looking for insulation/construction input on shop construction

[9Fingers]

Hi there, I'm getting ready to build my workshop. It is going on top of an existing concrete pad that is 24'6" wide by 32' 8" long. I live in Ontario, Canada so the winters here can get pretty cold.
I'm looking for some input or thoughts on the insulation and maybe even the construction.
I will be building 10' high walls out of 2x6. I'll put 1" foam board on the outside of the studs to give me a thermal break and then spray 2" of foam on the inside of the studs, drywall and call'er done.
I had planned on using scissor trusses for the roof. I'll put plywood on the trusses, synthetic underlay and then barn metal on top of that. I was planning on putting batt insulation in the ceiling and venting the space (about 18 - 24") above the batts with a ridge vent but after speaking to a representative from an insulation company (let's say they are Holmes approved), I was told that I can just spray the underside of the plywood roof decking with 3" and not worry about venting.
I'm surprised to hear that I only need 2" on the wall and 3" on the ceiling given that spray foam generally provides between 5-7 R value per inch thickness.
Is anyone familiar with this type of construction, no venting that is? There will be a space between the drywall and the foam of about 18-24". I plan on heating the shop with a radiant tube heater about 1' down from the top of the ceiling.

Anyone done this for their shop or something similar?
Thanks for your time and input.

 

[KRKWW]

Yes, similar. I built a 9'x30'x40' building with 2x4s with metal on the outside. I spray foamed 3-4" in the walls and 4-5" on the roof with no venting almost 4 years ago. I use electric radiant heat panels on the ceiling to heat the shop.

For Canada's climate, I would think you may need more insulation than you've mentioned. I'm in Arkansas so summer heat is also suppressed by my construction.

 

[Brent Taylor]

I used to build with SIPs, they make a great building envelope. B

 

[rizzoa13]

Re the roof- I like having the airflow that the plastic stand offs give in unison with a ridge vent and vented soffit. I've removed enough roof sheathing on roofs that were not vented and had to replace due to dry rot and even charring. The heat can build up and destroy the sheathing.

Since you have the space I would install the standoffs, use vented soffit and have a ridge vent. Then spray foam right on top of the standoffs to whatwver r-value you decide. This gives you airflow across your sheathing and still gives you the insulation and air sealing of foam. May as well pack the rest of the space with unfazed fiberglass or cellulose while your at it for added insulation, no reason to leave the cavity unfilled.

 

[Holmz]

I have no practical experience, but I understand thermal physics...

1) Sips are good.
2) offset 2x4 or 2x6 that break a direct path from the outside to the inside is good from both a noise and thermal sense. So a 2x4 wall that is 2x6 thick or a 2x6 wall that is 2x8 thick.
3) in France and Australia, (and likely Canada) Danpalon is a thranlucent roofing/wall material that has a moderate R value. So you may get light and heat in during the day, that offsets losses at night. Obviously an 8" (200-mm) this ceiling with pink glass will be very high R, but maybe a 2" (50-mm) clear ceiling with heat input during the day is better overall? Somewhat it depends whether you have a big heat sink in the floor/slab, etc?... To store the heat.

At some point you need to understand solar input seasonally, and have a sharp pencil for the physics/math(s).

 

[antss]

Rizzoa - have you torn off any unvented roofs that had 5+" of spray foam attached to the underside of them ?

Foamed roof assemblies don't get vented.  Would you cut a slot in the lid of your cooler and on sides too ?  You're advocating the same thing. Besides, fingers is using a metal roof, so cooked shingles isn't going to enter into the discussion here. 

I'm not looking to kick you in the shin, spray foam insulation is just now getting some critical mass ( in the USA anyway) and the detail of how it works as part of the overall system are confusing to many.  Couple that with issue that it's not as simple as substituting material.  Using foam , and how it's applied is not as easy as substituting say blown in cellulose instead of fiberglass batts.  Or using recycled bluejean fibers instead of blown fiberglass. 

Spray foam isn't just plug and play for the builder.