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Can you quantify the air leakage savings of spray-foamed above-grade walls vs. batt insulated?

energysmarts | Posted in Green Products and Materials on

We are looking to convince a client that it’s worth it to spray foam their exterior walls and basement. Does anyone have quantifiable data that shows the ACH@50 difference between two houses. One insulated with conventional batts, and another with sprayfoam?
Perhaps you’ve done air leakage tests on similar homes with the two different techniques?
Toby Smith

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  1. GBA Editor
    Martin Holladay | | #1

    It's possible to build a fiberglass-insulated home that is very tight. But on average, fiberglass-insulated homes are leakier than homes insulated with spray foam.

    The best data on this issue were collected by Bruce Harley; I reported on his findings in the April 2005 issue of Energy Design Update, in an article titled "Fiberglass-Insulated Homes Are the Leakiest." Here are some excepts from the article:

    "Recently Bruce Harley, the technical director for residential energy services for the Conservation Services Group in Westborough, Massachusetts, was able to study correlations between airtightness and insulation type in a much larger number of buildings. Harley assembled airtightness data on Energy Star homes (including single-family and multifamily homes) completed in 2004 in Massachusetts and Rhode Island. All of the homes were blower-door tested after completion.

    "The number of houses in the data set differs depending on whether the houses are divided by wall insulation type or ceiling insulation type. (There are several reasons for this, including the fact that a small number of houses, including homes with SIP walls or roofs, use a variety of insulation types or unusual insulation types. There was no easy way to segregate the houses with more than one type of insulation from those with a single insulation type.) Harley was able to look at airtightness data for 906 homes divided by wall insulation type, and 702 homes divided by ceiling insulation type.

    "Harley found that houses with walls insulated with spray polyurethane foam were significantly tighter than those houses with walls insulated with cellulose, and that houses with walls insulated with cellulose were significantly tighter than those insulated with fiberglass (see Table 1). Sorting the houses by ceiling insulation type yielded similar results to those obtained by sorting the houses by wall insulation type.

    "Although the data clearly show a consistent correlation between insulation type and airtightness, the reasons for the correlation are unknown."

    [Click on the image below to enlarge it.]


  2. energysmarts | | #2

    Thanks Martin, that is very helpful. I don't suppose you could look at the ACHnat reduction as a straight % decrease and apply that to ACH@50? What I mean is if spray foamed homes are 30% tighter as those numbers suggest would the ACH@50 drop by 30% too? Just trying to find something to do a cost/benefit analysis for the customer.

  3. GBA Editor
    Martin Holladay | | #3

    Rules of thumb for calculating ach50 from ach(nat) vary by climate. In Minnesota, ach50 equals approximately ach(nat) multiplied by 17, while in Florida, ach50 equals approximately ach(nat) multiplied by 30.

    So, yes -- a 30% reduction in ach(nat) roughly corresponds to a 30% reduction in ach50.

  4. Expert Member

    While the average spray foamed house may be tighter than the average batt insulated one there is no guarantee that this will in all circumstances be the case. Tightness is the result of successfully implementing one of many air sealing strategies. Don't forget that the tightest house in the world doesn't use spray foam and includes a poly air barrier.

  5. ntisdell | | #5

    Yes i would say the choice in-wall insulation often is more of an indicator (to me anyhow) of how the home was built.

    If a someone mentions spray foam or dense pack - likely the home was built with an eye for energy, quality, and results in tightness. Not necessarily true... but more likely? YES

    If someone mentions 2x6 walls with fiberglass batts.... I would bet good money it is a code built home - and just enough airsealing to meet code/inspections. Which these days in MN goes a decent way (sprayfoam sillplates, spray foam all wiring holes in studs/exterior boxes, gasket-ed elec boxes., acoustic caulked vapor barriers....etc.)

  6. GBA Editor
    Martin Holladay | | #6

    As I noted in my original 2005 article, "It is possible that builders who choose cellulose or spray foam insulation — both of which have a reputation for resisting air flow — may be more meticulous in performing air sealing tasks than builders who choose fiberglass batts. Perhaps builders’ attention to detail in certain areas unrelated to insulation performance — for example, careful sealing between wall bottom plates and subfloors — partially explains the measured differences in airtightness. If this theory is true, the extra dedication to air sealing may be motivated by the builders’ desire to justify the added cost of spray foam or cellulose over fiberglass batts.

    "However, the principle of Occam’s Razor favors a simpler explanation: that the measured differences in airtightness are due to differences in the material characteristics of the different insulations."

  7. ntisdell | | #7

    I agree martin - a large component of the tightness in a spray/dense house is also related to the insulation (especially spray).

    restrict flow through the wall with foam/dense and even with holes at the top/ isn't going to flow the same as it would through free space or a fiberglass merv1 furnace filter... i mean wall filter....oops, i mean wall batt.

    Been tested i would assume? A perfect lab wall with each material.... then drill small holes in the sheathing. test airflow in/out under various natural conditions (sun/cold) and also under a set static pressure (wind/stack).

  8. wjrobinson | | #8

    Standard batted walls of the last few decades... do not compare to airtight walls you can offer today.

    batt walls can be improved by foam caulk... at least two systems out now. Our local insulator uses the Owens Corning system.

    Any builder can tighten up a home in many many ways. No matter how you do so, it does cost more, for me it costs twice as much as straight batt work costs. And to insulate twice the code costs twice that.


  9. davidmeiland | | #9

    I wouldn't look to "convince" a client that foam is worth it because of the increased air-tightness. Closed cell foam is great if you need (a) a hot roof, or (b) more R value in an assembly than other materials can provide.

    Foam, especially flash-and-batt wall insulation has caught on to some extent around here, and I have done blower door/IR inspections and concluded that it doesn't lead to dramatically greater air-tightness. There is often still air leakage between the bottom plate and the floor that gets into the wall behind the drywall, and/or straight into the room. There is also often air leakage between the top plate and the drywall that gets into the wall, via the roof venting. I remember testing a house not long ago and being surprised when the builder told me the walls were foamed, because there was air coming out of his electrical outlets just like there would be with batts.

    I would rather try to make the air barrier at the outside of the sheathing, if you can. Otherwise, judicious use of sealants and drywall adhesive can get it done. Don't just shoot foam insulation and think you're gonna be airtight.

  10. Expert Member
    Dana Dorsett | | #10

    I'm with David Meiland- the best that cavity insulation can do is affect the air-tightness of the cavity, which is necessary, but not sufficient.

    Foam insulating a studwall while leaving the stud-plates uncaulked makes no sense if the stated reason for using foam was air-tightness.

    One location that spay foam does better than other approaches is air leakage at the foundation sill, especially in retrofits. Cheap foamy sill gaskets are pretty dysfunctional, but some EPDM sill gaskets can be pretty air tight, and recommended in new construction.

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