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Community and Q&A

Spray foam installation on underside of roof sheathing–existing house

TFMdMgCRGe | Posted in Green Products and Materials on

We own a circa 1920s renovated farmhouse in eastern Nebraska. Over the years we have been making improvements to the home’s performance. This summer we finally knocked a hole in the second-story ceiling to get a look at the attic space for the first time. The second story was always bitterly cold in winter and extremely hot in summer.

What we discovered was haphazardly thrown-in batt insulation on top of 1″ thick 1920s insulation (black on one face). In a large portion of the roof, only the 1″ thick insulation was present. The second floor is not a real second story, so the walls slope due to the gabled roof. Where the walls are sloped, only 3″ of space exists between the sloped ceiling/wall and the roof deck, and even this is broken by the rafters and protruding roofing nails.

The spaces available are so small (especially that 3″ space) that the “standard” techniques of blown fiberglass fill and ventilate are really not an option to achieve the R-values that we desire. We were shooting for R-38 to 40 with the blown-in fiberglass, but would be willing to accept a lower “R” with foam with the understanding that reducing infiltration boosts the actual performance as compared to loose insulation.

So now we are thinking of tearing off the ceilings to allow clear access to the underside of the roof decking, eliminating any ideas of ventilating the attic space, and having a contractor install several inches of open cell foam to the underside of the roof.

Several questions:

1) Should we install a vapor retarder against the decking before the insulating foam?
2) What about the protruding nails? Can we snip these shorter?
3) Whats a reasonable number of inches to install on the underside of a roof in Nebraska, both from the standpoint of any limitations of the foam and reasonable R-value when using spray foam.

I thank you in advance for responses–cold weather is coming soon, and I think giving that 1920’s stuff an R-value of 1 would be generous!


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Should we install a vapor retarder against the decking before the insulating foam?"

    A. No.

    Q. "What about the protruding nails? Can we snip these shorter?"

    A. Yes, but nobody does, because there is no real benefit.

    Q. "What's a reasonable number of inches to install on the underside of a roof in Nebraska, both from the standpoint of any limitations of the foam and reasonable R-value when using spray foam?"

    A. Because you are visiting the GBA Web site, I'm going to assume you care about energy performance. That means that no matter what type of insulation you use, you'll do your best to do a good job of air sealing. So I wouldn't give any undue credit to spray foam for its air sealing ability. In other words, R-value is R-value.

    It looks like you are in Climate Zone 5, where the IRC requires a minimum of R-38 ceiling insulation. I advise you to use this code requirement as your minimum goal. I understand that achieving that goal isn't always possible -- but that's the right answer to your question. Exceeding the minimum code requirement is even better.

    Since you plan to demolish your ceiling, you will be able to furr down your rafters or sister new framing to your rafters to provide the necessary depth for your new insulation.

  2. Byron Bargas | | #2

    Typically you can achieve approximately an R-6.5 per inch of urethane insulation. As for the vapor barrier you may wish to talk to a roofer although the urethane foam will spray over anything/everything, including protruding nails/holes, sealing everything it is sprayed over, as such I wouldn't necessarily see the value of a vapor barrier. As for removing the entire ceiling; you may only be required to provide openings to accommodate the ability to access the roof area so that they can get their spray wand up in the area in which to spray, this could save you quite a bit as far as drywall replacement cost, etc. Look for GREEN GUARD certified urethane product and installers in your area as this would be your best bet.

  3. TFMdMgCRGe | | #3

    I think the ceilings are going to have to come down, especially in the sloped area where there is only 3" of space between the drywall and the roof. Also, I am motivated to remove the original 1920-era insulation, some of which is decomposing presumably from getting wet once in a while over the past 80 or more years. The presence of it in the space will subtract from what the foam can fill, and it's material composition is unknown--is there any reason to leave it?

    As for getting to an R-38, I've read that there exists limits to foam thickness based on fire testing. The two contractors I've looked at so far had foam with R values of 3.5 and 5.1 per inch, respectively. There's one in the area that advertises 6.8 per inch that I will call tomorrow. At 5.1 per inch, R-38 would equate to 7 1/2 inches; at 6.8 this is ~5 3/4. Is that depth really feasible?

  4. GBA Editor
    Martin Holladay | | #4

    Is it feasible? An excellent question. The answer to that question is somewhat uncertain, due to ongoing disputes about the relevancy and reliability of current fire-safety testing procedures.

    For the time being, the answer is, "it's possible -- if you can find a contractor, foam manufacturer, and building inspector to all agree that it's possible."

  5. Riversong | | #5


    If you're tearing out the ceiling and have only 3" framing in the roof, then it's almost certainly inadequate for the snow loads. Have an engineer or highly experienced builder check out the roof framing and load paths before even considering how to approach the insulation.

  6. J99aAMQzYo | | #6

    Urethane R-value - You also need to determine what type of SPF (Spray Polyurethane Foam) you're talking about. Open Cell (ocSPF) will only get you to about R 3.5" per inch. The guys who are talking to you about R5 - R6+ per inch are talking about Closed Cell (ccSPF). These two behave very differently when it comes to moisture, they tend to have very different Global Warming Potentials (GWP), they can offgas at very different levels, and the code required fire protections can be very different. You can read more about all of this in multiple articles and reviews elsewhere on GBA.

