attic insulation question

phil.grass

Building a new house and was have sprayed 3" of closed cell spray against underside of roof w/ lightly covering all of the 2 x 12 rafters, the idea was to do this and make it air tight and then back it w/ R30 fiberglass batts, giving me over a R50. I have recently hired a HERS energy star rater to evaluate the house and he has told be that I now will need to add an air barrier to the inside face of the fiberglass batts, he told me I should scrape down the overspray on the joist and sheetrock the attic w/ 3/8" board. Seems like a lot of work for an attic that will only house a gas furnace and air handler. Do I need this other air barrier?, isn't 3" of closed cell good enough? what else could I use for an air barrier that is not expensive and easy to get up into an attic that only has drop down stair access? Also, should i consider adding insulation between the 2nd floor ceiling and attic? Seems like a waste to have nothing there.

Look forward to hear what people think

Wildie

You didn't say where you are building and that would be an important consideration.
It seems that you have been well advised. I assume that you will install a vapour barrier over the fibreglass and then close it in with 3/8" drywall!
What type of roofing will you use? I am given to understand that asphalt roofing warranty is voided, if its not vented underneath!

phil.grass

Wilde

Building in MA. I donn't want to add the drywall as the spray foam has already been coated for fire resistance. The plan was to back it w/ fiberglass only, The HERS rater wants the drywall as an air barrier but I would not be able to get sheets of it up there easily. I was thinking about just adding 6 mil poly or worst case senerio spend big $$$ an ad some of the foil reflective air bubble material.

I have asphalet shingles from certainteed and have already contacted them about a "hot" roof and have been advised that this will not void the waranty.

ccarlisle

IMO, the 3" spray-foam, plus the fibreglass, would have been enough (in terms of insulation, air barrier and vapour barrier) had the attic not housed the furnace. And I think that's the kicker; fire regulation perhaps dictate that it be covered with drywall, I don't know, but precaution may be the reason behind it...And even if the furnace weren't up there, some codes might say to cover it anyway. But it's moot and I'm assuming the HERS rater know his local building code.

But from an energy loss perspective, yes, I'd cover it. Just how you get drywall up there doesn't change that opinion just makes it a challenge.

No need for a vapour barrier with that foam and no real need to insulate the attic floor. Air-seal would be better time spent. And you don't want it too tight, the furnace has to breathe.

I wouldn't bet on any shingle warranties helping you out; in any claim, the most Certainteed would do for you with that set-up is refund you the cost of the shingles if you can prove they were faulty. You'll get in the neighbourhood of 15 years, that's it.

PS: Oh yeah: you say you are building a new house? I thought new codes insist on roof venting (i.e a gap between the roof and the insulation).

phil.grass

Code does not call for drywall as the closed cell spray has been treated w/ a fire retarder. Drywall was because the HERS rater wants an air barrier. Will be using mechanical air exchange, still haven't decided if putting a forced hot air furnace in attic or going w/ hydro heat with putting gas boiler in basement and piping up to air handler. What else could be used as an air barrier besides insulation baord or drywall?

ccarlisle

Well then, the recommendation to cover the ceiling in your unvented attic has more to do with the fibreglass needing covering (from an energy losss viewpoint) being most efficient in an enclosed cavity.

Polyethylene sheets can act as air barriers but then might also violate code if you have a furnace up there.

Gary in WA

When you figure you R-value, don’t forget to allow for thermal drift with SPF: http://www.kwikbuildpanels.com/pages/article_facts.html

And the whole attic ceiling value, including the rafters heat loss: http://www.coloradoenergy.org/procor...f/r-values.htm

You may end up with only R-42 or so: http://www.rvaluehomes.com/rvalue.htm
Check with your local Building Department on approved materials for air sealing: http://www.energysavers.gov/your_hom.../mytopic=11300 Click link at page bottom.

http://www.buildingscience.com/docum...wall-approach/

Radiant barriers are mostly for hot climates: http://www.ornl.gov/sci/roofs+walls/radiant/index.html

I’d do more research on the heating source, and where to install it.

Be safe, Gary

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