Insulation for Detached Storage Loft Garage in Extreme Variable Climate

[Asclepiadae]

Hey everyone, just looking into some advice here on insulating my garage. First off - specs!
The cross-section of the garage is attached (yes, the drawing shows interior insulation on the side walls - there isn't any at the moment). The cross-section shows only one vent - there are two, both on the same side (south) about 4ft in from either end. As you can see, the attic has knee walls and a ceiling joist in the trusses to create a storage area. The total footprint dimensions are 20ft by 22ft. It is fully detached, and will not be conditioned (no central heating or cooling), and there are vented soffits (standard/code here with typical construction).

I am in Edmonton, AB Canada so I believe that’s climate zone 3b (USDA) / 4a (Natural Resources Canada). Here we have a LOT of variability of temperature. Our mean daily min/max (1991-2020) range from 23.0°C (69.3°F) to -18.1°C (-0.6°F) but our extremes range from 35.0°C (95°F) to -48.3°C (-54.9°F). Sidenote: As I write this tonight, our measured air temperature is -31°C (-23°F). In the winter, our cars will bring in snow/ice after driving, and the engine block is of course warm, so there will be some humidity created. Additionally, in the winter I would like to use a small (5120 BTU) electric space heater to prevent fingers and toes from getting too cold.

My primary concern is keeping the attic storage space cool-ish during the summer months, and then a bonus would be keeping it any closer to zero than -20 in the dead of winter. When we have our summer days with >30°C the direct sun on the asphalt shingles raises the attic temperature to the point where LDPE items stored up there have deformed (I haven’t measured to be certain, but what I’ve read suggests the plastic will soften and deform around 50°C).

I’ve read on here (and other sites) that simply installing more passive vents won’t help with cooling (not sure if whirlybirds would help enough...) so I'm looking at insulating (and obviously vapour barrier). Somewhat confusingly, when I asked the garage builder for advice it was suggested NOT to fully insulate the garage unless I am going to install full space conditioning (gas furnace/radiators etc) for the winter. Here the overall consensus seems to be "insulate, but don't do a half-job" which makes sense to me!

My big concern is, with the vented soffits, I don't want to allow ice dams to form, but I also don't want issues with not having enough air movement that the moisture lingers in the winter with the snow/ice brought in on the vehicles.

The conditioned space would possibly be one zone - I've attached two concepts I've seen for this construction, not sure which would be best... or if for the radiant heat reduction, should I try and “shield” the inter-truss space on the underside of the roof with some kind of foil lining (possibly the "Double Bubble" foil sheets?) and then use the typical semi-rigid (e.g. Comfortbatt with R24 or something) on the exterior walls and under the "floor" joists.

Thanks for any advice you can offer!

 

[billconner]

Some quick thoughts.

Light colored shingles will do much more to keep it cooler in summer than all the soffit and ridge vents in the world.

I think the double bubble stuff is fraudulent.

I see no advantage to insulate knee walls and floor/ceiling near eaves. Less square feet. Work out what you'll do on rafters and run it from storage ceiling to eaves. Spray foam depth of rafters would be good without venting - so called hot roof - but rigid foam on rafters, leaving rafter space for vents will work. Just read of someone cutting strips of foam, installing them on either side of each rafter, and then rigid foam panels on them, flush with rafters. All with spray can foam adhesive. Foam should be covered or painted to make it flame retardant. Tape at joists and you have vapor barrier.

Agsin, lighter shingles will do more to keep it cool than all this venting.

You might consider an exhaust fan in one table on a thermostat for high summer heat. Needs to be sealed for winter.

I don't think you need to worry much about ice dams in an unheated building.

 

[Asclepiadae]

Light colored shingles will do much more to keep it cooler in summer than all the soffit and ridge vents in the world.

It's a brand new garage (~2 years) built in a new neighbourhood - the garage is matched with the house (which is ~5 years old), and I'm not interested in re-roofing the garage.
Also of some consideration, light shingles aren't as frequently used here (and home builders working on standard builds don't use them at all) so they would look very out of place. Not to say you aren't correct in terms of it being a strong solution, but it's just not one that's a practical option.

I think the double bubble stuff is fraudulent.

Well, that's a very vague answer! /s

Any particular data (or discussions) where I can read more on why Double Bubble / Resisto M2M is junk? It seems conceptually it would be beneficial... so I'm interested to learn why it just doesn't work in practice.

I see no advantage to insulate knee walls and floor/ceiling near eaves. Less square feet. Work out what you'll do on rafters and run it from storage ceiling to eaves. Spray foam depth of rafters would be good without venting - so called hot roof - but rigid foam on rafters, leaving rafter space for vents will work. Just read of someone cutting strips of foam, installing them on either side of each rafter, and then rigid foam panels on them, flush with rafters. All with spray can foam adhesive. Foam should be covered or painted to make it flame retardant. Tape at joists and you have vapor barrier.

So essentially: treat it like a vaulted ceiling. I can't get a crew in to do a full rafter depth spray foam install (if I could afford that, it wouldn't be DIY, I'd just have them do the whole garage from ridge to floor).

Now, for that install you mention: do you mean 6" (well, 5½") wide strips on the rafter faces? So that between the rafters is a nice insulated tunnel? Is there any data on the benefits of the extra work of wrapping the rafters rather than just putting the rigid foam panels on the edge and then taping?

You might consider an exhaust fan in one table on a thermostat for high summer heat. Needs to be sealed for winter.

Hmm - would that be significantly better than swapping one (or both) of the passive turtle-back vents to a whirlybird or fan-powered? Again if I can avoid reworking any of the still-new construction that would be ideal (so no new penetrations etc).

I don't think you need to worry much about ice dams in an unheated building.

The general conversation from builders in the area would suggest otherwise - in the spring it's a big issue with the freeze-thaw cycle day after day, and tracking moisture in on the vehicle along with the radiant heat.

 

[billconner]

My point about the shingle color was not to suggest changing, but to warn you how much all the other things would do relatively.

Double bubble

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I appreciate the cost obstacle of spray foam, and I'm not a fan of it. Many here are, big fans.

I'll try to explain my foam idea better. Your building vent baffled or chutes. Rip strips of foam 1 1/2 or 2" wide, adhere to roof deck against rafter. Them rip a piece of foam - as thick as you can afford but probably 2" but 4" would be great - to fit between rafters against the small strips. You could use 1x2s instead of small foam strips. Or I think you could skin the rafters with rigid foam but your loosing the space for the thickness.

With the foam insulation - a good tape like Zip over rafters and foam - it will be a very good air barrier which in combination with the insulation and venting should prevent ice dams. Instead of tape, you can seal the foam to rafters with foam on a can. Your choice. I always end up with a mess using spray foam so olean to the tapes.

Beware to nuch ridge vent can defeat some of the air sealing by creating negative pressure in the vented spaces. 60% soffit 40% ridge is my preferred balance.

I think the gable exhaust fan - 12" 1500-2000 CFM or smaller - on a thermostat - will get the warm air out of the storage loft. If the garage is really tight you may have to open a window or door for make up air.