Want a Warm House? Don’t Just Insulate, Also …

by Erik North on December 17, 2012

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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.

Holes in the attic air barrier

Probably goes to the basement

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.

{ 6 comments… read them below or add one }

Eric January 4, 2013 at 9:10 pm

I had my home “air sealed and insulated” by a pro who, it turns out, skipped or at least skimped on the air sealing part.

A laundry chute which passed through the bathroom was still open to the attic. Hm, where’d that humidity come from up there?

And of course, once you’ve got 14″ of cellulose in the attic, it’s a lot harder to do the air sealing that should have been done beforehand. That stuff is hard to push around, it doesn’t stay put. 🙂

If I had it to do over again I’d have grilled the contractor about exactly what they planned, to be sure they did it right. It’s a shame when even the “pros” get it wrong!


Erik North January 5, 2013 at 4:17 pm


Thanks for the comment. Your situation speaks to the challenges within the industry. So many incentives line up against best practices.

Start with an insulation contract bid versus one which includes air sealing. An insulation only bid can be thousands of dolalrs less expensive. If one bid is $2000 and another is $4000, which will a homeowner choose? Even with great information, cheaper is all too often the overriding selling point.

If I’m not involved with the project, I always tell customers to make sure their insulation contractors are air sealing and testing with a blower door before and after work.


KrS January 7, 2013 at 6:12 pm

Hi there,

It’s great to see you writing about this important issue.
I need a little more technical information about “air sealing”.

Maybe you’ll be able to suggest something for my little house. I’m re-doing everything. 915 sq ft, built around 1906, balloon framing. You’ve got the sills on brick columns, then the joists, then 3/4″ t&g pine flooring. That’s it! 3/4″ board between me and the ground.

Half of the house I’m putting down a new 3/4″ sub-floor and new hard wood flooring.
That’ll help some.

Interior walls & ceilings are t & g heartpine, true-dimension 2 x 4 studs & cross-bracing, and heartpine siding. Metal roof on lathe over rafters. Nothing else between the bottom of those metal panels and the ceiling – a 3/4″ ceiling board and a panel of metal between me and the sky.

But all of this has been about insulation – the lack thereof. But for starters, before I tackle insulation [& I don’t know now what all that will entail], besides gaping holes, what else, specifically, might you be referring to when you say “air seal”?

Or maybe I should ask, what suggestions at all might you have for me ending up with a better insulated house?

Best Regards.


Erik North January 13, 2013 at 11:31 pm


Thanks for the comment…air sealing is the sealing with caulk or foam sealant of any flaws in the building’s air barrier. If the flaws are larger (like those open wall partitions above), this may include using foam blocking as well.

Here’s Energy Star’s thermal bypass checklist for all those common major flaws in the air barrier:



Mark January 11, 2013 at 11:02 am

I live in south east South Dakota, our 1959 house has a two sided masonry fireplace in the middle of the ranch house main floor. We just installed a wood burning insert, Drolet, made by SBI in Canada. This unit comes ready for an outside air duct and I know having outside air would be best. We presently are using inside air to combust and are aware of backdrafting potential with our natural gas furnace and DHW– both in the basement. Eric, have you see any masonry fireplaces modified to be penetrated with an air duct?


Erik North January 13, 2013 at 11:19 pm


Thanks for writing…I have not, unfortunately, seen masonry fireplaces modified for air ducting. Doesn’t mean they don’t exist of course.



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