Sto-Emerald Coat for SIP roof? (below battens & standing seam)
In the near future I am going to be building a SIP over timber home and have been doing a LOT of research lately on best practices for sealing/wrapping/coating SIPs. I have a construction company that does most aspects of remodeling and building, but we specialize in roofing. However, the use of SIPS in my area accounts for about .05% of new homes (which there are not many of), so I have little exposure to them.
I know that a cold roof situation with SIPs seems to be the best method, and that you do not want to cover the roof panels with an impermeable layer (like an ice & water membrane). There are many options for roof underlayments, but I am looking for something that I can possibly do that will last 3-4 months in case the weather turns and I cannot get around to installing the roof. The other idea, is trying to reduce air-leakage through the roof panels (same as the walls, of course). Regular underlayments are not meant to be left exposed and are not considered a 100% leak-proof barrier for driving rains. So, the Sto-Emerald coat (which I plan on using for the walls) seems like it should work for the roof panels as well. It will allow internal moisture to still vent out, but keep the rain/snow off it until the roof is ready to go on.
My question is: Is this LWRB good enough to be used on a roof? Or are there reasons why it should not be used? Any thoughts, insights or previous experience with this is greatly appreciated.
– Nick
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Replies
Nick,
I don't recommend that you use a liquid-applied WRB like Sto Emerald Coat on a roof unless the manufacturer recommends it for that purpose. (As far as I know, Sto doesn't.)
Building codes require the installation of roofing underlayment, so that's what you should install. (That way, the manufacturer of the roofing underlayment will supply a warranty.)
I know of at least two brands of synthetic roofing underlayment that are (a) vapor-permeable, and (b) rated for 6 months of exposure to the weather. The two brands are PermaR PermaFelt and GAF Deck-Armor.
For more information on this topic, see Synthetic Roofing Underlayments.
-- Martin Holladay
Martin,
Thank you for the reply. I have used both products in the past with asphalt shingles. I guess one of the main goals I'm aiming for is to have an air barrier on the roof, but with some permeability. Would taping all of the seams and fasteners on the Deck-Armor be worthwhile (akin to taping Tyvek seams on walls) ?
-Nick
Nick,
As you probably know, it's essential that you tape your SIP seams on the interior.
If you also want to tape your SIP seams on the exterior, use a vapor-permeable European tape like Tescon Vana. Here is a link: Tescon Vana tape.
-- Martin Holladay
I think that once you tape the top (say Siga Wigluv 100) and bottom SIP seams, there would be little more to gain by taping the underlayment. At least once the roof is complete.
The SIP seams are all going to be taped inside and out, regardless of what goes on. From what I understand though, OSB sheathing is not an airtight layer, and coating the walls with the Sto product will create a permeable, but air-sealed layer. This will help reduce the ACH rating, so why stop at the walls if the roof is the same material? Or is a SIP not susceptible to air infiltration the way a traditional framed house is with OSB sheathing?
I am just trying to max-out all of my options for air sealing while keeping an outward breath-ability. If doing a Tyvek layer with taped seams is worth it for walls, or doing a Prosoco or Sto LWRB, then why stop at the walls?
Thank you,
-Nick
Nick,
OK, so you've got interior SIP tape and exterior SIP tape. You've also installed canned spray foam along the SIP seams, because that's what most SIP manufacturers require.
And you've installed roofing underlayment (taped or untaped -- your choice) because the roofing underlayment is required by code.
You also want to spread some expensive pookie (as Joe Lstiburek calls gooey substances) on the exterior side of the SIP? Be my guest. I think it's unnecessary, though -- especially if you include a vented air channel between your roofing underlayment and your roofing.
-- Martin Holladay
So you feel that the SIPs, when all sealed with tape, are enough of an airtight barrier? I guess I was under the impression that they needed more sealing to get the most benefit from them. I know it would already be much better than an old fashioned stick-built home that just meets code, but nobody visiting GBA wants to do the minimum. I also realize there is a point of diminishing returns with most anything.
So maybe I should just wrap the exterior the "normal" way and put the savings toward the triple-glazed windows?
-Nick
Nick,
If you seal the seams with canned spray foam, and if you tape the interior of the seams religiously (including the seams that fall on timbers -- a step which requires advanced planning and head scratching) with high-quality tape, and if you tape the exterior of the seams with vapor-permeable tape from Europe, and if you seal all penetrations well, you'll get an envelope with a very low rate of air leakage.
Air leakage through OSB is irrelevant, because the OSB is glued to rigid foam. The OSB/rigid foam/OSB sandwich is an air barrier, so all you have to worry about are seams and penetrations.
Of course, you need to verify your work with a blower door.
If you include a ventilated rainscreen gap on your walls, and a ventilation channel under your roofing that is at least 1.5 inch high, you will avoid the SIP disasters that you read about on GBA.
-- Martin Holladay
Martin,
I appreciate your reassurance that keeping it simple and using the exterior ventilation will be adequate for making the SIP envelope last for decades. Blower door testing will certainly be done because I am curious, but also due to the fact that it is now code required for completion. My uncle built a home next door to where I will be building and used ICF's for the basement and stick-framed the upstairs. He spray-foamed the walls and roof and was very particular. He was able to get a .6 ACH, so now I have to try to beat his number of course!
-Nick