Furring 2 x 8 for insulation
Stillwater Lefty
| Posted in Energy, Heating & Insulation on
I have framed up a new garage with a habitable space above. The garage has 12/12 pitch with 2×8 rafters. Code requires R-38 insulation which needs 12″ for bats (my preference). Does anyone have thoughts on the best way to fur out the rafters to accomodate the R-38 insulation? I thought of adding 2×4 on edge to get the depth but am concerned about the weight of the sheet rock pulling on the furring. Nailing 2x4s does not seem to be strong enough yet screwing them in would require 6″ screws. This question is similar to others that I have read recently but I think I ambetter off since this is new construction.
Replies
Pocket holes?
You might try pocket hole screws, and drilling holes for the heads halfway through the 2X4s with a pocket hole drill.
Another option would be 3/8 or 1/2-inch plywood gussets.
And yet another option would be to use Simpson connectors, and run the 2X4s perpendicular to the 2X8s to eliminate the thermal bridging, and use blown in fiberglass insulation.
Yeah, run your furring at right angles to the rafters. You can use Simpson anchors or toe-screw.
Tip on Toe Screwing:
Use round washer head or pan head screws - the kind that DON'T countersink into the wood. Countersinking will just cause the wood to split.
Take a 1/2" wood boring bit and set the sharp tip about an inch from the edge. When you activate the drill, use the tip to pivot the drill into the wood sideways. This creates a "pocket hole" that you can use #9 or #10 sized screws with.
If you are dead set on using bats, you can fill the bays with the x8 bats, then fill the cross furring with x4 bats.
If you really want to use batts, using 6" screws to attach the 2x4's isn't that hard if you use the right screws. These will melt right in with an impact driver, avoid bridging, etc. and strong enough that you don't have to use very many. Otherwise, use PL Premium adhesive & nails.
http://www.mcfeelys.com/product/TLS-4060/6quot-TimberLok-Hex-Drive-Timber-Screws
Use interior rigid foam
Stillwater Lefty,
You need a 1-inch ventilation channel under the roof sheathing. That leaves about 6 inches. Fill the 6 inches with fiberglass batts -- your preference -- and you'll get about R-22. So you need another R-14. If I were you, I would install 2 inches of polyiso rigid foam under your rafters. That's easier than furring out the rafters -- and it stops thermal bridging through the rafters.
Install 1x4 strapping under the polyiso to help hold it in place. The foil facing on the polyiso faces an air space, adding about R-2 to the assembly.