Instead of SIPS hipped roof, can I do it with traditional bulk insulation and still achieve airtightness?
Last Post 17 Jan 2020 01:48 PM by cmkavala. 5 Replies.
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Dee_WadkarUser is Offline
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18 Sep 2019 11:47 PM
Instead of SIPS hipped roof, can I do it with traditional bulk insulation and still achieve airtightness? P.S. I am going with SIPS walls for sure.
newbostonconstUser is Offline
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19 Sep 2019 11:22 AM
On an ICF house with traditional roof, I DIY spray foamed along each truss and around all electrical/light boxes from the top side.

Then used a $200 inferred camera to look for leakage....Then I drilled small holes through the drywall before they mudded it and pumped in more foam till everything looked good.

Got .79 ACH on a 7000 sqft house. It wasn't that hard with the thermal camera; just a couple hours a day for 3 days and 2 days to spray foam the ceiling.
"Never argue with an idiot. They will only bring you down to their level and beat you with experience." George Carlins
DilettanteUser is Offline
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22 Sep 2019 03:32 PM
Another possibility is to look into something like Zip system roof sheathing. Then either tape or liquid flash (or both).
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24 Sep 2019 08:31 PM
Posted By Dee_Wadkar on 18 Sep 2019 11:47 PM
Instead of SIPS hipped roof, can I do it with traditional bulk insulation and still achieve airtightness? P.S. I am going with SIPS walls for sure.


Using a fully adhered membrane over standard roof sheathing is as-tight or even tighter than a SIP roof. SIPs are at high risk of leakage at the seams, particularly at the ridge seam which is prone to movement and flexing from the wind.

It's impossible to adequately vent a hipped roof when the insulation is at the roof deck layer. A raftered roof with fiber insulation between the rafters snugged up to the roof deck, and an adequate amount of insulation above the roof deck for dew point control (which varies by location/climate) is cheaper and a heluva-lot greener than doing it with all foam insulation (as with an EPS core, or even works a polyurethane core SIP.) Depending on climate zone it may even be reasonable to do the above-deck insulation cost effectively with rock wool, but in most cases rigid foam board under a 5/8" plywood/OSB nailer, or nailbase panel (sort of a half-SIP with wood cladding on one side only) would be less expensive than 4-8lbs rock wool.

Foam insulation is really not very green, and while there can still be some rationale for using it, the minimum amount necessary for dew point control on some other insulation layer in the assembly is being nicer to the planet.

There is one SIP vendor (AgriBoard: http://www.agriboard.com/ ) using high density straw core for insulation, but the fattest ~8" thick product is only good for R18 or so in an ASTM C518 test. But due to the high density and material type it has a substantial amount of thermal mass which translates into a very high thermal diffusivity, which lowers both the peak and average heat flow through the SIP and would substantially outperform an R18 EPS core SIP. The thermal diffusivity of high density straw is comparable to that of fiberboard insulation such as Gutex or SonoClimat ECO4:

https://gutex.de/en/product-range/product-properties/insulation-in-summer/

For a roof assembly using AgriBoard supported by rafters with fiber insulation in the rafter bays could still meet code anywhere in US climate zone 6 or lower on a U-factor basis and still have adequate dew point control on the conditioned space side of the SIP. But like all SIP roofs it's critical to getting religiously zealous about air sealing details.

The R18 AgriBoard SIPs would also meet code-min for walls on a U-factor basis in zone 5 or lower with no other insulation layers. High density straw core SIPs have a very low embodied energy or other environmental impacts relative to EPS SIPs, and becomes sequestered carbon, with a NEGATIVE, carbon footprint.

http://www.agriboard.com/carbon/Embodied%20Energy-AgriBd.pdf

As with insulated concrete form (ICF) vendors there's a fair amount of marketing hype about it's' "dynamic R value" due to the substantial diffusivity, but it's a solid R18 just for the SIP, and with both interior and exterior finish cladding it would have no problems ducking under U0.060 to meet IRC 2018 for zones 5 & lower. In locations with high diurnal temperature swings it could approach R22 ICF performance.

Some criticism of the marketing claims lives here (and yes, that WAS more than 20 years ago!):

https://www.buildinggreen.com/news-analysis/agriboards-inflated-r-value-claims
InnovaUser is Offline
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14 Jan 2020 10:10 PM
We are an MGO SIP manufacture and we fabricate many projects for contractors that are using MGO SIPS to replace masonry block. The MGO SIPS are used with standard wood trusses. Spray foam the underside of the sheathing down to the wall. You should be in 1.5 - 2" ACH blower door test range when complete.
cmkavalaUser is Offline
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17 Jan 2020 01:48 PM
Posted By Dee_Wadkar on 18 Sep 2019 11:47 PM
Instead of SIPS hipped roof, can I do it with traditional bulk insulation and still achieve airtightness? P.S. I am going with SIPS walls for sure.



No never as tight as a SIP roof
Chris Kavala
[email protected]
1-877-321-SIPS
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