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Community and Q&A

Insulating a Shed Roof

hamiltonworks | Posted in GBA Pro Help on

Hello GBA folks,

I am going to apologize ahead of time as there are probably more articles, questions, and answers to this type of question than any other topic on here. However, after studying several of them, reading Martin’s book, and being a member for years, I’m still not confident in my solution. Here goes:

I’m installing a detached, prefabbed, 200 sqft shed/studio behind my workshop for material storage, work overflow, my office, and a finishing room. I will also have my veneer press in here for large assemblies. That being said, this building will need to be a “mildly conditioned” space via a window AC unit in summer, space heater in winter, mostly so that woodworking adhesives and finishes can cure properly. Here’s the context:

– Climate zone 5b 
– Walls: 2×4’s, 16″ O.C. Tyvek WRB, LP smart side sheathing/siding (basically T111). Four 3’x3′ dual pane windows, and a double door.
-Roof: 2/12 monoslope pitch “shed roof”. Rafters are 2×6’s, 24″ O.C. There is rafter blocking between every rafter at the top plates. (I have never seen a roof, rafters or truss, without full height blocking between them at the top plates here in the west, perhaps its for seismic reasons? Most drawings in the detail library on GBA don’t have this blocking between the rafters/truss, not sure why?) Continuing on, roof sheathing, felt paper, and asphalt shingles. Overhangs are 2′ all the way around and with soffits. 

-My plan is to insulate the walls with formaldehyde free batts and drywall over them. Keeping in mind airtightness. 

-I’m hung up on insulating the roof… and spray foam is not an option. I’m going for low-impact, local materials & DIY style. My thought is I will be basically doing a vented insulated cathedral roof, only it may not quite qualify for “cathedral” I suppose. Most articles indicate that a 2/12 pitch is basically nearing a “flat roof” scenario and traditional venting from the eaves to the ridge becomes ever more difficult with the lower slope. Code indicates min. 1″ venting below roof sheathing, but most builders on here seem to be more inclined to go with 1.5″-2″ vent channels.

Q. Since the low slope is working against me on the airflow, shall I go more?

Q. I can fur out the rafters, with the various methods proposed in other articles such as ply gussets and additional 2x members deepening my rafter cavity, or scabbing on more material etc, but do I really need to try to achieve an R-49 roof in this building? I would lose a fair amount of head height, which is useful storage for me. I’m ok with being a bit cold and hot.
I am open to using some rigid foam board if necessary. Perhaps I use a hybrid approach- combining fluffy and rigid, but in what capacities/thicknesses and in what orrientation? 

– My main goals are to provide some kind of thermal barrier and not rotting out my roof. Air sealing the interior side of the roof seems absolutely essential, so I will aim to make sure my vent channels are sealed well and do an airtight drywall approach, to mitigate air migrating into the roof assembly. 

Q. Venting the roof- since I have full height blocking between the rafters at the top plates, I guess I will need to drill adequately sized holes through this blocking in every bay at the eave and ridge. The holes will have to be in the upper portion (perhaps notches even) of the blocking so that they are in line with my vent channel. 
Any suggestions on what size holes, how many, and what size/depth my vent channel should be? Shall I go with a deeper vent channel because of the low slope? (I’m sure someone will kindly suggest the 1/150 rule.) I was thinking of 1/4″ Luan panels nailed to my spacers for the channel. And sealing them well. 
For the soffit and ridge vents, I was thinking of using a continuous lineal soffit vent??

-To summarize, what are some good options and details for insulating and venting this type of roof (without the use of spray foam), providing some thermal comfort, and not rotting it out? R-49 seems redundant given the type of building it is and purpose it serves…? Is that unreasonable? Anybody anybody?

Thank you so much to everyone! 
Cheers 🙂

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