The Building Envelope
The envelope is the continuous shell separating the heated inside of your house from the outside. Continuous is the word that does the work.
Think of it as a bucket
A bucket with thick walls and one hole in the bottom is not a slightly worse bucket. It is not a bucket. The thickness of the walls stops being the interesting number the moment there is a hole.
Building envelopes behave the same way. A beautifully insulated wall assembly with an unsealed rim joist underneath it leaks, and the wall does not save you.
Where the envelope usually breaks
- Rim joists - the leakiest few inches in most Maine houses
- The attic hatch - often a sheet of ply with nothing on it
- Top plates - where interior walls meet the attic
- Every wire and pipe penetration
- Knee walls in a half-story, insulated on the wrong plane
- Where a cathedral ceiling meets a flat one
Every one of those is a place where batt insulation was cut around something and a gap was left. Individually small; together, a hole in the bucket.
The first job is deciding where the envelope is. Is the attic inside it or outside it? Is the crawl space? Houses that are uncomfortable are often houses where nobody ever decided, and the insulation ended up on two different planes doing half a job each.
Why foam suits this
Because it is applied as a liquid and expands, it follows the shell rather than being cut to fit it. It goes around the pipe instead of being notched near it. That is what makes a continuous envelope practical rather than theoretical. The underlying physics is here, and the moisture side is here.
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