Rigid foam insulation for roofs - need product recommendations

[ysette9]

Hi all - I want to put rigid foam board insulation under the new roof we are going to have installed. My hope was to do spray foam insulation in the attic, but there isn't enough physical space to have it installed. I'd rather do the rigid foam board under the asphalt shingles to prevent the attic from getting hot to begin with, rather than putting in a bunch of blow-in fiberglass in the ceiling that will get blown out with the attic fan over time.

That said, I'd trying to do research online and am finding a bunch of info about how to install the stuff, but nothing that mentions specific product names. I need to get some specific products to provide to the roofers so they can quote, and I'd like to read up on pros and cons as well as prices for various options. Can anyone point me to some websites to educate myself? I am in climate zone 3 which apparently means that my target attic R-value is 50.

[MDM]

Don't know if there is any particular difference among brands.  Dow Chemical's Styrofoam is one, Owens-Corning's Foamular is another, etc.

[ysette9]

That is something helpful at least. I did come across the Insulfoam product line on my online searches but I couldn't for the life of me figure out any other products targeted to the residential market, including the Foamular that you mention. Thanks for the leads.

[LostGirl]

Are you planning to put it on from the top, and sheath over it?  If you install from the bottom you will have thermal bridging at the stud gaps. R-50 would be pretty thick. Search for John Mansville specs. I think its R3.5/inch. The new CA standards are R30 at the roof level, the gains from R30 to 50 would be incremental in heating and cooling costs here.

[ysette9]

The plan is to install it from the top. Good to know about R-30 versus R-50. I guess I got bad info on the site I was on.

The question apparently is still outstanding whether we could make an attic fan + blown-in fiberglass work, or even whether a passive system of vents at the eaves for intake and eyebrow vents at the roofline to dump the hot air would work. I'm open to any experience or thoughts folks might have since this is all new territory for me.

[bacchi]

I used 4" of Mansville polyiso (~R24). If you go that thick, your roofers will need long screws with caps to secure the foam to the sheathing.

There is a minimum of rigid foam in order to avoid attic sorption (aka condensation). You'll also need to maintain a safe ratio between under-sheathing and above-sheathing insulation.

http://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top-roof-sheathing

[ysette9]

I'm getting advised that 2-3 inch mac is what we can handle while still disguising the extra thickness without major alterations. I think that means I'll need some hybrid solution to get the R value high enough.

[Fishindude]

If you have a typical sloped shingle roof with attic, this does not seem like a very good plan.
Standard blue board & pink board foam runs about R-5 per inch, so it would take 10" of this stuff to get to your R-50.
This stuff is also expensive and highly flammable.   Thermax makes a fire retardant, more costly foam that has a little better R value per inch at approx. R-7.5, but that would still take 7" of material to get to your R-50.

This going to be very expensive and impractical.
You would have to somehow sandwich the foam between two layers of decking so that you have something for the foam to sit atop, and something for the shingles to nail to.

Really, the only practical way to achieve the R-50 would be with blown in or multiple layers of roll fiberglass.
Depending upon your location, R-50 may be a little overkill too, something like R-30 may be plenty.

[ysette9]

In your opinion, what is the best way to get to R-30?

[Fishindude]

In your opinion, what is the best way to get to R-30?

Fiberglass rolled material of appropriate thickness laid on top of your ceiling, two layers; one parallel and between the joists, then another running 90 degrees the other direction on top.  Two layers of 6" will exceed R-30 a bit.   Then a typical vented attic.

Some would argue in favor of blown in because it is cheaper and there would be less likelihood of gaps.   This would be fine too.