The purpose of this post is just to give one more data point to folks who are considering sealing a vented attic with spray foam.

I live in climate zone 4a. Sealed my walk-up attic with open cell spray foam last summer. Depth of spray foam was about 6″ so somewhere around R-22. Not ideal, I know — but it is what it is.

Anyways, performance has exceeded expectations. Before spray foam, when attic was completely unconditioned, the temperature in the vented attic stayed pretty close to the outdoor temperature (no surprise). So far this winter, with overnight lows as low as about 20F, the sealed attic has never dropped below 60F. So the sealed attic might be 30-40F warmer than the unsealed attic was during the coldest parts of the day.

Needless to say, the affect of this on my heat pump is very noticeable. I have a Trane XV18 that displays what capacity the compressor is operating at. Last winter, the compressor would be operating between 80-100% capacity as soon as outdoor temperatures hit freezing. And if it dropped below 25F, indoor temperatures (thermostat setpoint 68F) slowly started dropping and the aux heat would eventually kick on.

This winter, even when it gets down to 20F, the compressor never exceeds 70% operating capacity except when recovering from a defrost cycle. So far the system has never had even the slightest trouble maintaining my 68F setpoint. Also, defrost cycles are much less noticeable. Last winter a defrost cycle would tend to result in an indoor temperature drop of 0.5 to 1.0F. This year, the defrost cycles result in a temperature drop of 0.2 to 0.5F.

Anyways, TL;DR — sealing the attic and bringing the HVAC into the conditioned envelope of the house has had a dramatically positive effect on winter thermal & HVAC performance in my house.