Dehumidification Dry Kiln 

Construction Plans

Neal Bennett

United States Department of Agriculture

Forest Service Northern Research Station General Technical Report NRS-188 March 2019
Abstract

Small, complete dehumidification dry kilns under 5,000 board-foot capacity are not available 

for sale. Dehumidification drying equipment is available, but an enclosure must be built to 

house the equipment and dry the lumber. A dry kiln needs to be operated completely filled 

with lumber. This forces airflow through the tiers of lumber instead of around the ends or over 

the top of the stack(s). This construction plan is for an 800 board-foot capacity dry kiln structure 

that is designed to work with an EBAC LD800TM dehumidification unit. This size is suitable for the 

small woodworking shop or for portable sawmill owners.

The Author

NEAL BENNETT is a general engineer for the USDA Forest Service, Northern Research Station, 

Princeton, WV.

Cover Photo

Completed 800 board-foot capacity dehumidification dry kiln. Photo by by Neal Bennett, 

USDA Forest Service.

Quality Assurance

This publication conforms to the Northern Research Station’s Quality Assurance Implementation 

Plan which requires technical and policy review for all scientific publications produced or funded 

by the Station. The process included a blind technical review by at least two reviewers, who were 

selected by the Assistant Director for Research and unknown to the author. This review policy 

promotes the Forest Service guiding principles of using the best scientific knowledge, striving 

for quality and excellence, maintaining high ethical and professional standards, and being 

responsible and accountable for what we do.

The use of trade or firm names in this publication is for reader information and does not imply 

endorsement by the U.S. Department of Agriculture of any product or service.

Manuscript received for publication 28 August 2018

Published by

USDA FOREST SERVICE

11 CAMPUS BLVD SUITE 200

NEWTOWN SQUARE PA 19073

March 2019
1

INTRODUCTION

Portable sawmill owners recognize the advantage to kiln-drying the lumber they produce. 

Having the ability to provide properly kiln-dried lumber opens new market opportunities and 

can increase profit margins. However, the construction and operation of a small dry kiln must 

be economical and simple. A small dehumidification kiln with an 800 board-foot capacity was 

constructed by the USDA Forest Service, Northern Research Station. Construction and initial 

use of the kiln were discussed in an article in Independent Sawmill & Woodlot Management 

Magazine (Bennett 2012). The cost of the dry kiln included $2,500 for the dehumidification 

unit and controller and $500 for construction materials. Scrap materials were used when 

possible in constructing this original model to keep material costs down. The performance of 

this first kiln constructed by the author as well as an accounting of electricity consumption 

measured during the drying of two stacks of hard maple (Acer saccharum L.) was documented 

in Bennett (2013).

Since the design and construction of the initial dry kiln, three additional kilns have been 

constructed for research use. Incremental design improvements were achieved between the 

first dry kiln and the fourth based on experience gained in usage. Among the improvements 

that are incorporated into the current version of the dehumidification dry kiln is a redesign 

of the doors to make them easier to open to check drying sample boards. Weather-stripping 

around and between the doors is improved to make the structure more air tight and thus more 

energy efficient. The plans provided in this report were developed to assist with construction.

Similar to the earlier versions of this kiln, the final dry kiln design is sized to dry 

approximately 800 board feet of 4/4-thickness hardwood lumber with board lengths up to 

12 feet, 2 inches. While this construction plan is designed to work with an EBAC LD800TM

dehumidification unit, other units are available to achieve similar results. The exterior 

dimensions of this kiln are 15 feet long × 5 feet, 5 inches wide × 5 feet high. This size is 

suitable for the small woodworking shop or portable sawmill owners. The length of the 

lumber stack of 12 feet, 2 inches was an arbitrary decision based on the average lengths of the 

logs we normally process in our wood shop. Also, commercially purchased lumber 12 feet in 

length is usually 12 feet, 2 inches long. This dry kiln is normally loaded with a forklift, so 4 

inches of extra length was added to the drying chamber. Boards shorter than 12 feet can be 

mixed in with longer boards to construct a 12 foot, 2 inch-long lumber stack. It is important 

for the lumber stack to fill the drying chamber in order to force air through the tiers of boards 

and minimize airflow over and around the lumber stack. The EBAC LD800TM owner’s manual 

(http://www.ebacusa.com/downloads/manuals/TPC230.pdf, pages 8-17) also provides basic 

guidelines for building a dry kiln. If a different-length lumber stack is preferred, then all of the 

dimensions in this plan would need to be altered.

