Insulation considerations for new construction

razlDecember 3, 2007

I'm in the planning stages of a new home in the northeast (RI) and am researching various insulation types (high performance fiberglass, cellulose & spray in foam).

I like what I've read so far about spray-in foam so the question is, would it be better to go with 2x4 or 2x6 construction under these scenarious?

A) 2x6 = R3.6 x 5.5, so about an R-20

B) 2x4 = R3.6 x 3.5, about R-13, plus add 1" rigid foam boards to exterior which adds another R-5 for R-18

overall.

I guess the benefit of 2x4 constructions with the rigid foam boards would be that I'm providing insulations to the otherwise naked studs.

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greif

razi
If it were me I would go with 8.5" wall, use 2x4 24" on center and stager them so they are half toward inside and half toward outside for thermal break(12" apart). Yes this does cost more but it will pay for itself over time. spary foam and 1" board on the outside.

go to gary's site it is full of lots of great info.

Gary Reif

    Bookmark   December 3, 2007 at 1:33PM
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razl

Hi Solarguy,

Thanks for the chock full of info. For the ornl.gov site, I'm not getting the same numbers. Did you assume polyurethane or something else for insulation? Either way, I think you are right that I should aim for a much higher R-value. Owens Corning recommends R7 rigid foam exterior sheathing and R21 for the wall cavities for R28 total.

Right now for walls, I have cedar shingle siding, standard OSB sheathing, R-15 fiberglass batts, 6mm vapor barrier, then finally drywall. The air infiltration was below min standards so they added a heated air-exchange. I like the idea stopping air before it enters the wall cavity, so rigid foam sheathing and foam spraying the cavity perimeter makes more sense. I mean, I'm stopping the air from passing thru the houses interior, but I'm sure I've got air movement in the wall cavity which kills my fiberglass R-value.

Gary Reif,

If you could provide a link for the 8.5" wall contruction you are mentioning, that would be great. This along with solargary's insulation upgrade calculator should arm me with enough info to calculate various ROI. Thanks guys.

    Bookmark   December 3, 2007 at 2:45PM
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greif

here is a link that shows what I mean, the link is for home theather as it stops sound also. the studs should be 24" apart on each side ans stagered 12". the picture shows 2 x 6 bottom plate but i would go with 2 x 8 plug 1" of foam board for 8.5' total

another way to do it is to put up a 2 x 6 wall and then put 2 x 2 purlins across the studs to get 7" thickness of wall, the nice thing about this is it makes a strong wall. plus 1" foam bord gets you 8"

both of these cost more but think of it as an investment as you will have lower energy bills for the life of the house

your builder might tell you it's not worth it but only a small percentage know or care

Here is a link that might be useful: staggered studs

    Bookmark   December 3, 2007 at 8:40PM
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Brewbeer

razl,

Have you researched SIPs? (Structural Insulated Panels)

    Bookmark   December 3, 2007 at 9:01PM
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coffeehaus

We thought about 2x6 walls, 1" foam board, and spray foam, but then found that the windows we planned to use would require special jams for the wall thickness, increasing our costs by a considerable margin. Just something to keep in mind when planning your wall thickness.

    Bookmark   December 3, 2007 at 9:57PM
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greif

hey the "gimme shelter" business is in my neck of the woods, about an hour away. I met one of their workers a few months ago when I took a PV solar class at the MREA.

one nice thing about the spray foam is it really seals the house good and is an awesome soundproofing. I have several coworkers that did it in their houses and you can't hear wind blowing untill it gets really gusty.

now the other problem is where do you find really great windows

    Bookmark   December 4, 2007 at 10:10PM
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greif

another thing to think about is roof overhang, you will want to go with more overhang to shade your windows in summer to reduce cooling bills but in winter the sun is lower thus giving you passive heating.
I have never found any sites that tell you want the overhang should be depending where you live but should be easy to figure out, just find on the web at what angle in the sky the sun is for summer and you should be able to figure out how far to over hang to get shading
Gary, do you have any info about this on builditsolar.com?

    Bookmark   December 4, 2007 at 10:32PM
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jegr

Historically in your part of New England, a 16" roof overhang is very common. With the rafters sitting on a wall facing south that overhang will shade about 5' of wall from mid June to mid July. Lots of caveates there...
Plan on porches, shades, awnings, trees, arbors to shade you as the sun moves. The angle of the sun in April and May will also be the angle in July and August. Spring sun shine is welcome, summer sun you would rather keep out.

    Bookmark   December 6, 2007 at 3:22PM
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