Insulation Does NOT Stop Infiltration

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Big holes in the building envelope make your air conditioner work harder.

Speaking of things that drive me crazy (we were, weren't we?), here's another one - people confusing the roles of insulation and air barrier. The people I'm talking about are not homeowners, though. I'm talking about contractors, builders, and journalists who should know better but don't.

This is a simple article because it's such a simple misconception. See that photo above? The insulation is dirty precisely because it does NOT stop the movement of air. That's why they make filters out of fiberglass.

A house is made mainly of structure and control layers. There's the framing that holds everything together and keeps it from falling down - the structure. Then there are the control layers, meant to control the flow of heat, air, and moisture across the building envelope. This is Building Science 101. The two control layers we're concerned about here are:

  • Air barrier
  • Insulation

Most of the time, houses are insulated with materials that do NOT stop air leakage. Fiberglass and cellulose are the two most commonly used insulation materials, and they do not qualify as air barriers under the standards of the Air Barrier Association of America. Cellulose will slow it down a lot more than fiberglass, but you still can't think of it as an air barrier.

The cause of this rant was my watching a home improvement show recently (always a dangerous thing to do). During the walkthrough, they said that the homeowners had complaints about drafts, so they needed more insulation. Really?! You're supposed to be pros telling people how to fix their homes properly, and you don't know that more fiberglass in the attic won't help them here?

It's pretty simple really. Reduce the air flow. Reduce the heat flow. Two separate processes. Most of the time, it's done with two separate materials.


Josh Lloyd

Great article! I can't tell you how many homes just have a piece of insulation laying over big gaping holes.

Tory T

You pointed out the problem but didn't give the solution. What is the answer if it isn't insulation?

Allison Bailes

Tory, the solution is to seal the leaks with rigid materials, caulk, and spray foam - materials that stop air leakage. I sealed the hole in the photo at top, for example, by covering it with a piece of foam board and then sealing around the edges with spray foam. The type of air sealing you do depends on the size and location of the leak as well as when you do the air sealing (during rough-in of new construction or fixing an existing home).

Tory T

Thank you. You always have fantastic information.

Carl D. Clark

This is the biggest misconception I run into.  
And a difficult one to overcome....

Walter Stachowicz

Thanks, Allison. Can you recommend a foam board that is fire-rated? Most manufacturers require that a fire barrier be placed over the foam board (like 3/8" drywall, etc.). The product sheets that I've seen state that the foam board should never be left exposed.

chris brown

You don't have to use rigid foam boards at all for these applications, in fact I wouldn't. Try duct board or something similar. It just has to be rigid material to cover the opening. Then seal around the edges to complete the job.


Excellent explanation of the differences - even I, a person who does not touch any type of home building or repair without a note from my mother, understood it. I thought insulation did it all, so dang me!

Allison Bailes

Walt, the only foam board I know of that has the fire rating is Thermax, made by Dow.  
Chris, with duct board, you just have to be careful to seal the foil side, since that's the air barrier. If you get that right, it's a great material to use for this application.


If the thermal by-pass is on the attic floor we use plywood, I don't want someone, especially the homeowner to be in the attic and step through and end up in the bath tub, actually happened to someone i know and the wife was in the tub at the time. If it is on a cathederal ceiling side wall we will use the foamboard w/radient barrier on the outside and tape the seams with foil tape and foam the edges. Oh what a difference!

David Butler

Another home run, Allison!  
Two places to look out for this:  
* wall sections that back up to a shed roof -- sheathing installed after shed roof is framed, so wall sections behind shed roof 'attic' remain open 
* attic knee-walls -- typically built with no back, no bottom plate, no top plate. Bad news. 
Here's a great resource on this topic (Energy Star Thermal Bypass Guide):


You know what drives me crazy...when journalists confuse "kiloWatt" with "kWh". I can't tell you how often I see this. It sounds like a minor mistake, but it is actually a major difference. This is why our blog on should be required reading for everyone!


The plus side of the issue you're calling out is that you can use dirty insulation as a forensic tool. Get into the attic, lift up the insulation. If you see dirt, you've probably found an air leak that you can now seal!


I have this discussion with my customers and potential customers at least 3 times a week.  
What I always tell folks is that insulation is like a sweater on a cool windy day. It will keep you warm--- out of the wind only. Put a windbreaker on (the air barrier), you will not feel the cold wind and be warm in the wind. Wear a gore-tex (breathable) jacket and you will stay dry too (vapor barrier).  
The light seems to go on with this explanation.  
It's amazing how many people think insulation is the answer to every cold issue in a structure. Professional builders and contractors included.  
This analogy helps me get my point across about the importance of air sealing BEFORE insulating.

Harris Woodward

Richard, you nailed it with the sweater/wind breaker analogy. People appreciate non-technical explanations. 
I use the $400 Northface Parka example... what good is this elaborately insulated coat if you leave the zipper down? The lightbulb goes on immediately.


I will work on improving the insulation at my home soon, before it gets cold! Last winter was brutal, i could feel the cold coming in on windy days and nights. This time I want to be prepared for winter, keep my family warm, and lower wood cost by having energy efficiency. It’s great to have this resource to consult and gather useful information stating which materials will be best for different cases. I am planning on using a plastic “covers” for the windows on the inside as air barrier. The doors are sized incorrectly leaving a gap at bottom, any suggestions? The property is a modified and remodeled adobe garage with air leakage all over, built in the early 1900s in the now historic district of Santa Fe NM.