Hi Folks,

First time poster and I'm admittedly rather ignorant and inexperienced with matters relating to home construction, structural design, and SIPs (so I apologize in advance if this post reflect that).

I've read exhaustively online and at the local library regarding roof construction, specifically with SIPs.

My question is this...

If SIP roofs built with OSB SIPs are extremely sensitive to moisture (introduced by condensation or air penetration), and we are assuming that the homeowner uses mechanical air ventilation to supply fresh air and also lower internal humidity...

Why not install a quality air barrier on the internal side of the SIP - with a cold roof on the external side?

My suggestion is predicated on the following assumptions:
1. SIP panels are installed on site and involve joints - which may invariably leak small amounts of air due to age, poor execution, weather.
2. During certain times of the year, the interior OSB will likely be the surface where condensation will occur (even with mechanical air ventilation).
3. In the event that the condensation occurs on the air barrier - no harm is caused to the OSB, and the condensate may be removed by the ventilation with time.

Thoughts?

If the air barrier is on the interior side of an air-conditioned building in hot-humid climate the interior side OSB can still rot- it depends on the vapor permeance of the material used, and the rate at which the condensation occurs.  The ONLY way the interior side air barrier would have condensation is when it's temperature is below the dew point of the exterior air, so that it strictly a hot-humid climate scenario.

If it's a high-perm flexible material like housewrap that can work- sorta, but in a cold climate the high-permeance may allow frost/ice to form in the joint at the outer skin, separating it further &/or rotting the outer skin, but air-tight is still better than leaking-air.

But it's, damn-near impossible to maintain perfect air barrier at a roof joint to last for 100 years. It's always better to build it resiliant to minor air leaks when possible.  It's more expensive, but the best overall solution I've seen to date is to add a nailer deck above the SIP with segmented 2x furring to allow cross-ventilation of the cavity which protects the SIP from roof leaks, and dilutes/vents away wintertime moisture drives from the interior. That still leaves some modest risk from minor leaks in the hot-humid/air-conditioned problem, but it's a much-reduced risk overall.

Or, you can build the roof with steel SIPs, which don't rot when exposed to moisture.

Thank you so much for the response!

The silence was deafening in here.

The noise level often varies, but stick around- there are some pretty smart practitioners with good tips on this site.

Thanks.

Have practitioners experimented/recommended the use of structural expansion joints with 50%-100% movement instead the common practice of using splines + sealant (or expanding foam) + tape?

Posted By AC-Bayard on 07 Apr 2012 11:21 AM
Thanks.

Have practitioners experimented/recommended the use of structural expansion joints with 50%-100% movement instead the common practice of using splines + sealant (or expanding foam) + tape?

Instead of experimenting, try going with a sure thing

With all due respect - if I limited my consideration to a "sure thing" - I would stay away from SIP and considering an air tight house.

If the technique can be improved with relatively low cost measures imported from non-residential applications - why not consider it?

Posted By Dana1 on 29 Mar 2012 06:05 PM .

Or, you can build the roof with steel SIPs, which don't rot when exposed to moisture.

The sure thing is to choose something that does not rot

A mere detail, Chris, just a detail ... :-)

Vented nailer decks are pretty much sure thing too. Tape adhesives fail, spray foam can shrink & crack. Acoustic sealants are messy to apply, but seem to hang in there over time. If building anything with OSB, give that OSB a drying path. (Not that there's anything wrong with steel.) No matter what the material, air-sealing will be an important consideration for whole-structure thermal performance.

Dana1; some of the newer adhesive tapes will last a lifetime, especially on metal.

I'm still a skeptic on the real definition of "lifetime" for these products, but I'm sure they have improved a lot over past 30 years. When possible, designing for 100+ year lifecycle on critical elements seems prudent, but I'm doubtful that either of us are going to stick around long enough to see if the noo-improoved tapes last that long.

I'm not 100% convinced that the EPS cores of SIPs will make it that long with no long term issues related to shrinkage, either, but so be it. They'll likely make it to the remainder of MY lifetime, so maybe that's good enough? :-)

Dana1; I have some accelerated aging tests done on metal SIPs according to the rep at the time, they estimated the life expectancy to be 300 years

That's pretty good! How much do they give the tape?

Posted By Dana1 on 10 Apr 2012 05:17 PM
That's pretty good! How much do they give the tape?

Dana1, tape wasn't part of the test, however they did do a simulaneous OSB SIP test, if you would like to see it drop me an email and I will send it to you

I reviewed the materials provided by Structall Building Systems, and while the Snap n Lock fastening looks promising (eliminating foam altogether), and steel insulated panels look quite appropriate for roof paneling in particular, whether using steel or OSB panels, I still have concerns regarding conventional wisdom of sealant/foam + tape found in SIP literature.

Some foreign literature in Scandinavian seems to discuss a lot re the use of structural gaskets for sill plates, wall plates, roof/wall panels. P-gaskets also look promising. It appears that while structural gaskets (capable of handling 2 by 6 plates) and P-gaskets (1 inch) are relatively expensive (70-90 cents per feet), they do seem allow one to account for the expected movement of panels (whether caused by expansion, settling, environmental loads, what have you).

Wood roofs require "breathing" or venting. The reason is that even when detailed correctly, the possibility of moisture being introduced into the wood is there. If the wood cannot be given a chance to dry out, it will rot, mold and eventually structurally fail.

The same goes for wood framed walls. If not detailed correctly and moisture gets in, it will cause the wood to rot unless it can be dried out.

OSB wood roof SIPs have quite their share of rot problems. This is almost always in the area where the 2 panels meet.  OSB SIP ROT STUDY

For me my choice would be a steel roof SIP or a vented wood attic. No OSB roof SIPS for me, no way.

Using a study of the risk in a foggy-dew semi-cold Juneau AK climate is a bit of cherry picking on data, and doesn't reflect the level of risk that somebody in AZ or TX might experience with OSB clad SIPs. The drying season in Juneau is short, the condensing-season long, and the exterior moisture drives EXCEPTIONALLY high- it's practically a rain forest, where the outdoor dew point is only rarely more than 5F away from the outdoor air temp. Most places in the lower 48 have far more favorable conditions.

But even there using a vented nailer deck above the SIP skin would protect it. Whether that's ultimately cheaper than a steel SIP roof just depends, but it's a good solution for almost any insulated wood clad roof.