How do I insulate this garage correctly and efficiently?

[Butkusrules]

I’ve had this garage for about 5 years. You can tell the previous owner had stapled in insulation at some point but then ripped it out. No idea why. Anyway I’d like to insulate and hopefully do it on the cheap. There are no soffits or ridge vents. What should I do?


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[dcg9381]

Where are you located?
To me, the cheapest way to do that is to create an "attic" and blow in cellulose or other traditional insulation. Most of your heat loss / heat gain is going to be out of that roof (subject to some northern climates with substantially exposed foundations).

You'll have to drop that heater or other wise build some sort of cavity for it.
Can't tell what's in the walls.
Wiper seals on the garage door.

Foam would be better, but it's not cheap.

 

[kj_mustang]

He would have to vent the attic space to do that. He has no venting. If you want to continue to use above bottom chord of the rafter space for storage, than you will have to insulate at the roof/upper chord of the rafter with something sealed that will not allow water vapor to pass through it to the roof deck given the no venting.

 

[juddspaintballs]

In that case, I'd cut a ridge vent, use the foam airspace slots for insulating the rafter space, then fill the rafter voids with insulation and put something up to hold the insulation in place like OSB or sheetrock.

 

[dcg9381]

He would have to vent the attic space to do that. He has no venting.

I *think* I see some sort of a fan/vent on that roof... He could also do it on the ends of the ridges.

 

[sz0k30]

Since the OP doesn't indicate where he lives, I'm going to assume he lives in Hawaii & doesn't need to do anything!

 

[larry4406]

Since the OP doesn't indicate where he lives, I'm going to assume he lives in Hawaii & doesn't need to do anything!

My thoughts as well.

 

[Butkusrules]

Sorry. Was at work. I 10 miles outside of Chicago. There is an electric vent behind that heater. There is also what looked like an exhaust pipe for a wood stove and a passive vent.
The garage is 22x44.
I’d like to keep that rafter storage if possible.

 

[acer66]

Where are you located?
To me, the cheapest way to do that is to create an "attic" and blow in cellulose or other traditional insulation. Most of your heat loss / heat gain is going to be out of that roof (subject to some northern climates with substantially exposed foundations).

This is what I just did, my rafter ties were all 8’ high, sagging and falling apart so I took them out placed the new ones 10’ high which kept them in the lower third of the roof and used 2x12’s so I could fit r38 bats in there and also so I could put some ply up there for some light storage.

 

[wanderer]

I would start by cutting in ridge and soffit vents so that you have continuous ventilation in every bay. Ventilation is much more important for an uninsulated space. Then I would insulate with XPS foam. Perhaps rip some down to 1 1/2 inches furring strips at the edge of each joist and then use two layers of 2 inch foam on top to be even with all the joists, then a third layer of continuous foam over all of it. this would give you 6 inches of XPS foam and an inch and a half air space without losing any loft access. Make sure to tape your seams on the last layer of foam.

 

[Butkusrules]

I would start by cutting in ridge and soffit vents so that you have continuous ventilation in every day. Ventilation is much more important for an uninsulated space. Then I would insulate with XPS foam. Perhaps rip some down to 1 1/2 inches furring strips at the edge of each hoist and then use two layers of 2 inch foam on top to be even with all the joists, then a third layer of continuous foam over all of it. this would give you 6 inches of XPS foam and an inch and a half here space without losing any loft access. Make sure to tape your seams on the last layer of foam.

Should have those tunnels underneath the foam venting up to the ridge vent?


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[juddspaintballs]

Should have those tunnels underneath the foam venting up to the ridge vent?


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If I had to guess, he's assuming your rafters are 2x6's. He's saying to put 4" of foam board in between the rafters and another 2" of foam across the bottom of the rafters. That would leave 1.5-2" of airspace between the underside of your sheathing and the "top" of the foam boards in between the rafters. No foam tunnels needed that way.

 

[wanderer]

If I had to guess, he's assuming your rafters are 2x6's. He's saying to put 4" of foam board in between the rafters and another 2" of foam across the bottom of the rafters. That would leave 1.5-2" of airspace between the underside of your sheathing and the "top" of the foam boards in between the rafters. No foam tunnels needed that way.

Correct. The air space should be continuously vented from the soffit to the Ridge.

 

[Butkusrules]

Correct. The air space should be continuously vented from the soffit to the Ridge.

Thanks guys.


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[jvitez]

That's a whole lot of work, and will it actually be enough ventilation to prevent rot?

Cry once, pay once: high density "2 lb" polyurethane spray foam sprayed on the underside of the roof. Do this and you can eliminate the power vent, and leave everything else the same.

 

[u3b3rg33k]

doesn't sound like the ventilation will be useful. skip it entirely and spray foam/staple up batts. just don't have any roof leaks.

 

[banjopete]

Not sure around you but also double check the soffit vents are actually open. My 60 year old garage had every soffit covered entirely in plywood so no air was coming through the vented soffit sections. Lucky i checked before getting too far along.

I just completed a similar job in my garage and ended up with basic thick batts, and some rigid foam. It's been awesome, no ceiling and no insulation to both was a huge change on its own in a similar climate to yours.

Foam is great and easy but $$, we used it indoors before and for us it is $3/sqft. It definitely costs more but I'm certain it's a superior option. One of the biggest benefits that isn't mentioned ofren enough is you can put it almost anywhere without additional needs for framing, furring, vapour barrier, venting etc. All things that bring the effective costs down and ease up, plus it's a hire out job.

Good luck whichever way you go, and lastly a good heater can overcome bad insulation [emoji3]

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