Old Garage Ceiling Insulation

[MikeyW]

Hi all,

First time poster.

I have a 1929 vintage brick dethatched, and unheated garage, with a nearly flat roof. I want to reduce the heat level in the summer, and I’m wondering if insulating the rafters would make any real significant difference in the Denver, CO area? Should I use faced insulation or not?

I’m not interested in insulating the exposed brick walls, just the inside roof area. I may use a space heated in the winter and a portable AC in the summer.
At $490, is this worth it? Will I really feel any difference?

Size: 16’ x 18’
Ceiling height to joist bottom: 8’ down to 7’5” at lowest end
Joists spacing: 22”
Roof: White PVC membrane, with no insulation over plywood (8 years old)
Windows: one facing East, and one facing North (wood, original construction, single pane glass)
Garage door: metal facing West (I plane to insulate using garage door insulation kit)

Thanks

 

[The Cobbler]

largest heat gain/loss is thru the roof. it will help keep it cooler in summer and warmer in winter .
what r value are you planning on using?

 

[MikeyW]

R30

 

[MikeyW]

These joists are 2x10

 

[billconner]

I just don't know what the "venting" stragies are for flat roofs but I'd think you'd want a very good vapor barrier below insulation. I wonder if it's the ideal application for spray foam. I'd just be a little leary of condensation on roof deck.

Most flat roofs seem to have rigid foam on top of deck under membrane roofing. I guess too late for that.

 

[MikeyW]

bill conner,
Yes a little late for the rigid foam. I don't have any plans to add any venting. Pretty large gaps at the walls, so it will not be air sealed. I want to DIY this project, so going with bats of faced R30. You think this may still be a problem for moisture build up?

Thanks

 

[billconner]

Sorry for delay. Family funeral has kept me busy.

Mike - not sure where you are but if a heating climate, yes, I would be concerned about warm air inside passing through batts and condensing on underside of roof deck and on metal purlins. A heavy duty vapor barrier under new batts might protect it. Not even sure how to do that with minimal and sealed penetrations.

 

[MikeyW]

Bill, thanks, and sorry for your loss.

I am located in Denver, where the other day temps were 100 and the inside temp in garage was 119. No metal purlins in construction. 1929 flat roof with PVC, ply over original 1 x 8 planks. In order to provide a larger air gap between the insulation and deck, I've decided to go with R22 instead of R30.
See attached pictures.

 

[billconner]

Mike-
I'm not sure of best or even good solutions for your conditions. I think faced with same problem, I focus on the vapor barrier. Might consider batts as you propose and then rigid foam across joists with joints sealed with foam in a can.

 

[MikeyW]

Thanks Bill

 

[BombShelter]

We have 100 degrees F this week and my garage is 10-15 degrees cooler with rock wool on the ceiling. Spend the little dough and you'll be so happy, get a dehumidfier as well!