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Insulation and air sealing around exposed rafters and beams

Our new build project with have exposed 4x8" Douglas Fir Beams cantilevering over the front and rear elevations of the property with a low-slope "warm" shed style roof. I've previously posted on the roof assembly and comfortable with our approach there.

I know that cantilevered beams penetrating the assembly is far from ideal, but it is what is so I am trying to come up with the best solution with some input from your kind selves. I can't seem to find anything in the articles so I've purely come up with the below based on a best guess.

The assembly and my initial thoughts for build up are as follows from inside out:
1) Internal 2X8 Douglas Fir blocking between the 4x8 beams sitting 1" or so proud of the drywall. Drywall would go up to the level of the underside of the 4x8 beams.
2) 4" Fiberglass/Rockwool compressed in by the above blocking.
3) 3" rigid foam. Leave a gap around the foam to be filled with expanding foam to the 4x8 beams on the sides, the wall plate on the bottom, and the 2x6 D Fir T&G decking on the top.
4) Outside face of the foam can be taped with high quality tape to ensure the air seal to the adjacent beams and decking (or do we do this on internal side or both?).
The bottom of the rigid foam would be integrated with the house wrap or Zip panel via tape. My understanding is that the foil faced products are much easier to tape effectively or is that not necessary?
5) External 2X8 Douglas Fir blocking sitting 1" or proud of underlying Cedar and metal cladding.

On the 'sides' of the building we'll have a 4x8 D Fir beam running along the wall plate and thus it would be exposed internally and externally. We can seal the bottom and top of the beam during installation to provide an air seal, but I'm not sure what to do about the thermal bridge of just having 4" of timber across the building envelope. Your thoughts would be appreciated!

FYI - wall assembly is Membrain, 2x6 studs, fiberglass cavity fill, ply sheathing and Tyvek OR 7/16 ZIP System, 2" Roxol Comforboard, Strapping, Cedar and Corrugated metal siding.

I hope my explanation is clear enough for your input on the matter.
Thanks in advance.

Asked by Quinn Sievewright
Posted Apr 11, 2017 3:50 AM ET
Edited Apr 11, 2017 5:19 AM ET


2 Answers

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Your approach is probably as good an approach as you're going to get.

One of the problems with this type of penetrating beam is that large pieces of timber tend to shrink, check, and twist more than 2-bys. Even if you get a good air seal at the time of construction, the shrinkage, checking, and twisting can come back to bite you a year or two later.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 11, 2017 4:55 AM ET


Thanks Martin.

Answered by Quinn Sievewright
Posted Apr 12, 2017 5:15 PM ET

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