Plastic free timber frame addition
My Partner and I have a 3 story brick house in Philadelphia (4A) and we’re adding a room above the single story kitchen. Demo is all done and we’re just waiting for permit amendment approval. She has a green architectural coating business (linseed oil paint and pine tar) and we’re both obsessed with greener building materials. Our goal is to avoid the use of plastics or petroleum based products when possible. We don’t think these materials are appropriate for a building we want to last 100+ years.
The addition is going to be Timber framed out of big green red oak beams. with tongue and groove sheathing, Pine coated in linseed oil on the sides and Red oak T&G on the roof. this will also be visible in the ceiling. The roof will have exterior rockwool insulation and the walls with have 2″ of continuous exterior rockwool board and 3.5 of R-15 batts between the studs. We’re doing a standing seam metal roof, its is fairly low slope 12/2. We will have 2″ of vent space between the metal and insulation. We think we’ll have a Layer of Ice and water between the metal stand seam and the sheathing it attaches to but we’re not sure where and if we should put a layer of something else like wax paper directly on the Oak Tongue and Groove to help with air tightness.
attached is a cross photo section of the assembly that we’re thinking of doing. Also included a photo of the floor of the addition timber frame room that we’re so excited about! So is there anything wrong with this or are there better green products we could use? Are we being foolish?
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Replies
Lindsay,
- The place for I&WS, or other self-adhered membrane, is over the T&G ceiling as the primary air-barrier. This needs to be connected to whatever layer you are using as your air-barrier on the walls. It’s very important you identify where these are on both assemblies, and figure out how that continuity is maintained where the T&G rests on, or continues past, the exterior walls. Below the roof panels you can use any sheet underlayment rated for metal roofing.
- The 2” mineral wool layer doesn’t not have sufficient compressive strength to be used continuously between the rafters and strapping above.
- The 2”x8” rafters running perpendicular to the roof slope will need periodic blocking to stop them overturning..
We'd be using comfort board 80 for the 2" with Rothoblaas DRS Timber to Timber Spacer Screws through the strapping. Does that not have the compressive strength? it looks like its used that way on Rockwool's site in drawings.
Lindsay,
It may. Could you provide a link to the detail?
What advantage are you looking for by using T+G lumber rather than plywood as your sheathing?
https://www.rockwool.com/north-america/products-and-applications/roof-insulation/pitched-roof-insulation/
Lindsay,
The Rothoblaas screws may help, but the two sections in your link are not analogous to your situation. On both of them the strapping is continuously supported. On yours the loads from the roof above are transferred by the strapping though the rockwool to the rafters below as point-loads. It's very similar to why Zip-R is not rated for use on roofs.
We like t&g because it will be visible as the finished ceiling in the addition. Also it allows us to avoid the adhesives in ply or osb.
Lindsay,
I agree with you entirely about using T&G for the interior ceiling. Where I'd suggest thinking about plywood is for your roof sheathing.
I don't see waxed paper being a practical air barrier here. I see a few problems: 1- it will probably be easy to tear or otherwise damage during installation, just like kraft paper is on insulation batts, 2- how are you going to seal between sheets? Tape isn't going to adhere well. I'd be concerned with waxed paper holding up over time too.
In this case, I would stick with traditional polyethylene sheet. If your concern is longevity, what is the problem with a plastic material? Usually people trying to avoid using plastics are concerned that they don't degrade over time, and you seem to want the materials you use to last a long time? There are places where modern materials usually are the best choice.
You could potentially install a layer of drywall under the T+G and use that as your air barrier, but it would be a lot of extra cost and work to do. Thin panel products would work too, perhaps 1/8" or 1/4" masonite with taped seams. You wouldn't need a thick material, just something that could act as an air barrier and that would be durable enough to avoid being damaged during construction. Another possibility would be 1/2" polyiso with taped seams, but that is likely to have a higher materials cost than drywall, although it would require less labor to install.
Bill
Thanks for your reply. Masonite is an interesting option. We do not want to use any newer poly or plastic things. All plastic degrades over time and ends up in a landfill eventually. The manufacturers of all the stuff only back it for 30 years. Plus in the event of a fire the chemicals created when the stuff burns is super toxic.
What will the R value of your ceiling be? Seems like code is at least R38 that puts you at what 10 or 12 inches of rockwool?
You do understand the green beams height and width will shrink about 15% or so and twisting is likely and they are certain to develop rather large cracks.
For the air barrier over the T&G you could cover it with exterior gypsum and tape and mud the seams.
Walta
2” of comfortboard r-8.4 + 7.25” of batts at r-30 =38.4. We expect to get cracks in the beams and shrinkage
Walta,
The complication is that the rafters above bear on and are fastened to the structure below, so something more robust that gypsum is necessary.
You don't need the layer of rigid mineral wool, it just makes your build harder. Bump up the rafters to 2x12 or TJIs and thicker batts. If the batts are less than the TJI height, the gap can form the vent space, if not you can strap out the TJI to add extra height.
If you want better, install first 2x6 purlins with batts, over that run 2x8 rafters with 2x6 batts. This is about just as efficient as continuous insulation without any monkeying with long screws. You can extend only the rafters to form the overhangs like a regular roof.
As for the air barrier bellow, any solid good works. I would go for CDX/OSB/ZIP with taped seams.
Roofers like to install I&W over everything but it really doesn't buy you all that much and very hard to remove if you ever need to get in there. Unless your metal roof is low pitch, I would skip it.
T&G can work for the roof deck under the metal but I think regular OSB/CDX would be simpler and cheaper install. T&G makes sense for any overhangs if you want to keep the underside exposed.
Akos,
That's a much better assembly.
I've brought this up with you in the past - this habit of suggesting more practical solutions rather than just answering the question. It really has to stop :)