A recent blog by Fine Homebuilding asked the question: “Are Insulated Concrete Forms Airtight?” On clicking through to the full article – the title read: “ Should Insulated Concrete Forms be Air-Sealed?” Both are questions about the product, and for that – the answer is simple: the finished, installed Insulating Concrete Form product is airtight by nature. Concrete is airtight.
But that wasn’t really the question which was addressed in the article. The discussion was about whether or not to build houses airtight, and if this is necessary in ICF construction. Again, it seems both answers are straightforward. We long crossed over the decision to tighten houses (ie Build Tight, Ventilate Right). The question about air tightening to the level of Passive House standards is one of really that of choosing the best balance between the investment of materials/time/money into the initial construction of the house vs supplementing with materials/time/money in renewable energy sources. It’s a question of embodied energy and economics.
From my experience in the ICF world, the real question is about the air sealing of ICF construction. The advantage is that the air leaks are in concentrated more easily identifiable areas. There is no need to worry about leakage at the corners, the rim joists, or around every electrical receptacle, if the construction is continuous ICF from the footing to the top of the wall. That alone is one headache gone. But the rest of the leakage areas still remain.
The typical Achilles heel of any construction is the lack of continuous air sealing and insulation at the wall to ceiling/ roof. ICFs are no exception. The interior wall is the air seal, but the exterior layer of EPS foam needs to be continuous through the top cap. This can be a raised heel, or “energy” truss, a cathedral ceiling which carries forth the outside edge of insulation, or a myriad of other solutions. The key is in the detailing, and the sequencing of trades.
The other major point of leakage in all construction is the installation of the windows. The detailing of which falls into a no-man’s land, which Barry Hardeman was spearheading through an ASTM committee before his untimely death. Who designs the details and ensures an install which both provides for a drainage channel, continuous exterior barrier detailing, thermal breaks, and interior air sealing? Not the window companies, but not the ICFs. It is a matter of assumed liability. I would encourage ICF manufacturers to take up this ASTM work again as an industry, and provide the appropriate training to their builders.
It also makes sense to leverage the benefits of the materials of ICFs in choosing the material for the windows frames, the bucking material, and the trim work. Concrete and EPS foam have very low coefficients of expansion and contraction. This means they don’t shift around much when temperatures change. Unlike wood, they are stable, don’t shrink and aren’t affected materially by moisture. Fiberglass is the window frame material which has similar properties, which are essentially the same as the glass panes. This is important, since windows typically fail (ie become leaky) first at the seal of the glass to the frame, due to the different rates of expansion of materials such as vinyl, metal or even wood, compared to glass. The second point of failure is typically the connection of the window to the wall.
Face frame with EIFS seems the simplest to install and seal, but is a single layer of protection against both moisture and air leakage. Recessed windows are harder to install, but offer more opportunities to build in redundancy in air tightening through rope caulking (mechanical) covered by wood trim (structural), itself caulked and painted. Mechanical or structural approaches can withstand the test of time. Caulk will fail. Remember that air tightening is typically on the interior, and water protection on the outside.
So, YES, ICFs themselves are airtight. And YES, the ICF construction should pay attention to air-tightening at the points of connection to other materials. And YES, this is the same type of detailing which is necessary in all construction.