Air seal! The most boring word and concept that can make all the difference in energy saving and comfort. Granted, I once said rising damp was the least interesting but important building science term, I think we can make a case for air sealing. Why is it important (and possibly more important) than just adding more insulation?
What Folks Think the Problem Is
When a house is cold and uncomfortable, the first thought is adding more insulation. This is a compelling thought for a few reasons. First, it’s a problem most homeowners can solve in a weekend. Assuming we’re talking about an open attic, a trip to the local big box for some rolls of fiberglass or a blower machine is all the solution needed.
Second, it’s a known and understood problem. The average homeowner may not have a clue about installing a fuse box or replacing the wax ring on a toilet but we all (we think) understand that more insulation = warmer.
This thinking causes a common issue: a fluffy pink fiberglass band-aid over a gaping wound that allowing heat to stream out. Folks pile on the insulation but haven’t air sealed. I’ll say this as clear as I can: Do not add more air permeable insulation until you air seal.
What The Problem Actually Is
The building enclosure has several control layers: the thermal layer (insulation), air and vapor controls. If there are paths for air to escape, your walls and ceiling are only doing a part of the job.
Attics have loads of little leaks (light fixtures, electrical penetrations, around vent fans) but sometimes have ridicously huge ones. Check out this photo.
The homeowners were complaining of a frigic second floor and didn’t understand given that they had a ton of fiberglass in the attic.
I had to pull back 18″ of crossed fiberglass batts to find this gaping hole. The house was built in 1797 with rough timber balloon framing. All of the interior and exterior walls were open at the top, allowing warm conditioned air to escape all winter. You could add R-100 insulation on top of the open cavity walls and it wouldn’t change things much. As long as there are giant holes in the attic floor, the insulation won’t work how you’d want.
Since then, the homeowners have had a contractor work through the attic, sealing the caps of the interior and exterior walls and all other penetrations through the ceiling plain. All the insulation was replaced afterward and owners reported it was waaaaay more comfortable. So remember to air seal then insulate.