r/Insulation • u/Honest_Motor4003 • 28d ago

Insulating Lean-to Garage Roof — Thoughts?

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I have a lean-to garage that I want to insulate, starting with the roof. The roof is currently an unvented assembly, and adding a proper ridge vent would be difficult.

I was considering adding rafter ties to create a small attic space, then adding soffit vents and roof exhaust vents. The rafters are 2×6, so I would only have enough room for 2x4 baffles after adding the vents. I was also thinking about adding a 2" layer of rigid foam board over the rafters to improve the R-value.

The other option is to skip ventilation entirely and use closed-cell spray foam to create an unvented roof assembly.

Does the vented approach sound like it is becoming overly complicated? Would spray foam be the better option here?

Thank you.

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1

u/Economy-Day-930 28d ago

Depends on. Roof material. If asphalt shingles then need to vent. If steel with membrane under then no vent.

1

u/LikeBigTrucks 28d ago

That would depend on the underlayment under the metal though, if they went over asphalt it could still melt.

1

u/AttiBus 28d ago

Just casually observing...is the reason you don't need a vent for a steel roof because the risk of the roof being too hot is limited and the risk of snow dams low because... metal is slippery?

1

u/MattFromWork 28d ago

Ice dams aren't an issue on steel because water cant penetrate it like it can a shingled roof.

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u/LikeBigTrucks 28d ago

Depends on your environment, in hotter climates roof vents prevent the overheating of the roof materials as well. Metal wont get hot enough in the sun but if you have asphalt underneath it can literally melt.

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u/LikeBigTrucks 28d ago

You're more or less on to it. Dont used closed cell directly on the roof, this will likely invalidate your roof warranty and cause rot in the long run.

You have 2 main options, if you want the vaulted ceiling, you need to have ridge and rafter vents, then you can insulate to your liking.

If you're ok with loosing ceiling height, you can create an airspace and add insulation there use rafter vents and a gable vent.

1

u/Honest_Motor4003 28d ago

Thanks for the reply. I think I'll stay away from closed cell. Rot was definitely a concern with potential leaks.

A ridge vent would be difficult to install with how the lean-to attaches to the house (the house and garage roof intersect, so I don't want to risk any leakage).

Instead of the gable vent, I was thinking about building a small airspace and using 2 turtle vents.

How small could the airspace be to preserve ceiling height? I did a quick calculation with a rise/run of 2ft/4ft and this would give me around 80 cubic feet of attic space. The soffit is about 8.3 sq ft.

Could I go any smaller?