I am currently planning a 2200 sq ft addition for my personal home located in eastern Nebraska. I am a remodeler/gc that has been in business on my own for 3 years after working for a custom builder for 5 years. The house upstairs (1200 sq ft) will have vaulted ceilings, lots of windows, skylights, and can lights. (I cannot convince “the client” that can lights in vaulted ceilings are a bad idea.) It will also have in-floor radiant heating w/ forced air for backup and AC.
Does anyone have experience/opinions about installing 1″ or 2″ XPS (Dow Blue Board) on top of the rafters prior to installing the 19/32″ roof sheathing? I think this system would work as a hot roof application, but I still think I need to install strapping (3/4″ or 1 1/2″; parallel to rafters) on top of the xps to provide a vent space for the sheathing. Continous ridge and soffit vents would complete the vent system. I believe the science of the hot roof system; I just don’t know if we can complete the perfect air-barrier system with multiple can lights and electrical penetrations.
The roofs will have a 10/12 pitch with multiple dormers and several complicated valleys connecting old roofs to new, so strapping on top of the xps would simplify the venting. I understand that there are connection issues between the strapping or sheathing through the foam and then into the rafters, but I think that can be solved fairly easily (with an engineers approval). Applying the xps over the rafters should provide an outstanding thermal break in the system and seems easier than applying to the underside of the rafters which complicates hanging the drywall and installing soffits.
The overall system starting from the outside working in would be: Asphalt architectural shingles, roofing paper, 19/32″ sheathing, 3/4″ or 1 1/2″ air space, 2″ xps foam board, dense pack cellulose held in by stapled & glued netting (similiar to Mooney wall system), 5/8″ drywall, textured drywall mud finish (ie no vapor retarding paint).
I am not opposed to using closed cell foam instead of the dense pack cellulose, but based on my calculations and conversations with an insulation contractor I can have 9″ of cellulose put in (2×10 rafters) for R35 plus an additional R10 from the 2″xps. For the same money, I can only get about 2″ of closed cell foam sprayed in.
I suppose that I am missing the obvious (compression of foam) with this system and hope someone can help me determine whether or not this system would work or provide ideas for another system that would work in this scenario. Thanks for your time!
Replies
Why no vapor retarder on the room side? Cellulose is very porous to water vapor diffusion. You'll have somewhere around R30 in cellulose, then R10 in XPS, a low-perm VR. I would expect the potential for damage by condensation under the XPS to be high in cold weather. Poly under the sheetrock, no. But a good coat of VR paint would be a good idea.
Look at the article at http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation/?searchterm=roof about roof ventilation. The article implies that you need more bearing surface for the foam board than the 2x rafter. However, I've thought the same as you that it would work, but I'm not an engineer.
Here is what I did... I put a 2x4 flat on the top edge of my 2x12 rafters. I then closed in each rafter bay with 14-1/2" (+/-) OSB rips stapled to the bottom of that 2x4. You could use the foam instead of the OSB to do that. That 2x4 on top of the rafters gives me a nice 1.5" air space, and leave the 2x4s an inch or so short in the valleys and you'll get good airflow up the valley as well. I then netted and blew with cellulose. I strapped the inside rafters with horizontal 2x2s to flatten things out and had 1" xps foam between the strapping.
I won't say your idea won't work... but I will say that I wouldn't do it as you proposed.
Edited 11/25/2009 9:37 pm ET by Doobz26
My question on this one is what happens when you don't have roof sheathing as a sructural component. Nailing the sheathing over 1 1/2 or 2" foam leaves room for a lot of squish when the wind gusts up to 90 mph as it sometimes does. I've always been a proponent of having the building elements connected tightly to one another. There are other ways to beef up insulation values and eliminate thermal bridging. Mostly they are done on the interior side, which has the dual advantage of retaining the heat where you want it [I'm in Minnesota] and reducing the chances of seeing your roof all over your neighbor's lawn.
Hmm .... one conern is the structural integrity of the roof not having the OSB in direct contact w/ the joists. That may be easily solved w/ metal cross strapping, though.
I wouldn't use XPS ... actually you suggest using EPS (extruded polystyrene). I would use foil faced polyiso for this. Higher R-value, reasonable price. You may have fire code issues w/ the EPS foam boards, too.
XPS is eXtruded and EPS is Expanded. XPS is the right stuff, but I agree he's better off using polyiso on the roof.
Billy
I don't.Here is my reasoning - roof areas are subject to moisture, whether during construction, or from roof leaks as the roof wears out in the future, or from moisture rising and condensing.Polyiso is slightly better insulation when dry, but it is subject to damage from moisture which it will absorb.In the type application being presented here, it is also less able to withstand compressive forces.
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I didn't weigh in on where to place the insulation. I think sheathing should go on the rafters followed by the insulation board, followed by the final sheathing and roofing material (in spite of OP's desire to skip the sheathing attached directly to the rafters).
It's possible for water to get in the polyiso if the foil face is punctured (which it will be) or the seams aren't taped correctly. In that sense the XPS is safer around leaks. I think the OP is in trouble if he has a leak problem with this roof, whether it is XPS or polyiso.
Billy
agreed on all that, esp position of the foam, regardless which kind he uses.
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Wow, lotta info there.
few comments here...
The foam on rafters means you lose a lot of shear value in the assembly. Wind more likely to rack it and cause cracks in teh ceiling finish etc. Normally when doing foam in cathedral roof assemblies, we do it interior side, then strapping, then the sheetrock.
When on top, we sheathe first, then the foam.
I don't know what you think you would need vent space above the foam since it is a VB when installed correctly.
Since they are requiring can lights in a cathedral ceiling, my answer would be that the ONLY way I would be able to do that and give them a proper insulation and ventilation package is with sprayed in place polyurethene closed cell foam like Corbond. Might seem expensive but saves a lot compared to the labour of what you are proposing with all the dormers and valleys
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I just worked on a roof that was partly done with a similar detail. We strapped the rafters with 2X4s (flat) and filled in between with sheets of 1.5" extruded polystyrene. We then put 2X4 rafters (on edge) on top of the strapping and then sheathed. The extruded is rated for about 30 psi, IIRC, so how far apart the strapping would need to be is an interesting question.
We just did the areas near the eaves with the extruded because it was a retrofit and we only had space limitations there. I think a similar detail would work on an entire roof.
Seems I frequently get it backwards ... and just when I think I've figured it out.