Posted By DaveJJ on 18 Nov 2014 12:15 PM
One more Canuckistani question

Dana if understand your previous post about leaks....would it be fair to say that leaking would be equally detrimental to both a straight 6" ccSPF setup and the one you are proposing? Which I guess brings me back to the question about membranes and their use on an unvented roof.

Thanks

At 6" ccSPF becomes a "dries never" sort of thing in your climate, making it slightly less resilient than a 2" solution.

At 6" closed cell polyurethane also has a significant lifecycle greenhouse gas impact, since it's blown with HFC245fa, which has a global warming potential about 1000x  CO₂.  From  (Rigid XPS is somewhat worse, blown with HFC134a, at ~ 1400x CO2, but EPS & polyiso are both blown with pentane, at a comparatively modest 7x CO₂.)  For those reasons it's probably better to go with the minimum amount that actually works from a vapor retardency or dew-point control/condensing surface point of view.  

Rigid foam above the roof deck at a sufficient R for dew point control, and using standard latex paint as the interior vapor retarder makes for a more resilient assembly, but in cold climates at high-T that's a LOT of rigid foam.  At a total of R50 foam + fiber it takes R30 to R35 of exterior foam to get there in a cold Canadian climate, which is on the order of 8" of foam thickness using EPS, at which the R15 to R20 fiber layer is just the icing on the cake.  The 2" ccSPF  (R12) + fiber (almost any R)  + smart vapor retarder solution is both cheaper & easier to build.

Posted By Dana1 on 18 Nov 2014 12:21 PM

http://www.buildingscience.com/documents/bareports/ba-1001-moisture-safe-unvented-wood-roof-systems

While the article doesn't specifically mentioned "SIP" roofs, it is basically applicable to a polyurethane core SIP roof type of lay-up, correct?

With a 8" PU SIP roof with OSB skins, in a Zone4B climate, there is an almost zero chance of moisture build-up/mold/rot on the interior, as per the studies. As long as attention is paid to the SIP joints/penetrations there shouldn't be any moisture issues. Am I correct in my assertion?

What do you think of the following R-60 roof build-up for a Zone4B ?:

• 8" PU SIP with gasketed tongue and groove joints and all joints and ridge beams areas will be taped
• 3.5" Roxul Batts underneath the SIP and ceiling finished with 1/2" drywall
• Exterior finished roof will be standing seam metal with a Grace-type of peel & stick membrane underneath
• There will be NO roof penetrations (plumbing stack will vent out of side of wall from 2nd floor wall)

Posted By Lbear on 19 Nov 2014 01:52 AM

Posted By Dana1 on 18 Nov 2014 12:21 PM

http://www.buildingscience.com/documents/bareports/ba-1001-moisture-safe-unvented-wood-roof-systems

While the article doesn't specifically mentioned "SIP" roofs, it is basically applicable to a polyurethane core SIP roof type of lay-up, correct?

With a 8" PU SIP roof with OSB skins, in a Zone4B climate, there is an almost zero chance of moisture build-up/mold/rot on the interior, as per the studies. As long as attention is paid to the SIP joints/penetrations there shouldn't be any moisture issues. Am I correct in my assertion?

What do you think of the following R-60 roof build-up for a Zone4B ?:

• 8" PU SIP with gasketed tongue and groove joints and all joints and ridge beams areas will be taped
• 3.5" Roxul Batts underneath the SIP and ceiling finished with 1/2" drywall
• Exterior finished roof will be standing seam metal with a Grace-type of peel & stick membrane underneath
• There will be NO roof penetrations (plumbing stack will vent out of side of wall from 2nd floor wall)

A SIP differs from the simulated roof layup in that document in one important way: The SIP has an interior side OSB skin as the "first condensing surface", whereas with the fiber right up against the foam, the foam is the condensing surface.  Since the OSB is moisture-sucseptible and the closed cell foam is not, one still has to pay attention to the temperature of that interior-side skin much more closely than you would the surface of naked-foam.

But for climate zone 4B with about 75% of the total R outside of the interior OSB you have HUGE dew point margin, so there would be no need for vapor retarders on the interior side.

I'm not a fan of PU SIPs in general due to the high  global warming potential of the blowing agent (HFC245fa, about 1000x CO₂). In zone 4B you can get away with as little as 30% of the total R being outside the condensing surface, and you have more than 2x that.  An 8" EPS core SIP comes in at about R30, and Roxul makes R28 & R30 batts (designed for 2x8 framing) which would put you at ~50% of the R outside the condensing surface, which is also huge margin.  EPS is blown with pentane, with a GWP of about 7x CO₂, a much lower environmental hit.

The key issue to long-term integrity of the SIP roofs is absolute air tightness of the seams, particularly at the ridge.  Using a peel'n'stick membrane bridging the ridge keeps that side air tight, but you also need to do something similar on the interior side of the ridge seam- a shot of can-foam or caulk in the gap simply doesn't cut it. The other interior side seams can be painted with acrylic latex primer paint for a bonding surface and taped with housewrap tape (or taped on bare OSB then over-gooped with fiber reinforced duct-mastic) for long term air-tightness integrity.

Posted By Dana1 on 19 Nov 2014 12:19 PM


The key issue to long-term integrity of the SIP roofs is absolute air tightness of the seams, particularly at the ridge.  Using a peel'n'stick membrane bridging the ridge keeps that side air tight, but you also need to do something similar on the interior side of the ridge seam- a shot of can-foam or caulk in the gap simply doesn't cut it. The other interior side seams can be painted with acrylic latex primer paint for a bonding surface and taped with housewrap tape (or taped on bare OSB then over-gooped with fiber reinforced duct-mastic) for long term air-tightness integrity.

SIP TAPE PIC

Taping interior joints and beams is a must.

A T&G SIP panel joint connection is also helpful in preventing air leaks

people live in better houses, are not dependent on energy anymore and have power over their own energy. Energy became so politicized… who do you know who drives in a 1970s car? In housing stock, it goes a lot slower but you need to get them to 21st century also.