Construction Details

The dry kiln walls are framed with 2 × 4s on 16-inch centers, insulated with R-13 fiberglass 

insulation, and constructed with 1⁄2-inch exterior plywood or T-111TM siding on the outside 

of the kiln and 1⁄2-inch exterior plywood on the inside of the kiln and on the roof. All nails or 

screws used are corrosion resistant. Plywood seams are caulked. Doors are constructed with 

1⁄4-inch plywood on the inside and 3/8-inch plywood or T-111TM siding to reduce weight. 

Doors are hinged to be bi-fold to ease opening and closing and loading of lumber (Fig. 1).
2

Cost savings realized from using scrap wood, etc., in the construction of the first dry kiln 

were not realized subsequently because scrap was no longer at hand. Material costs excluding 

the dehumidification drying units were about $1,200 per dry kiln. All materials (Table 1) for 

each of the three latter dry kilns were purchased at a local home improvement store to reduce 

construction time. The dry kiln can be built with lumber produced by a portable sawmill to 

save money. In that case, the builder would have to exercise care in making sure all joints are 

sealed to prevent water from seeping in and wetting the insulation. Dimensions are given for 

almost every component; however, the builder will have to measure and compensate for any 

minor variances that occur during construction.

Table 1.—Dry kiln construction materials list.

Material No. of units

1⁄2-inch plywood 19 sheets

1⁄4-inch plywood 2 sheets

T-111TM 19/32 inches 5 sheets

T-111TM 11/32 inches 2 sheets

1⁄4-inch peg board 4 sheets

6-inch x 6-inch x 16-foot timbers 3-4

2-inch x 4-inch x 8-foot timbers 49

2-inch x 4-inch x 16-inch timbers 10

Mortise hinges 14

Barrel latches 8

Shingles 4 bundles

Plastic 6 mm 10 feet x 100 feet

Caulk 6-10 tubes

Nails 16p and 8p Corrosion resistant

Corrosion resistant screws #8 11⁄2 inches and 2 inches long

Drip edge for roof 44 lineal feet

Lag screws 8 inches x 1⁄2 inch 12-15

Fiberglass insulation R-13 7 rolls

Figure 1.—Completed 800 board-foot capacity dehumidification dry kiln.
3

Bottom

Build the floor frame as shown (Fig. 2, Drawing 1). Nail 1⁄2-inch plywood to one side. The 

plywood will help square the frame. Flip the bottom over. If it is anticipated that the dry kiln 

will be moved, it is advisable to attach 6-inch × 6-inch timbers long-ways under the bottom of 

the dry kiln. In this case, the timbers were spaced to accommodate the 48-inch long forks of 

our forklift. If the dry kiln is not going to be moved, then it must be elevated off the ground to 

keep the bottom dry. If 6-inch × 6-inch timbers are used, attach them now (Fig. 3). Lag screws 

inserted through the bottom joists into the timbers are suggested. Add insulation with the 

vapor barrier turned up between the 2-inch × 4-inch joists (Fig. 4). To protect insulation from 

water seeping through the plywood, add a layer of 6-mm plastic film (Fig. 5) and then the top 

layer of plywood, which will be the floor of the dry kiln (Fig. 6). Seal the plywood seams and 

nails in the floor of the dry kiln with construction caulk to further seal them from moisture 

seepage.

Figure 2.—Bottom of the kiln.

Drawing 1.
4

Figure 5.—Plastic (6 mm) stapled 

to frame covering insulation.

Figure 3.—Timbers in place under 

the dry kiln floor.

Figure 4.—Insulation installed in 

the bottom of dry kiln.

