Does infiltration really reduce the r-value of our insulation?

Previously posted at:

But Sean and I couldn't come to a full agreement.

"When we add up the heat loss from a home we need to be sure we are only counting those BTUs once.

How often have you heard the statement that air leakage reduces the effectiveness of the insulation?  It's one of those things that's easy to see.  Turn on the blower door on a cold day and watch the cold areas grow as the infiltration chills the walls.  We are even supposed to look at the resulting patterns and reduce the grade of the insulation accordingly.  Well, would you be surprised to learn that infiltration and exfiltration do not degrade the performance of the insulation when that air passes through?  If it is r-19 with no air passing through, it is still r-19 with the leaks.

Here's where the distinction comes in.  If the infiltration on a cold day was headed all the way into the house, it was going to get warmed up to room temperature at some point anyway and those BTUs will be accounted for when dealing with the somehow determined air leakage number.  To further degrade the wall assembly insulation value would be penalizing that home twice for the same heat loss.

There are examples like attic venting where incoming air blows through the insulation and then carries that stolen heat out the high vents, but since no other calculation is accounting for that loss, then degrading the insulation is probably appropriate."

This is truly shooting from the hip as even I have concerns about my explanation.  Any help appreciated.


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Actually, I think the net effect is improved performance compared to considering conduction and air leakage separately. Air moving through the insulation recovers some of the conduction losses as it makes its way inside. A similar effect happens as air passes through buffer zones like crawl spaces, basements, and attics- infiltrating air gets warmed on the way in and exfiltrating air leaves some heat behind on the way out. The overall effect is kind of a low efficiency HRV.

Hi Michael and thanks for the input.

I was playing with some air leakage numbers from the air sealing thread (: (I'm in Maine) and remembered my son's new home.  Very detailed fiberglass install, yet the IR view with the blower door was scary.  Without having seen the almost perfect installation one would easily conclude the rating of the wall should be degraded.  Yet the energy loss seemed to be fully accounted for with the leakage numbers.  If one were to reduce the r-value of the wall assembly it seemed a double penalty.

I doubt there are enough BTUs to worry about, but it is nice to have a better understanding of what is happening.

Thanks again,


It has been found that roof application of spray foam will reduce infiltration and reduce ceiling heat transfer and duct losses. Wall and floor applications will also create better thermal and air barriers, and make better use of engineered products.

Joe's comment on roofs adds to my preference as an upgrade to remove the siding, add insulation board boxing out for windows-doors-vents, adding an air gap to allow condensation to drip to the ground behind the siding that's re-installed.

This removes infiltration and removes conductive heat-loss to the sheathing that can become radiative heat-loss to the wind doing the infiltration without the board.

Agree with Bud it's double jeopardy to degrade the rating for batting, an detraction is the dew point, where it is determines heat-transfer within the batting so when there is infiltration it can degrade the rating from moist fibers, may be minor but too often it's wet enough to keep the bottom plate damp, so, adding the exterior insulation forces that the dew point by design will most of the time be somewhere within the board and never reaches the batting in walls.

Consider that another big reason to do this beyond the heating bills is that it helps reduce fungal & rot problems, walls have too little air flow to clear moisture well enough in too many corners and crannies to deal with dew point condensation very well here in the Pacific NW, so this was always in the picture of why to add the board since becoming aware of the advantages possible.

Of course infiltration reduce the performance of insulation. 

been in Home Performance field for 8 years.

I've tracked my customers energy savings and savings on my own house.

We've all seen or built infiltration demo boxes like this one: 

R-value is a lab experiment performed under lab conditions in a lab box to measure heat loss through the lab box. 

R-value indicates the potential performance of the product not the actual performance. 

Insulation is a system and only works right if all parts of the system are installed and maintained correctly.

Its our job as Home Energy Pros to educate homeowners and tradespeople about this.

Andrew, the question here is in what column do you place the heat loss.  In a wall assembly where air is infiltrating we have already accounted for all of the extra heat exchange with the infiltration.  That leaves just the r-value of the insulation to calculate the loss through the traditional wall with no reduction in performance.  If we were to reduce the r-value of that insulation we would be double dipping, penalizing the wall assembly for more heat loss than is actually occurring. 

If the air filtering through the insulation is not being counted as part of the infiltration, like attic venting, in the soffits and out the ridge, then a reduction in r-value would be appropriate.  Another that I have earmarked to watch for is board siding.  After years of drying the gap between those boards can exceed 1/4" and the wind just blows in and out without entering the house.  Since that heat loss is not part of the infiltration number, it needs to be accounted for by reducing the performance of the insulation.


Let's consider a knee wall in an attic with open faced fiberglass.  With no air barrier on the outside surface, the insulation is not performing like it would if there were barriers on all sides.

By sealing the back of that wall, there would be minimal reduction in the infiltration, but an improvement in the performance or the R value.

In places like that, there should be a "double penalty

However, if the infiltration is coming between plates and floors, it has little opportunity to have an impact on the insulation value.

Hi John,

We basically agree.  My primary concern is the air that passes through the envelope, infiltration and exfiltration.  This is of particular concern as a blower door and IR inspection will spot when this is happening and lead one to conclude that the insulation is performing well below its rating.  However, air leakage numbers are already accounting for the BTUs being lost/gained, therefore an adjustment in r-value becomes a double penalty.

The kneewall and other wind washing areas that are not part of the in/ex filtration calculation do indeed need some form of accounting.  Unfortunately, despite computers that can carry a calculation to an infinite number of decimal places, much of our work boils down to good old guessing.


That's true Bud. We can't trust the software completely and try to make our guesses good.

Bud: plez see : thank you! 

Tom, too much reading in that link for me to search for whatever point you are trying to make, be more specific.  But note, I'm aware that infiltration will reduce the performance of insulation, however I'm specifically pointing out that at times the heat loss assigned to that infiltration as air leakage has already accounted for the loss.  To derate the insulation further would be double dipping (of sorts).


Here in Austin, there are more things to consider other than heat loss or heat gain, which I do believe occurs, but indoor air quality suffers because of of allergens and dust from vented attics.  Our cooling season is much longer than our heating season and with hot humid air infiltration, moisture issues are a concern.


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