Vapor barrier between ceiling and EPS?
Planning on building a timber-frame. SIPS on side walls. 1×4 or 1×6 T&G run vertically on top of purlins for ceiling. 2×6 or 2×8 stood on edge around perimeter of roof, like a box. ESP the same thickness as 2x material around perimeter screwed down with 1/4″ gaps at seams for foam. Seams are foamed and top of seam is taped. 2×4’s laid flat, run vertically from eave to ridge, 24″ centers screwed down thru foam. OSB or plywood sheathing on top of 2×4’s, one of the Grace underlayments next, then fiberglass shingles. Ventiiated soffet and ridge vent.
My question is: what would I need for vapor control between ESP and T&G ceiling? What are your thoughts on vapor getting into T&G joint and following it up to the ridge? Any concerns there or am I over thinking this?
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Replies
John,
There is no need for a vapor barrier under your EPS. If you plan to install 5.5 inches or 7.25 inches of EPS on top of your ceiling, your EPS is already a strong vapor retarder -- and you don't really need a vapor retarder in any case.
What you need under your EPS is an air barrier. Tongue-and-groove boards are not an air barrier. If exfiltrating air gets between the seams of your EPS, your roof sheathing can rot. This type of failure happened on dozens of SIP-roofed homes in Juneau, Alaska.
Your plan to seal the seams of the EPS with spray foam and tape is a good one, but it probably isn't enough. Rigid foam can shrink over time, and your sealing attempts may fail. So you need a bullet-proof air barrier under the EPS.
Under your EPS, you could install synthetic roofing underlayment with taped seams, or housewrap with taped seams, or one of the European membranes with taped seams. However, since roofers will be walking on this surface, I would install OSB or plywood between the tongue-and-groove boards and the EPS if I were you. Don't forget to tape the seams of the OSB or plywood.
If you are installing a thick layer of EPS, you want to install the foam in at least two layers, with staggered seams, to limit air leaks and thermal bridging at the seams.
Martin, thanks for your reply. I read about Juneau. Poor workmanship gave SIPS a bad name. I am located in Lancaster, Pa. a more temperate climate then Juneau. I have a copy of Joe Lstiburek book about building with SIPS. I cant get straight in my mind the difference between vapor barriers and air barriers. I have been looking at so many products online it is driving me nuts! Some products are described as being both an air and vapor barrier. I was thinking of using Grace Tri-Flex under the shingles. It has a permeability of .04. What excatly is this telling me? Would a higher or lower number be better? Unless you roll out poly plastic above the ceiling I cant imagine there will not be some air leaks down the road. That is why I have the mindset that you need a ventilated roof. I am planning on an airspace above the esp with ventilated soffit and ridge. My real concern is even with a proper air barrier on top of the t&g the air will get in the t&g, travel uphill to the end of the board at the ridge. The ends meet at the peak right above the ridge beam. If the esp at the peak is foamed and taped like it should be then what happens with this air that collects in this small void? I know of a local fellow who had a timberframe addition added to his house. It had the planked ceiling. Water started dripping from the ceiling. They ended up removing the roof and redoing it. I do not know all the details but it is on my list to find out. Thank you for your help. You have a great site with a wealth of information. I grew up in the era of leaky houses but I do understand the concerns of building a tight house.
John,
You wrote, "I know of a local fellow who had a timberframe addition added to his house. It had the planked ceiling. Water started dripping from the ceiling. They ended up removing the roof and redoing it."
You have described the classic symptoms (and the results) of air leaks through a tongue-and-groove ceiling. That's what you are trying to avoid.
Although you seem to believe that polyethylene will be the best air barrier, it won't. Plywood with taped seams or Zip sheathing with Zip tape is likely to be more airtight and more durable than polyethylene, which is fragile and easy to damage.
A permeance of 0.04 perm is quite low. That would be considered a vapor barrier. To get an idea of the permeance scale, see the examples in this article: Do I Need a Vapor Retarder?
You asked, "Would a higher or lower number be better?" The answer is, "It depends." Sometimes you want a membrane that allows a building assembly to dry out; that would be a vapor-permeable membrane. And sometimes you want a membrane that prevents moisture from entering a building assembly; that would be a vapor barrier.
For more information on the difference between an air barrier and a vapor retarder, see these articles:
Questions and Answers About Air Barriers
Vapor Retarders and Vapor Barriers
Forget Vapor Diffusion — Stop the Air Leaks!
Air Barrier or Vapor Barrier? - Building Science Podcast
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A request for comment, as much as a comment: I am currently building a (somewhat) similar roof on a timber frame. I had similar concerns re air sealing. I used Ice and Water Shield instead of 6 mil poly over my T&G boards, under my rigid insulation.. I ran it long and will lap it over the wall panels.. OK?
Michael,
Yes, that method will work. It is the traditional method used for PERSIST construction. (For more information on PERSIST, see Getting Insulation Out of Your Walls and Ceilings.)
Be careful about how far you extend the peel-and-stick membrane over the walls, however. You really don't want a vapor barrier on the exterior side of your walls. If you cover 6 inches at the top of your walls, no harm will be done. But don't extend the peel-and-stick much further than that.
Michael
Just courious how you are doing your roofing system. What did you do at the ridge where the ends of the t&g come together? Maybe I am wrong but I am thinking air carries moisture so if air travels up the along the t&g to the ridge it will be trapped. Maybe this is a case where the moisture will dry towards the interior. Like I said sometimes I think to much and this isn't even a concern.
John,
It's OK if interior water vapor is "trapped" on the interior side of your air barrier. After all, you don't want the water vapor to travel through your roof assembly. The whole purpose of your air barrier and your thermal barrier is to "trap" your warm, humid air indoors, and to keep that air entirely separate from the cold, dry air outdoors.
A timber framer here with decades of roofs behind him tapes two layers of foil faced foam and vents above.
I never ever would build this type of roof unvented. Spray Foam is the only type of unvented roof I approve of.
T&G ceilings act as if they are not even installed as to how much air passes through them. I have stood in an attic above such an install and felt a warm breeze hitting me coming from that type of cathedral ceiling install.
Also as to water dripping from an unvented buildup I have witnessed that too. Expensive to do the roof twice plus demo it and do all this while living in an expensive home. Ugly situation.
AJ,
Good advice. Tongue-and-groove ceilings are responsible for a great many roof failures, so a belt-and-suspenders approach makes sense.
John,
Nothing wrong with thinking things through. I didn't do anything special with my T&G at the ridge, just more Ice and Water over the top. That said I'm not worrying so much about R - value as I am about air sealing. So with my cathedral ceiling I used Ice and Water over the T&G, all the way up and over ridge. It's a small, simple building. I've run my wall panels down the outside of the timber frame and put a ledger board on my sill where the panels will rest, using gaskets and low expansion foam to seal to the sill, the timber frame and the T&G and ceiling panels. Lap the Ice and Water a little ways over the wall panels. Hopefully it will all workout and I'll end up with a snug little camp.
Michael
That is how I am doing my sill area also, except there will be a termite shield in there.. Did you use SIPS on the roof or did you use a different system? Right now I am thinking not to use SIPS because of the extra two layers of osb I feel I do not need. I am a firm believer of a ventilated roof so that is why Im planning on esp with 2/4's flat on top with sheathing and shingles or metal. It is one of the systems I saw in Tedd Benson's Timber Frame book.