Figure 6.—Plywood installed over 

the plastic moisture barrier.
5

Sides

Build the frames of the sides as shown (Fig. 7, Drawing 2). Dimensions are given for the 

outside studs; the middle studs must be cut to fit. Nail the sides to bottom. The outside edges 

of the sides are flush with outside edges and back of floor.

Figure 7.—Sides attached to the floor of dry kiln.

Drawing 2.
6

Back

Build the back frame as shown (Fig. 8, Drawing 3). Nail the back to dry kiln floor so it is flush 

with the rear of floor and the sides.

Figure 8.—The back and sides attached to the bottom of the dry kiln.

Drawing 3.
7

Front or Header

Build the frame of the header as shown (Fig. 9, Drawing 4). Nail the front to the top of the 

sides of the dry kiln. Make sure the front or header is level and is not sagging before attaching 

plywood. Prop it up into a level position or attach the plywood to one side of the header to 

keep the header from sagging before nailing it to the side walls.

Next, insulate all walls with the insulation vapor barrier facing the inside of the dry kiln. Nail 

1⁄2-inch plywood to the inside of the dry kiln, and attach 1⁄2-inch plywood or T-111TM siding to 

the outside of dry kiln. Apply caulk to the seams between the sheets of plywood.

Figure 9.—The header attached to the front of the dry kiln.

Drawing 4.
8

Top

Build the frame for the top as shown (Fig. 10, Drawing 5). Nail 1⁄2-inch plywood to one side. 

Use the plywood to help square the frame. Turn the top over and place it on top of the dry 

kiln. Apply caulk as a sealant between the top and the walls. Nail the top to the dry kiln walls, 

allowing an even overhang of all sides. It is best to toenail through the top joists into the top 

plate of the walls. Insulate the top with fiberglass insulation with the vapor barrier turned 

downward. We used R-13 but other R-values may be appropriate for different-sized chambers 

or climatic conditions (a guide is provided in the EBAC LD800TM owner’s manual). Staple 

6-mm plastic over the insulation and nail 1⁄2-inch plywood on top of it (Fig.11). If the dry kiln 

is to be stored or used outside, then roofing must be added. Steel roofing is not recommended 

due to the danger of cuts while loading and unloading the dry kiln. Consequently, shingles 

with drip edge are recommended.

Figure 10.—Framing for the top of the dry kiln.

Drawing 5.
9

Figure 11.—The dry kiln with the top attached.

Inside Parts

The inside parts of the dry kiln are designed specifically for an EBAC LD800TM

dehumidification (DH) unit. Instructions for the inside parts are also included in the EBAC 

LD800TM owner’s manual. Cut out inside parts A and B (Drawing 6). Add 3⁄4-inch square 

softwood stock to fasten parts A and B to the dry kiln wall, floor, and ceiling. Fig. 12 shows 

parts A and B installed with the EBAC LD800TM in place. Attach the peg board diffuser across 

the back of the dry kiln using 3⁄4-inch square softwood stock (Drawing 7, Fig. 12). Also use 

3⁄4-inch softwood stock along the vertical seams between pieces of peg board. The DH unit is 

supplied with a foam baffle to reduce air velocity. Attach the baffle with glue or screws to the 

back of the first sheet of peg board (Fig. 13). It is to be mounted about 161⁄2 inches from the 

DH unit and 7 inches above the floor.

Drawing 6.
10

Drawing 7.

Figure 12.—Inside view of dry kiln with EBAC LD800TM dehumidification unit in place.

Figure 13.—View of the peg board diffuser after installation.
11

Doors

Build the door frames as shown (Drawing 8). Size the doors by measuring the total opening 

dimensions. For door height, subtract 1⁄2 inch from the opening height. Door width will be the 

total width of the opening divided by 4. Subtract another 1⁄4 inch from the width of each door 

for hinge clearance. The last door installed may have to be further reduced in width to allow 

clearance plus room for weather-stripping. Nail 1⁄4-inch plywood to inside. The plywood will 

help square the frame. Insulate with the vapor barrier turned toward the inside of the door 

and add plywood or T-111TM siding to the outside of the frame. The hinges on the outer doors 

are placed on the outside of the dry kiln and visible when looking at the kiln (Fig. 1), whereas 

the hinges that connect the inside doors to the outside doors are mounted on the inside of the 

dry kiln. This results in two sets of folding doors. To save money, hinges and latches used were 

not stainless steel. Corrosion was not an issue until a load of green red oak was dried. If acidic 

species such as red oak are going to be dried in the kiln, then it will be necessary to treat the 

hardware with a rust preservative or use much more expensive stainless steel hardware.

