Gable Insulation

[GA_Brown]

How to I insulate the gable ends in my garage. The roof is a 8/12 pitch with the trusses made for a loft. The gable trusses have the 2 x 4 flat, so only the 1 1/2 inch part of the 2x4 is available to add insulation to. I want to have the loft insulated as well.

I don't want to use sprayed foam. I have looked at the foam boards but the R value is so low. Do I just frame out a wall in order to add the insulation? What has every one else done?

 

[spudley]

Frame out the wall.

 

[stm317]

With just 1.5 inches of depth foam is really the best/only option. It could be rigid foam panels or spray foam.

If you frame the wall out to get deeper wall cavities then your insulation options increase.

I still think I'd use rigid foam for the 1.5 inch depth that you've got right now. That will stop alot of air infiltration/draftiness and then you can insulate with cheaper more traditional means inside of that to get the desired R value.

 

[blkhonda1991]

are you insulating the rafter bays as well? insulating the ceiling is much better/easier unless you want to have conditioned attic space...

 

[anythingyoucanimagine]

Yeah just frame it out. Mind your stud spacing so the insulation fits. You'll notice the biggest bang for your buck by (properly) insulating the roof. Will make much more of a difference than the walls.

 

[GA_Brown]

Yes, will be insulating the rafters, that is pretty straight forward. Just did not know what to do with the gable ends.

 

[anythingyoucanimagine]

Yes, will be insulating the rafters, that is pretty straight forward. Just did not know what to do with the gable ends.

You are down in GA so I'm assuming climate zone 3. I'm 5/4marine. I think you need R-38?? (I could be wrong on that) for the bottom of your roof deck.

When I did mine it was in steps. I used 1-by furring strips and stapled them (crown stapler) to the insides of the rafter cavities, up against the bottom of the roof deck. Was a pain in the balls going up the ladder with the strapping, then measuring and marking, then back down to bandsaw to cut notches for the roofing nails. That was a tedious process I was glad to see finished.

Then install rafter baffles, insulate over the top plates, and because I did mine in steps (and I was dumb and cut soffit vents before I insulated), I then stapled strips of Tyvek to the furring strips. That creates a 1.5" air gap between your insulation and the bottom of your roof deck. So cool air can come in from your soffit vents and run up along bottom of roof deck then out the top (ridge vent, gable, etc.).

When I did mine, in hindsight, rather than waste time/money with the Tyvek and tons of rigid foam boards, I would have bought thin foam board (maybe R-5), stapled that to the furring strips then do spray foam on top of that.

If you need R-38 under your roof that's about 6" of R-6.5 spray foam. Assuming you have 2x6 rafters with a 1.5" air gap you'll have 4" of space to insulate and you need 6". If you are going to do drywall I'd rip down 2x4's to 2" strips and screw them to the rafters. Structurally you aren't changing the 2x6'es but dimensionally you would be changing them to 2x8's. Spray in the foam then put up sheetrock.

 

[Vehical13]

I'm up in Maine and am working on a 30x30x27 high gambrel garage. My contractor only used 2x6 trusses and R30 needed 2x8 construction. I ripped a 4x8x1 foam board into 1x1 strips, then glued them to the roof sheathing. By the way this is a cathedral ceiling. Each 2ft bay had the 1x1 strips down the center and then plastic baffles. I wasn't convinced the baffles wouldn't buckle in the center, that's why the 1x1 strips. at this point there was only 4-1/2" left. To make up the difference 2x3 studs were glued and screwed to the 2x6 trusses. That gave me the other 3-1/2" to accommodate 8 inch R30. I have ordered Dura Skrim® Fire Retardant Reinforced Plastic Sheeting - 6 mil, from Americover. Local code enforcement allowed me to use this plastic sheeting insted of 5/8 fire rated sheetrock, if I choose. I'm a glutton for punishment and refuse to do things half way, so then i'll put the sheetrock and 1x12 pine shiplap. I built the down stairs similar except without the 2x3's or the baffals. The outside will be finished with board and batten, making my downstairs walls 9" thick and my roof 9 1/2" thick. The roof ridge will be vented.
I'll use your idea with the foam boards to fill the 1-1/2 gap on the gables, stud out and put up R 23 Rockwool plastic, sheetrock and finally 1x12 shiplap.