Insulating an old garage

[Shivatron]

Insulating an old garage?

I'm in the process of renovating the detached garage on a house I just purchased in central Texas. The garage is 20x20 with a slab foundation, standard timber framing, and a gabled metal roof supported with rafters. (The house was built in 1930; I believe the garage is slightly newer, probably postwar.)

The structure is currently down to studs. I'd like to insulate and potentially air-condition the space with a mini-split system so that the indoor temperature can be lowered for working in the garage during the hot summer months.

For the walls, this probably means drywall over fiberglass or sprayed foam insulation. I'm not sure what to do at the top. There is no ceiling below the joists currently. The roof has no soffit vents (it's closed at the wall), but it does have a ridge vent. (The roof itself is metal over a membrane over the decking.)

My contractor recommends spraying foam onto the underside of the decking and air-conditioning the whole space. This is attractive as I'd get to keep the rafter (loft) space as conditioned storage. That said, I'm not sure how well that would play with the ridge vent. I'm also not sure how easy it would be to light the space evenly without a reflective ceiling above the lights, which would be presumably hung below the joists.

I'm going to talk to my contractor more about these matters, but I figured I would ask here as well. Anyone tackled a project like this and have any thoughts?

I'll try to document in this thread what ends up happening. As a starting point, here are some photos.

Demolishing the old, poorly installed drywall:

Looking up into the rafters. Hard to spot the ridge vent, but it's there:

South side soffit:

Ridge vent from the outside (along with questionable-looking ridge beam):

 

[SkidooRider15]

interesting remodel. Good luck.

 

[Shivatron]

Thanks! It's a cool space, and very rare in this city. Want to make sure I do it right!

 

[Voi]

Re: Insulating an old garage?

My contractor recommends spraying foam onto the underside of the decking and air-conditioning the whole space. This is attractive as I'd get to keep the rafter (loft) space as conditioned storage. That said, I'm not sure how well that would play with the ridge vent. I'm also not sure how easy it would be to light the space evenly without a reflective ceiling above the lights, which would be presumably hung below the joists.

I'm assuming your contractor's plan with spray foam is to seal up the ridge vent, thereby making it an unvented (aka "hot") roof.

If it were me, especially given the look of that ridge beam, I'd try to carefully remove the metal roof and do rigid foam over that shiplap looking roof deck.

Or I'd look into vented R board. If that plays nicer with your climate and local code. The concern with either of these is will the roof fit back on.

My reasoning for thinking this way is I just think the current rafters and roof decking just looks so cool on the inside. I'd hate to cover that with spray foam, especially when exterior foam would do more to eliminated thermal bridging and be cheaper (assuming you do much of the labor and roof can be removed and put back on) and allow you to address the aging framing once everything is exposed.

Another option is to add rigid foam between the rafters with a space so the underside of the decking is still vented. I think for this to be most effective you'd have to add some sort of soffit vent, but I'm the last guy who should be suggesting such things. I'm only offering a perspective of what I'd be looking into if I were renovating that space.

 

[DC73]

Re: Insulating an old garage?

My contractor recommends spraying foam onto the underside of the decking and air-conditioning the whole space. This is attractive as I'd get to keep the rafter (loft) space as conditioned storage. That said, I'm not sure how well that would play with the ridge vent.

Don't worry about the ridge vent. The spray foam will seal it very well from the bottom and you can always remove the ridge vent whenever the building needs a new roof.

DC

 

[Cyberbear]

If the conditioned attic space is precious to you than include it in your a/c calculations when sizing the unit. More air space means additional operating costs.
But if the storage space will not be holding items affected by summer heat, put in a ceiling and save money. At some point you have to ask yourself how many times do you want to spend the value of those items by cooling the attic year after year?

 

[NUTTSGT]

Like Cyberbear mentioned, I'd prefer to put a ceiling in and create an attic space. You have a ridge vent but you still need a place for the cooler outside air to come into the attic. Something to think about.

 

[Shivatron]

Some good answers in here -- thanks! I'm leaning toward the ceiling myself after spending some time looking at it this weekend. Will talk to my contractor and see what he thinks.