Shim the doors off the floor 1⁄4 inch before attaching hinges to facilitate opening and closing of 

doors. At this point, every effort has been made to build the dry kiln to provide a square door 

opening and to build the doors square to fit the opening. Nevertheless, the doors may have to 

be trimmed to fit the opening after the hinges are installed. Attach latches to the doors (Fig. 1) 

to keep them closed and add moderate pressure to the weather-stripping. Make sure any holes 

drilled for the barrels on the bottom latches do not penetrate through the plywood floor and 

expose the insulation. After the doors and latches are installed, add 3⁄4-inch stock softwood or 

aluminum angle across both the top and bottom of the kiln opening. This material serves as a 

stop for the doors and will need weather-stripping applied. Rubber weather-stripping works 

better here than foam weather-stripping. Add weather-stripping between the doors to seal 

gaps. For the gaps between the doors, foam weather-stripping is used because it is generally 

available in various thicknesses. Finally, add a pair of handles near the middle of the paired 

doors to assist with opening and closing the dry kiln doors (Fig. 1).

Drawing 8.
12

Control Box, Cables and Drain

Mount the control box for the EBAC LD800TM DH unit on the outside end of the kiln near the 

DH unit location (Fig. 14). Drill a 2-inch-diameter hole through the side of the kiln to pass 

the drain hose and power cables to the outside of the kiln. It is important to place the hole 

below the drain pan of the dehumidifier unit because drainage is by gravity. The control 

box has a temperature probe that passes through the same hole to the inside of the kiln. 

Mount the probe loosely about 2 feet above the kiln floor to Part B (Fig. 12). After the cables 

are routed, the hole can be sealed with expandable foam. Any gaps should be caulked and 

moldings can be added for cosmetic improvement. Finally, paint or stain the outside of the dry 

kiln to protect it from the weather.

Upon completion, the DH dry kiln is now ready to use to dry lumber. Use the EBAC LD800TM

owner’s manual, or the owner’s manual for whatever brand of dehumidification dry kiln unit 

is used, to begin operating the dry kiln. Reference some of the sources of information listed in 

the next section to refine your operations.

Additional Resources

Other DH drying units are available from EBAC and other companies that dry larger 

lumber charges (e.g., 3,000 to 15,000 board feet of 4/4-thickness lumber) but would 

require modification of the construction plan. In addition, a simple internet search of 

“dehumidification dry kiln kit” indicates there are options for kiln kits that include additional 

hardware such as vents, fans, and auxiliary heating required to improve drying results if a 

larger capacity kiln is needed. The dry kiln kit option typically requires the construction of the 

chamber for which the plans provided in this paper may be useful as a starting point.

One of the earliest efforts to document the construction and operation of a dehumidification 

dry kiln was based on an earlier version of the larger dehumidifier unit manufactured by 

EBAC (3,000 board-foot capacity when drying hardwood lumber with 4/4 thickness) (Boryen 

1995). This is a highly recommended resource as it provides overview information on kiln 

drying, strategies for operating and monitoring the performance of a kiln, and information on 

factors that should be considered in planning a kiln construction based on location, available 

capital, and operational constraints.