    Martin is also spot-on when bringing up the building inspector. We are in Maryland which does not have a Uniform Statewide Building code. As such we run into all kinds of personal preferences depending on which county we're working in. Some guys we can show the factory specs and proceed without a hitch and some make it such a huge battle we've just given up trying in their jurisdictions even though we have all the documentation to back us up. You want to look closely at what may be required in your area, for your assembly, and based on the type and manufacturer of the foam you're looking at. The requirements will change based on every one of those.

    You're right to think along the lines of pulling down all the ceiling and dumping the original material. Particularly if you're going with SPF, it will be important for an experienced installer to butter both sides of the rafters first before filling the field. You'd never accomplish this with access holes. You also run a very significant risk of overheating and causing charring (or worse) if the full face of the foam isn't exposed. Finally, many of the foams can expend aggressively enough that they could actually push the finish material (plaster, gyp board, etc.) from behind causing nail pops, cracks, etc. Access holes are options for blow in fiber materials, but never for spray foam - and any of the factory specs will tell you them same thing.

    The other advantage to complete ceiling removal is that you'll either find that A) the 3" may not be what you think it is (i.e. it may be deeper) or B) That Robert's right and you need to bring in a PE to verify that it is a safe and/or sufficient condition. Even with 2x4 Truss roofs you should have a depth exceeding 3". Here in the mid-Atlantic we're required to have a roof loading capacity of 20 lbs/SF and 2x4 just barely gets us there. I would imagine that it must be more in your neck of the woods given your annual snow fall totals.

  7. ERIC | | #7

    Depending where you are in the country, different depths are needed. First step determine if you get a discount for home fortification, if you do you will install 2 inches of a foam (secondary water intrusion) along all the cracks and osb boards, this tightens the racking strength and can lower your insurance. is a good start. Roughly 5.5 inches of a half pound foam, 4 inches of a 1.2 pound foam, or 3.5 inches of closed cell foam will do what is needed in 85% of the US.

  8. TFMdMgCRGe | | #8

    The rafters are actually 2x4s, 16" on center, on a pretty steep roof pitch (45 degrees). I only stated 3" because after one takes the actual 3 1/2" and subtract for the old insulation that's about how much was left if there was any idea to simply drill and fill.

    I am working on an "as-built" model of the house in Revit, and once I get up to that level of detail, the load analysis should be easy. I'll also check with our rather laissez-faire local building inspector. Until we get more of the gypboard pulled off, I don't know where any roof beams might be.

  9. Riversong | | #9

    Roughly 5.5 inches of a half pound foam, 4 inches of a 1.2 pound foam, or 3.5 inches of closed cell foam will do what is needed in 85% of the US.

    Except that foamed roofs are not needed in most of the US, and are questionable even in hurricane country.

    Perhaps such a "home fortification" treatment will reduce roof deck tear-off potential, but does nothing to improve the roof/wall connection which keeps the entire roof - now more of a continuous sail - from lifting off.

    A Florida Solar Energy Center study determined that the most "hurricane-proofed" roofs - those with self-adhering membranes above and closed-cell foam below - are the most vulnerable to decay once water finds its way through. And this they found to be true in both hot/humid climates and cold northern climates.

    There are always unintended consequences to apparent "solutions" to architectural problems. Beware of marketing schemes intended to convince you otherwise.

  10. JphaYjYFPw | | #10

    Florida does not insure unless you build "Fortified" or if they do insure, they give discounts for "Secondary water intrusion barrier" . When building homes (anywhere in the USA has "Tornados") the wind upload coefficient and the wall racking strength combined with proper building techniques (hurricane Straps) will protect better than older methodes of building, ie Joplin MO ALSO the R-38 R-value is incorrect when foam is applied to the back to the roof deck. (An R-38 is the Value to the attic floor for cellulose and loose fill type insulations) Go to Amazon and buy the New Code book: International Code Counsel, International Energy Conservation Code 2012. Then proceed to page C-32 the NEW CODE is an R-25-ci for that area (the CI means Continuos Insulation) to the "BACK OF THE ROOF DECKING". SO JUST BECAUSE ONE DOES NOT UNDERSTAND HOW TO BUILD A FORTIFIED HOME (TO PROTECT ONES VALUABLES ) DOES NOT MEAN THAT EVERYONE SHOULD BUILD USING OLDER METHODES. Sorry for yelling, but calling Home Fortification marketing a SCAM is ridiculous. Even if your area has adopted the new codes many people do not understand them (and even more do not install the products via the Manufactures specifications and that causes more issues) like spraying foam too deep can cause fire due to core burn (using 1.8 lb closed cell foam or this Foam Nightmare due to many factors. The building needs proper building to start. So if you do multiple types of building Fortification your home may even get DISCOUNTS on INSURANCE... WOW WHAT A GREAT IDEA (NOT A MARKETING THING)

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