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Combination of Insulation Types

Cynthia Hartley | Posted in Green Products and Materials on

To insulate our home near the coast in Long Beach, NY we are looking at a combination of rigid foam board for the exterior and spray foam for the interior. We wish to achieve an R-value of about 20.

Since we should not use a vapor barrier on both the outside and inside, which option would you recommend:

Option 1) 2″ close cell spray foam insulation (vapor barrier) for an R value of 14 on the inside with a 2″ expanded polystyrene foam board (non-vapor barrier) for an R value of 7 on the outside

Option 2) 4″ open cell spray foam insulation (non-vapor barrier) with an R value of 14 on the inside and a 1″ polyiso rigid board on the outside (vapor barrier) with an R value of 7

Option 3) some other option

Also, we are using 3″ close cell spray foam for the crawl space. Any suggestions for achieving an R value of 49 in the attic?

Many thanks.

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Replies

  1. Aj Builder, Upstate NY Zone 6a | | #1

    Have you contacted contractors? If you gave me a scope of work from here I would note that in the contract and be your subcontractor limiting liability to you alone.

    R49 in foams is not generally done for many past and present reasons. Many rigid foam roofs in my area have had rot failures.

    Those who have experience are the only contractors i would use. The last thing I would do is try to have a contractor do foam who does not have experience. Why? Because when I take apart builds with foam so far I have found lots of severe damage.

    Dana can give you some details as to your options. Next you need the right contractor.

  2. GBA Editor
    Martin Holladay | | #2

    Cynthia,
    Option 2 is better than Option 1.

    I assume that you live in either Zone 4 or Zone 5. (Here is a link to the zone map.) To determine the minimum R-value of your exterior rigid foam, read this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    The minimum R-value depends on your climate zone and your stud size. If you have 2x6 studs in Zone 5, the minimum R-value of the foam is R-7.5. If you use exterior polyiso, which has an R-value of R-6 or R-6.5 per inch (not R-7), then your exterior foam choice is almost but not quite thick enough.

    Q. "Any suggestions for achieving an R value of 49 in the attic?"

    A. Sure. Install 14 inches if cellulose on the attic floor.

  3. Expert Member
    Dana Dorsett | | #3

    "If you have 2x6 studs in Zone 5, the minimum R-value of the foam is R-7.5."

    Yabbut, Long Beach NY is on the southern coast of Long Island, well into Zone 4, where anything over R3.75 meets code for 2x6 framing.

    R6 iso on the outside of 2x6 framing 16" o.c. has a whole-wall R of about R20 but only with a FULL CAVITY FILL of about R19 on the open cell foam. If it's truly only R14 cavity-fill due to the 3.5" depth of 2x4 framing you'd be at about R16 whole-wall, not R20. You can achieve a higher whole-wall R at the same wall thickness using 2x4 framing 3" of exterior foam compared to 2x6 framing and 1" of exterior foam. The 2x4 wall can hit close to R30 whole-wall using 3" of 1.5lb density foil faced polyiso, but the 2x6 wall only hits R20 with 1" of exterior iso. Thermal bridging matters.

  4. Cynthia Hartley | | #4

    Thanks AJ Builder. I did contact a contractor, several. Most contractor's but not all, that I have dealt with over the years have wanted to sell me products or services that were more than I needed and more expensive, such as a 200,000 Btu boiler for a 1,200 sq ft bungalow. To mitigate that I am trying to educate myself so that I can make an informed decision.

    Thank you, Martin and Dana. I am indeed in Zone 4 and the framing is 2 x 4. I should have specified that. As such my minimum R requirement on the exterior is 2.5. I appreciate you both directly addressing the question I posed. Additionally, thank you Martin for this fantastic site. I spent all day yesterday researching insulation. The more I learned, the more I need to learn... most exciting.

    I suppose my most basic question is whether or not it is better to have the vapor barrier on the outside or the inside. Perhaps I should not be concerned with that, but rather be concerned with a high R-value, and put polyiso on the outside and close cell on the inside?

    Thank you, Dana, my math (not my strong suit) was wrong in calculating the R values. When you say ' 3" of exterior foam", do you mean 3" of close cell spray foam or polyiso foam board? If the latter, then you and Martin are in agreement - which gives me peace of mind, but I now realize that with 2x4 framing open cell which expands to 6" would be wasteful. If I went with close cell on the inside then that would be a double vapor barrier, which I understand is no good. Do you suggest cellulose as an option on the inside?

    If you mean 3" of close cell foam, then this contradicts Martin - causing me much confusion :-).

    I know all of you smart people have a lot going on, and I am deeply appreciative of the time you have given to my question.

  5. GBA Editor
    Martin Holladay | | #5

    Cynthia,
    The question about "whether or not it is better to have the vapor barrier on the outside or the inside" is the wrong question to ask. A wall needs to have adequate R-value; to address thermal bridging; to be relatively airtight; and to be able to dry in both directions from its most vapor-impermeable layer. Those are the issues that matter. The idea that you need a vapor barrier on one particular side of your wall is an old-fashioned idea that has been discredited.

    For more information on vapor barriers, see these articles:

    Do I Need a Vapor Retarder?

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

    It is better to put rigid foam on the exterior of your wall, not the interior, for two reasons: exterior foam does a better job of insulating rim joists, and exterior foam does a better job of insulating partition intersections. Exterior foam also does a better job of keeping your OSB or plywood wall sheathing warm and dry, lessening the chance of wall rot.

  6. GBA Editor
    Martin Holladay | | #6

    Cynthia,
    I'm pretty sure that Dana was talking about rigid foam on the exterior side of your wall sheathing -- not spray foam on the exterior side of your wall sheathing. For more information on installing exterior rigid foam, see How to Install Rigid Foam Sheathing.

  7. Cynthia Hartley | | #7

    Thank you so, so much Martin! I have read and heard a lot of contradictory information, albeit not the three articles you just provided. I will consider the matter of vapor barrier settled. I look forward to continued reading of all the great articles and watching the great videos on your site.

  8. Expert Member
    Dana Dorsett | | #8

    For clarity, I was indeed talking about 3" of rigid polyiso insulation on the exterior, as Martin surmised.

    In your climate R18-19 rigid foam over 2x4 w/open cell that would be near the limit of what's rational from a purely net-present-value on future energy cost savings, but any less than 1.5" would be skimping from a comfort-at-temperature-extremes point of view. See table 2, p10:

    http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

    Assuming you met pretty much all of the zone-4 row (note, those are "whole assembly" R valuves, with all thermal bridging accounted for) in that table you could be in pretty good shape for hitting near Net Zero Energy with a rooftop PV array that actually fits on the roof (if not in your pocketbook at today's prices.) If you're off the gas grid with only expensive fuel options the raw financial rationale for 3" of exterior iso might be there, which would put you at about R28-29 whole-wall. If you use recalimed roofing foam from commercial demolition with an exterior side layer of 1" foil-faced virgin-stock the material cost of foam-overs is cut by more than half, bringing the the financial rationale for more than 3" even more in line. (InsulationDepot.com, is one source of reclaimed goods but I keep finding other vendors on craigslist, etc. as well.) On a recent deep energy retrofit I was involved with (completed about a year ago), extensive use of reclaimed goods saved many thousands in cost.

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