Insulating the Roof Deck

[cls89]

I have a detached garage. I've insulated the walls. I still need to insulate the ceiling. The problem I have is that the spacing on the ceiling joist (horizontal surface that is parallel to the ground) aren’t standard. Please see photo for example. The gaps are pretty significant, so standard insulation wouldn't fit the openings. This is a 20x32 garage, so trying to correct the framing would be expensive time consuming.


The other option I can think of is to insulate the roof deck (sloped surface). However, from what I've read this isn't good practice because moisture can get trapped in the insulation when snow melts, rain, etc. which could lead to mold and premature failure of the shingles and framing. Is there any way around this? I came across a post where someone installed a ridge vent (see link) and then faced fiberglass insulation over this vent. Is this a good option? Are there any other options that I am not aware of? I would imagine a lot of people have this type of challenge when it comes to insulating a detached garage. I am just curious what some of the solutions are to this type of situation.

 

[billconner]

I can't quite tell what's going on but install a ceiling - maybe metal liner panel - and blow loose fill on it.

What's climate? Will you be heating? We can talk about venting and vapor barriers if necessary.

 

[cls89]

I can't quite tell what's going on but install a ceiling - maybe metal liner panel - and blow loose fill on it.

What's climate? Will you be heating? We can talk about venting and vapor barriers if necessary.

Wouldn’t the metal panels sag? In some cases the span is over 48”

Climate is Michigan. I would like to at some point get a electric heater.

 

[billconner]

I think you can find liner panels that will span up to 8', and 4' without sag easily. Worse case add some framing. Check manufactures specs.

 

[Jim greengo]

Run furring strips between the joists,then hang ceiling/insulate.

 

[strutaeng]

Maybe do what this guy did with Polyiso:

 

[cls89]

Run furring strips between the joists,then hang ceiling/insulate.

I consider this. I’m assuming you would just toe mail the furring strips between the joists? What gage mail would be appropriate for this?

 

[Jim greengo]

I consider this. I’m assuming you would just toe mail the furring strips between the joists? What gage mail would be appropriate for this?

Across the rafters,I'd say 6 penny nails would be plenty

 

[billconner]

If you're going furring strip route, fur across entire garage and shim to level.

 

[cls89]

Across the rafters,I'd say 6 penny nails would be plenty

How much space would you put between the furring strips?

 

[75gmck25]

During a recent renovation I had the roof deck on my house insulated with open cell spray foam, and its getting to be very common in this area. I also had them build an enclosed/conditioned crawl space it also got spray foam.

They have apparently resolved any moisture issues, since neither the renovation contractor or the roofing company had any issues with it. The crawl space did require positive ventilation that circulated air to the rest of the basement, or installation of a dehumidifier.

 

[gymman12]

this is some solid advice

Run furring strips between the joists,then hang ceiling/insulate.

 

[cls89]

Also if I run furring strips across the ceiling joists am I limited to metal panels for finishing the ceiling or would I be able to use drywall, plywood, or even PVC liners?

 

[billconner]

Use any of those. But metal panels are install and done and very bright, the others need many more steps to be as finished.

 

[Jim greengo]

How much space would you put between the furring strips?

Maybe every 16"

 

[mike93lx]

How much space would you put between the furring strips?

Either 16" or 24" on center, depending on the material you are using to finish

 

[cls89]

Either 16" or 24" on center, depending on the material you are using to finish

Insulation would ether be fiberglass batt insulation or spray insulation and the finished panels I’m undecided. It could be drywall, plywood, or metal panels.

This is one concern I have. Since the ceiling joists are spaced far apart will loading the joists with insulation plus drywall / some type of finished panel cause structural problems / sagging or will the furring strips add enough rigidity to support the drywall and insulation without any problems? Thanks

 

[mike93lx]

Insulation would ether be fiberglass batt insulation or spray insulation and the finished panels I’m undecided. It could be drywall, plywood, or metal panels.

This is one concern I have. Since the ceiling joists are spaced far apart will loading the joists with insulation plus drywall / some type of finished panel cause structural problems / sagging or will the furring strips add enough rigidity to support the drywall and insulation without any problems? Thanks

I wouldnt use 1x for a 48" span. At those distances, I'd use 2x4 for furring strips if drywalling. That said, I believe metal panels would be fine for that span

 

[cls89]

I wouldnt use 1x for a 48" span. At those distances, I'd use 2x4 for furring strips if drywalling. That said, I believe metal panels would be fine for that span

I figured 2x4 would be better for rigidity on large spans. I just thought it might be too heavy and cause load problems.

 

[mike93lx]

I figured 2x4 would be better for rigidity on large spans. I just thought it might be too heavy and cause load problems.

It's possible, but unlikely, IMO. Drywall and insulation is about 3lb/sq ft.

 

[cls89]

It's possible, but unlikely, IMO. Drywall and insulation is about 3lb/sq ft.

I figured. How about plywood? 3/8 to 1/2” thick? I would think same thing

 

[mike93lx]

I figured. How about plywood? 3/8 to 1/2” thick? I would think same thing

Same. About 50lbs per 32 sqft sheet.