Insulation and Decking Requirements over a timber-framed roof
Hello everyone,
I am a new member and want to thank you all for the great advice and insight you provide on this site. I am learning a lot! I am hoping I can get some recommendations on insulation, sheathing, and venting atop a timber frame roof. The structure will be in central Maine (climate zone 6A), and is a 24 x 36 frame with a 12/12 pitch with common rafter-timbers 3 feet on center. The interior will be a cathedral ceiling throughout the house so no attic or insulation below the sheathing is desirable. My tentative plan (depending upon your input) is the following.
1.Nominal 2 x 6 pine decking will serve as the interior ceiling and the exterior roof decking.
2. A layer of Solitex mento over the decking.
3. 10-20% above code minimum requirement using recycled rigid foam with polyiso as the first layer(s) followed by EPS, as in the recommendation on this post from 2015 https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing
Above this layer, I would like to explore the idea that was mentioned in the above linked post, namely not using another layer of sheathing such as plywood, zip-sheathing, etc., and installing screwed-through metal roofing to purlins placed over the rigid foam to create a vented channel connecting soffit vents to a vented ridge. My questions about this approach which seems to make it different from other posts I have seen on this site, has to do with the 3′ rafter spacing. In order to attach screws through these layers and into structural members I can only place screws 3′ apart.
Question 1: Is this spacing sufficient for fastening for the purlins or should I place intervening screws that would bite into the 2 x 6 decking (but miss the rafters).
Questions 2: If the consensus is that omitting the top layer of sheathing is not ideal in this setup, does the following sound reasonable?
Zip-sheathing (not Zip-R) over the rigid foam and screwed down into the rafters, with purlins over this to allow a venting channel, followed by metal roofing.
A third approach which may be necessary for cost saving measures, would be installation of asphalt shingles rather than metal roofing. If I went this route, I was thinking that the venting channel would be under the sheathing.
If it helps at all, I am more money-limited than time-limited and will be doing all of the work myself, so incurring costs for materials but none for labor. Thank you all for your time and expertise,
-Eric
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Replies
Won't the purlins be an obstruction to an effective soffit-ridge channel?
Daniel
Hi Daniel,
Thanks, I do share that concern as well. The other option is to run strapping vertically from eave to peak but of course this still leaves the issue of the underlying structural members being 3' feet apart. In addition, the few metal roof installs that I have seen have been over horizontally placed purlins. I am not sure if this is a detail specified by the manufacturer or if it is just status quo. I am open to alternatives that may be better!
Eric,
With a 12/12 pitch you shouldn't get any snow accumulation, and I'd imagine the risks of ice dams are minimal. That's the only reason to vent the roof assembly you are proposing. I would use strapping, l but not provide intake or outlets for ventilation of that small cavity. Any incidental moisture can make its way out through the profiles of the roof panels.
If you are dead set on venting, you have two opti0ns. Run the strapping vertically and then either cross strap with a horizontal layer for roofing attachment, or put on a second layer of plywood.
You could double strap the roof with 2x4; 1st level vertical on top of rafters, second level horizontal. I’ve strapped lots of roof horizontally, using heavy duty log screws or GRK structural screws and, so far, none have blown off as far as I or my lawyer knows. Probably would not do it with 1x3 strapping. And yes, go into the rafters. The 2x will span the space easily.
Thanks Bob,
Given Malcolm's comment regarding not needing to provide an extra venting channel due to the roof pitch, it seems like your suggestion of using 2x 4 stock for structural reasons makes sense but I am thinking that one horizontal layer is sufficient for attaching the metal roofing down. It seems the vertical layer can be omitted as the extra venting may not be required.
Malcolm,
Thanks a lot of this and all the advice you provide on this site.
Cheers Eric,
Thinking about these things is the best way I know of filling in the gaps in my own knowledge.
Hi again Malcolm,
I have a follow-up question if you have the time. If I go with the proposed idea where the 2 x 6 tongue and groove decking would be the only sheathing material and not worry about venting as you suggest, would you recommend that I reconsider the use of the Solitex mento and instead use a peel and stick over the decking? Either way this would give me the 2 x 6 decking, followed by either the Solitex mento or a peel and stick membrane, then my greater than code requirement layers of taped rigid foam, strapping, then metal roofing. I just wasn't sure if a peel and stick would be better in this scenario. Thanks again,
Eric
Eric,
The first layer above your t&g is all about air sealing. So a peel and stick WRB is a good choice. You don't need a self-adhered membrane like grace I&W though as that deep in the roof assembly there is no bulk water to worry about. Don't forget you also need an underlay for the metal roofing, which wasn't in your latest description.
One thing that's always problematic with t&g roof decks that extend out to form overhangs is how to air-seal them at the exterior walls. The most effective method is to interrupt them, and re-start them on the outside. But if they are cantilevered and that isn't possible, they should be bedded in sealant and sealant applied to the t&g as they are installed.
Malcolm,
I do plan to interrupt the decking as you describe so that the ceiling/decking is entirely inside the conditioned envelope. Thanks again for your quick response.
Hello again Malcom,
Hopefully you will see this response to my older post. Do you think that the peel and stick membrane or other air sealing layer above my nominal 2 x 6 pine decking should be an impermeable layer (in regards to water vapor)? My original post had mentioned products like Solitex mento but after your response it seems a peel and stick may be better here. I have been researching options such as Henry's Blueskin VP100 and the Grace Vycor enV-S peel and sticks. These are vapor-permeable which I had been thinking was desirable. However, upon more thought this excess vapor would then be hitting the layers of taped polyiso and/or EPS, hence my follow-up question. What do you think?
Eric,
I agree, although I'm not sure a semi-permeable underlayment would cause any trouble.