p>Climate Zone 3A in the South Eastern US is a predominantly hot and humid climate with a certain amount heating days, so you’re obviously doing more cooling than heating, which will impact the ‘<a href="https://www.ecohome.net/guides/2268/best-practices-for-basement-insulation-in-new-home-construction/" target="_blank">ideal basement wall assembly</a>’.  First read here, this page tells you all you'll need to know - </p> <p><strong><a href="/guides/3219/how-to-insulate-crawlspaces/" target="_blank">How to fix and insulate a humid crawlspace properly so it won't develop mold</a></strong></p> <p>Given the description of the wall assembly you listed, we realize you’re not likely to change course, but even just for other readers we need to point out that in an ideal world you would always insulate a basement from the exterior. Structures are like humans in the sense that they prefer balanced temperatures rather than constantly being in flux with seasonal changes.</p> <p>Having said that, when you’re already doing an <a href="https://www.ecohome.net/guides/2270/basement-renovations-how-to-prevent-mold-when-retrofitting-basements/" target="_blank">interior basement renovation</a>, we realize it makes sense to do your work on the inside rather than absorb the additional cost of insulating a basement from the exterior, as you would be looking at thousands in additional excavation costs to remove and replace dirt.</p> <p>So, if we were to be <strong>insulating a Cinder Block Foundation</strong> from the interior using Rockwool, here are the factors we would prioritize and how we would do it.</p> <p>First, the three main rules of durable basement construction are – water management, insulation, and airtightness.</p> <p>Our first suggested assembly for interior basement insulation would be a vapor barrier membrane directly against the concrete block foundation, with a channel at the bottom for drainage, either a surface drain in the concrete, or we would remove the concrete from the floor along the perimeter and install a French drain. That way you keep the moisture out in the first place, and you create a drainage route for any water that may accumulate on the foundation wall and drain to the floor. This would also have the advantage of creating a radon gas membrane (See our pages on <a href="https://www.ecohome.net/guides/3218/removing-radon-gas-from-basements-and-crawlspaces/" target="_blank">Radon barriers in basements for more information</a>). Then we would add <a href="https://www.ecohome.net/guides/2254/polyisocyanurate-foam-is-a-hot-new-building-product-learn-where-and-where-not-to-use-it/" target="_blank">rigid insulation panels</a>, a stud wall with batts, and drywall with latex paint.</p> <p>But - the assembly you mention would work, however, we wouldn’t include the smart membrane in that case. The predominant source of humidity would be the walls, not the interior air, so we would allow the moisture to dry inwards and manage it with either a dehumidifier, or and <a href="https://www.ecohome.net/guides/2276/choosing-between-an-hrv-and-an-erv/" target="_blank">HRV or ERV (Heat Recovery Ventilation / Energy Recovery Ventilation).</a></p> <p>The Rockwool insulation boards can go right against the CMU wall (Rockwool / mineral wool is not harmed by moisture, so you alleviate the risk or mold forming inside walls. Then add the stud wall (with batts in the cavities). Unless you need the smart membrane to appease a building inspector, we would omit it. Where the wall will be half-backfilled, it will only be able to dry to the interior, so let it keep doing that. Meaning, install drywall directly on the studs rather than including a smart vapor barrier membrane (Intello or Smart Membrain from Certainteed)</p> <p>The inconsistencies in a concrete block foundation mean there will be a small air space regardless of how you space it, and air spaces are always good to have. But, it is also very important to make it airtight. If you have a Basement wall with a high air leakage rate then you risk having a convection loop where warm, humid air is constantly being delivered to a cold surface behind your stud wall. So, we would also recommend a<a href="https://www.ecohome.net/guides/2298/what-is-an-air-barrier/" target="_blank">n Airtight Drywall Approach (ADA)</a>, which you can achieve with caulking, acoustic sealant and backer rods, and thick sill gaskets underneath the bottom plate of the stud wall.</p> <p>Regarding the smart membrane and how they work – they typically meet the requirements to be classified as code-compliant vapor barrier in cold months, when interior moisture is higher than exterior. In the summer months, the pores of the membrane are more open, which allows a small amount of moisture to pass through. They are designed more for colder climates than yours where vapour barriers are part of code but where cooling loads are also fairly significant.</p> <p>One of the best things about smart membranes is that they 'look' like typical vapor barriers when inspectors come by, but they are more breathable. Building Code has not kept up with building science, and you should <a href="https://www.ecohome.net/guides/2271/why-are-basements-moldy/" target="_blank">NEVER cover a basement stud wall with 6 mil poly</a>, yet many inspectors insist on it, so this can appease them, but it doesn't do the harm of a typical vapor barrier. </p> <p> </p