Figure 14.—Control box with the access hole through the side of the kiln.
13

A prior dehumidification dry kiln development project undertaken by the USDA Forest Service, 

Forest Products Lab with partners in the Wisconsin Department of Natural Resources and the 

Madison Area Technical College is described in Bowe et al. (2007). This kiln was built using a 

large home dehumidifier unit. The price cited for the materials used to construct this dry kiln 

is $2,703 which equates to $3,310 in 2018 dollars (using the CPI inflation calculator on the bls.

gov site). This kiln model had some humidity control issues as it lacked the automated humidity 

control unit that allows for regular and timely adjustments of the dehumidifier. This issue and 

a few others noted by the authors are discussed in the “Lessons Learned” section of the Bowe et 

al. publication. A related publication by Bergmann (2008) details operating costs for the Bowe 

et al. kiln and provides useful background information on the fundamental components and 

principles of dehumidification drying.

A highly visited Sawing and Drying Forum that is curated by a renowned lumber drying expert, 

Dr. Gene Wengert, is found in the WOODWEB system and referenced in the literature cited 

section of this paper. The “kiln construction” section of the forum contains many discussion strings 

related to dehumidification kilns with insights provided by Dr. Wengert and other kiln users.

Finally, the hardwood lumber drying manual (Denig et al. 2000) is an excellent general reference 

that all kiln drying operations will find useful in making decisions about drying approaches and 

methods to optimize the quality and value of dry lumber.

LITERATURE CITED

Bennett, N.D. 2012. The construction and initial use of a small-scale dehumidification kiln.

Independent Sawmill & Woodlot Management Magazine. March 2012. No. 107: 22-27.

Bennett, N. 2013. Drying hard maple (Acer saccharum L.) lumber in a small dehumidification 

kiln. Res. Note NRS-196. Newtown Square, PA: U.S. Department of Agriculture, Forest 

Service, Northern Research Station. 6 p. https://doi.org/10.2737/NRS-RN-196.

Bergman, R.D. 2008. Operation and cost of a small dehumidification dry kiln. Res. Note 

FPL-RN-0310. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products 

Laboratory. 20 p. https://doi.org/10.2737/FPL-RN-310.

Boryen, B. 1995. Early operating experiences with a small-scale dehumidification kiln.

Headingly, Manitoba: Woodworkers’ Sawmill and Kiln. http://www.cfs.nrcan.gc.ca/

bookstore_pdfs/22908.pdf (accessed November 13, 2018).

Bowe, S.; Molzahn, P.; Bond, B.; Bergman, R.; Mace, T.; Hubbard, S. 2007. Dehumidification 

drying for small woodworking firms and hobbyists: building your own lumber dry kiln 

with local building materials. PUB-FR-396 2007. Madison, WI: UW Extension–Wisconsin 

Department of Natural Resources. 14 p. https://www.fpl.fs.fed.us/documnts/pdf2007/

fpl_2007_bowe001.pdf (accessed February 8, 2019).

Denig, J.; Wengert, E.M.; Simpson, W.T. 2000. Drying hardwood lumber. Gen. Tech. Rep. FPL–

GTR–118. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products 

Laboratory. 138 p. https://doi.org/10.2737/FPL-GTR-118.

EBAC. 2013. Lumber dryer owner’s manual, kiln construction guide, troubleshooting guide, 

LD800. http://www.ebacusa.com/downloads/manuals/TPC230.pdf (accessed August 20, 2018).

WOODWEB. 2018. http://www.woodweb.com/KnowledgeBase/WDKBPPKilnConstruction.

html (accessed February 28, 2018).
Bennett, Neal. 2019. Dehumidification dry kiln construction plans. Gen. Tech. Rep. 

NRS-188. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern 

Research Station. 13 p. https://doi.org/10.2737/NRS-GTR-188.

Small, complete dehumidification dry kilns under 5,000 board-foot capacity are not available 

for sale. Dehumidification drying equipment is available, but an enclosure must be built to 

house the equipment and dry the lumber. A dry kiln needs to be operated when it is filled 

with lumber and forces airflow through the tiers of lumber instead of around the ends or 

over the top of the stack(s). This construction plan is for an 800 board-foot capacity dry kiln 

structure that is designed to work with an EBAC LD800TM dehumidification unit. This size is 

suitable for the small woodworking shop or for portable sawmill owners.

KEY WORDS: dehumidification, dry kiln

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