Heat Loss from Air Is No Big Deal, Right?

by Erik North on June 11, 2012

Air leakage


No, it’s a huge deal. That photo is of air streaming through recessed lights in a cathedral ceiling.

I often and exhaustively speak about air sealing like a universal good. And it is, right there with brown ale and Avengers movies. Audit customers often look confused when I address their insulation questions by bringing up air barriers and air leakage. I mean…I asked about the insulation?!?

But controlling air leakage is profoundly important for saving money, energy and improving comfort, especially in houses insulated with fiberglass – which means, you know, all of them.

Remedial Heat Movement

The concept of air sealing ties in with how insulation works and how heat moves. Thermal energy moves in three different ways: conduction, convection and radiation. Conduction is heat transfer between two solids, such as scalding your hand on a hot pan handle. Radiation is the infrared radiation that warms on contact; think of the sun beating down on your skin. Convection, the one we’re interested in, is heat movement in a fluid medium like air and water.

Stack Effect And You: Your Energy Losing Friend

There’s a building science concept known as stack effect, well known in the industry and not at all outside of it. Simply put, it is convective heat movement at work. Warm air is more buoyant than cool air, naturally floating upward through your house. When the outdoor temperature is colder, the warm updraft grows stronger. This updraft creates a slight but definite positive air pressure, escaping from every nook, crack and hole in the top of your house.

The air convection moves heat out of the top of your house, and as an added bonus creates a slight suction (negative pressure) in the bottom. For every cubic foot of warm air escaping from the top, a cubic foot of chilly winter air seeps in the bottom. I mean if you live in Maine. If you live somewhere warm, well, those 60 F winter are sure tough.

Fiberglass: You’re Not Helping

The air permeability of fiberglass compounds this problem. Or at least does absolutely nothing to stop it. Most homeowners think, “Hey I have two feet of pink fluffy stuff up there. That’ll handle it.” Fiberglass slows conductive and radiant heat flows but convective heat loss … It may not be as bad as an open window, but it is closer than you’d care to think. And I’ve seen a few hundred blower door tests to back this up.

Warm air is flowing out the top of your house, cold air is sneaking into the bottom. Pop quiz hot shot – what do you do? Air sealing is the answer here. In the attic, sealing around chimney chases, plumbing chases, recessed lights, framing seams…really everywhere. In the basement, sealing the sills, oil supplies, bulkhead doors and other penetrations.

The majority of new homes are insulated with fiberglass but absolutely no attention is paid to air flow and leakage. This is something that new building codes are working to rectify. Fiberglass insulation stops air flow not at all yet air is often the single largest source of heat loss.

Comfort is also a major factor. It’s a rare homeowner who has ever thought they were uncomfortable because of poorly insulated walls. Leaky windows or drafty rooms … another matter entirely. Air sealing homes is one of the most cost effective ways to save money and feel more comfortable in your home.

{ 2 comments… read them below or add one }

Forrest Fielder July 5, 2012 at 10:46 am

In new construction, esp in the Energy Star program, HERS raters are tasked with the inspection and testing of air barriers. For drywall air barriers, sealing must be done either before (in the inboard face of the framing members) or after (at the transition of drywall to framing) the drywall is nailed. Of these options, I suspect that the former is the most effective, if done properly. However, the HERS inspector will be unable to observe this, since it’s occuring concurrently with the naling of the drywall. Are builders required to use the post-nailing approach, so it can be observed, or do you rely on blower door testing to infer that it was done properly?
Thanks –
Forrest Fielder
Building Code official
Phoenix, AZ


Erik North July 7, 2012 at 1:39 pm


Thanks for the comment. I can’t speak to HERS standard, not being a HERS rater (HERS and Energy Star aren’t as common in Maine as other sections of the country).

As for testing an air barrier with a blower door…the testing depends on the installation of the air barrier. If the exterior sheathing is the air barrier, testing could be done prior to finish work. If the interior drywall were the air barrier, then testing would be after installation and taping.

And my understanding (I’ll talk about this next time I see one of my HERS rater friends) is that blower door testing for HERS is not about confirming the integrity of the air barrier. Rather it is for calculating the HERS rating and whether it meets Energy Star performance standards.


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