 

materials may need to be selectively integrated into new features. 

Walls: excavate, repoint masonry walls, add footing drains, and 

waterproof exterior subsurface walls; replace wood sill plates and 

deteriorated structural foundations with new materials, such as 

pressure treated wood, to withstand chronic moisture conditions; 

materials may change, but overall appearance should remain similar. 

Add dampcourse layer to stop rising damp; avoid chemical injections 

as these are rarely totally effective, are not reversible, and are often 

visually intrusive. 

Interior: Control the amount of moisture and condensation on the interiors of historic 

buildings. Most designs for new HV AC systems will be undertaken by mechanical 

engineers, but systems should be selected that are appropriate to the resource and 

intended use. 

Windows, skylights: Add double and triple glazing, where necessary 

to control condensation. Avoid new metal sashes or use thermal 

breaks where prone to heavy condensation. 

Mechanical systems: Design new systems to reduce stress on 

building exterior. This might require insulating and tightening up the 

building exterior, but provisions must be made for adequate air flow. 

A new zoned system, with appropriate transition insulation, may be 

effective in areas with differing climatic needs. 

Control devices/Interior spaces: If new climate control systems are 

added design back-up controls and monitoring systems to protect 

from interior moisture damage. 

Walls: If partition walls sit on floors that periodically flood, consider 

spacers or isolation membranes behind baseboards to stop moisture 

from wicking up through absorbent materials. 

I. Critically damp foundation walls were protected with a layer of bentonite clay to reduce 

moisture penetration. This work was in combination with new downspouts that were connected 

to drainage boots that deposited captures roof run-off away from the foundation. 

Photo: Courtesy, Larry D. Dermody and the National Trust for Historic Preservation. 

F. Triple glazed windows replaced the originals 

to control condensation. Photo: ©Isabella 

Stewart Gardner Museum, Boston. 

G. New sensors which monitor temperature and 

relative humidity are located throughout this museum and tied to a computer that controls the 

climate control system. Photo:© Isabella Stewart 

Gardner Museum, Boston. 

H. New computers tie a variety of monitoring 

and security features into a comprehensive 

system which provides warning and backup alerts 

when any of the system components are not 

functioning properly. Photo:© Isabella Stewart 

Gardner Museum, Boston. 

15 
Back Cover: The Diagnosing Moisture in Historic Building Symposium held in Washington, DC, May, 1996, brought together practitioners in the field of 

historic preservation to qiscuss the issues contained in this Preservation Brief Attendees are standing in front of the cascading fountains at Meridian Hill 

Park, a National Historic Landmark. Photo: Eric Avner. 

Reading List 

Conrad, Ernest A., P.E. "The Dews and Don'ts of 

Insulating." Old House Journal, May /June, 1996. 

Cumberland, Don, Jr. "Museum Collection Storage in an 

Historic Building Using a Prefabricated Structure." 

Preservation Tech Notes. Washington, DC: National Park 

Service, issue PTN-14. September, 1985. 

Jessup, Wendy Claire, Ed. Conservation in Context: Finding a 

Balance for the Historic House Museum. Washington, DC: 

National Trust for Historic Preservation (Symposium 

Proceedings March 7-8, 1994). 

Labine, Clem. "Managing Moisture in Historic Buildings" 

Special Report and Moisture Monitoring Source List. 

Traditional Building, Vol 9, No.2, May-June 1996. 

Leeke, John. "Detecting Moisture; Methods and Tools for 

Evaluating Water in Old Houses." Old House Journal, 

May /June, 1996. 

Moisture Control in Building~ . Heinz R. Trechsel, Editor. 

Philadelphia: American Society for Testing and Materials 

(ASTM manual series: MNL 18), 1993. 

Museums in Historic Buildings (Special Issue). APT 

Bulletin. The Journal of Preservation Technology, Vol26, 

No. 3 . Williamsburg, VA: APT, 1996. 

Oxley, T.A. and A. E. Gobert. Dampness in Buildings: 

Diagnosis, Treatment, Instruments. London, Boston: 

Butterworth-Heinemann, 1994. 

Park, Sharon C. AlA. Preservation Brief24: Heating, 

Ventilating, and Cooling Historic Buildings: Problems and 

Recommended Approaches. Washington, DC: Department of 

the Interior, Government Printing Office, 1991. 

Rose, William. "Effects of Climate Control on the Museum 

Building Envelope," Journal of the American Institute for 

Conservation, Vol. 33, No.2. Summer, 1994. 

Smith, Baird M. Moisture Problems in Historic Masonry Walls; 

Diagnosis and Treatment. Washington, DC.: Department of 

the Interior, Government Printing Office, 1984. 

Tolpin, Jim. "Builder's Guide to Moisture Meters," Tools of 

the Trade Vol2, No.1 (Quarterly Supplement to The Journal 

of Light Construction). Richmond, Vermont: Builderburg 

Group Inc. Summer, 1994. 

Acknowledgments 

Sharon C. Park, AlA is the Senior Historical Architect, Technical Preservation 

Services, Heritage Preservation Services Program, National Park Service, 

Washington, D.C. The author wishes to thank the following individuals and 

organizations for providing technical review and other assistance in developing 

this publication: The attendees, speakers, and sponsors of the Diagnosing 

Moisture in Historic Buildings Symposium held in Washington, DC in 1996 and 

funded by a grant from the National Center for Preservation Technology and 

Training, National Park Service; Hugh C. Miller, FAlA; Michael Henry, AlA, 

PE, PP; Baird M. Smith, AlA; Ernest A. Conrad, P.E.; William B. Rose; Rebecca 

Stevens. AlA; Wendy Claire jessup; Elizabeth Sasser, AlA; Bryan Blundell; 

George Siekkinen, AlA; Larry D. Dermody; Kimberly A. Konrad; Barbara J. 

Mangum and the Isabella Stewart Gardner Museum, Boston; Gunston Hall 

Plantation; Friends of Meridian Hill; Friends of Great Falls Tavern; The National 

Trust for Historic Preservation; Thomas McGrath, Douglas C. Hicks and The 

Williamsport Preservation Training Center, NPS; the staff at Heritage 

Preservation Services, NPS, Charles E. Fisher, Brooks Prueher, Anne E. 

Grimmer, Antoinette Lee, and especially Kay D. Weeks. 

This publication has been prepared pursuant to the National Historic 

Preservation Act, as amended, which directs the Secretary of the Interior to 

develop and make available information concerning historic properties. 

Comments about this publication should be directed to de Tee] Patterson Tiller, 

Acting Manager, Heritage Preservation Services Program, National Park Service, 

P.O. Box 37127, Washington, DC 20013-7127. This publication is not copyrighted 

and can be reproduced without penalty. Copyright photographs included in 

this publication may not be used to illustrate publications other than as a 

reference to this Preservation Brief, without permission of the owners. 

Normal procedures for credit to the authors and the National Park Service 

are appreciated. 

Cover Photo: Masonry repoi11ting in a wet environment. 

Photo: Williamsport Preservation Training Center, NPS. 

ISSN: 0885-7016 October, 1996