Open Cell Foam Is Not a Panacea

I love open cell spray foam. It is the best tool we have for managing the attic environment in hot, humid Texas, where attics have an outsized impact on whether a house holds up over time. Ventilated attics are a relic of the past here, and I will fight with swords over that point. But foam is not a panacea, and I have watched that truth show up in a dozen different ways over the years.

When spray foam first gained traction, it quietly became an excuse to cut corners. "Good enough" turned into the standard. Crews stopped blocking and sealing chases. Fireblocking at tub drains and subfloor decks got skipped. City inspectors do not catch this. Nobody does. But the attic being partially managed does not mean the fundamentals stop mattering. Blocking and air sealing are just as important as they ever were, and in a foamed assembly they may matter more.

What we found on a recent project

Late last year we got into a modern home with flat roof assemblies and a lot of cantilevered floor to balcony and roof conditions. Open web trusses run straight across the exterior wall line. There was foam in the cavities, applied in a just good enough fashion.

Here is what was actually happening. Heat and humidity were building up inside an isolated exterior truss cavity, and it was worst on the southern elevation, which, by the way, had a pool sitting underneath it. That buildup created relentless vapor drive pushing inward through the assembly. The vapor found the first cool surfaces it could reach and condensed on them. Recessed light housings. HVAC supply boxes. Air infiltration slammed into a supply box at the rimband, condensed, and mold followed.

The foam was there. But foam can be poorly applied, or it can fail to expand fully into every void, and in this case it was doing only part of its job. The cavity was never detailed to handle the load it was being asked to carry.

The rule of thumb

So how do we prevent this? We shift away from the "just good enough" mindset and treat detailing like it actually matters, because it does. We are hired to build great houses, and resiliency demands it of us. The climate demands it. Code changes have shrunk the margin for error to almost nothing. And our clients expect it, whether they have said so out loud or not. Trust me, when problems show up, they will loudly state they had those expectations all along, even if they never once voiced them.

Foam is the right call for our climate. It is also not a substitute for thinking. Use the foam, and still do the blocking, the sealing, and the fireblocking like the foam was never there. That is how you get a house that performs.

TONER Home Performance finds root causes, not just symptoms. Building or renovating in Houston, Austin, Dallas, or San Antonio? Loop us in at the design stage. It is where we do our best work.