Roof underlayment for vented roof with standing-seam metal roofing
Im nearing the end of construction of a high efficiency timber frame hybrid house I’m building for my self and have a low slope roof question that I would like some opinions on. Roof pitch is 1.5/12 (7 degrees)
Our main roof structure consists of timber rafters, 20″ I-joists, dense pack cellulose, 1/2″ plywood sheathing taped to the walls as air barrier. We drilled holes all over the roof to dense pack the cellulose from above (as our interior finish was installed on top of the timber beams as soon as they were installed.
We sealed the holes by installing Blueskin VP100 across the entire roof. Now to the tricky part. Blueskin VP100 is not rated as low slope underlayment. I figured we would either negate needing a low slope underlay with our vented over approach or install a proper underlayment when building the upper roof assembly.
So our shell is air tight with no overhangs, VP100 on the roof (lapped over #15 felt on walls). Next we install 4×6 timbers @5′ O/C across the entire roof. Then 2×4 “periins” at 2′ O/C above, then 7/16″ OSB on there.
So in conclusion, the roof material will be installed on the OSB, which is a total of 9.5″ above the VP100, with 6″ of continuous vent space between my secondary timber beams.
Roof material will be a New Tech SS150 standing seam profile (1.5″ tall) , single fold. The roofing company said they would prefer to install a Titanium PSU30 underlay. It appears to be self adhered, but only rated so 2/12 slope according to manufacturer website. Installer wants $1300 for this underlay on my 1500 square foot roof.
My question is, with our vented over approach, will the VP100 suffice as underlay? How much water will actually get through the standing seam roof with the fact that its vented above 20″ DP cellulose? I can’t see any heat loss occurring, especially enough to cause melt and ice buildup that would compromise the roofing. And if any water gets through occasionally, will the VP100 work any better since there is 6″ of vent space above and no capillary action keeping the water pushed against the underlay?
Perhaps its an issue of capillary action with the low slope?
Any advice would be appreciated.
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