Insulating attached garage walls
snydergc
| Posted in Energy, Heating & Insulation on
I would like to add 2″ rigid insulation to the interior garage walls in order to help the insulation value. Currently they are just 2×4 walls with fiberglass insulation. I live in Pennsylvania. The walls have drywall on them and I would like to just put the rigid insulation over the drywall and then put furing strips over the insualtion. Over this I would put another layer of sheetrock. Does anyone see an issue doing this? I would rather not have to remove the current layer of drywall. My garage is not heated and I don’t plan on making it a heating space. I will have to consider my electric needs so I may have to use 2×4 turned sideways for my furing.
Replies
Bear in mind that the wall between garage and living space should be a solid firewall, including attic spaces, etc. Some older homes aren't built this way, so you may want/need to fix that as a part of your project.
First make sure the existing wall is airtight, so that, should a fire get into the foam, no gasses from that will enter the house. Then make sure the drywall over the foam is also airtight, to keep fire from getting to the foam.
You are going to have to figure out how you are going to detail the bottom/top of the new wall where (potentially) you have created an open space that would actually suck a fire up behind your fire rated drywall.
I don't see a problem with what he intends to do. There is no vapor barrier issue since it isn't conditioned space, and the furring is horizontal so you don't have the same flame spread problems. Everything is outside the existing firewall.
You are assuming the furring is horizontal. You don't know that from the facts presented. and yes while it all would be outside the current fire break, the rigid insulation is much more flammable than normal wood construction.
Regardless of if it is horizontal or vertical furring, you are going to have to decide how you detail out either the top/bottom of the two sides. Horizontal still leaves an air gap for fire and also a very nice highway for rodents. Filling the gap between the furring would add to the insulation value and stop some of the issues with the void behind the drywall since it is not needed for a rainscreen.
Since you are looking at a build-up of 4 inches you are also going to have to find a way to detail any access doors into the house.
I misread the "2x4 turned
I misread the "2x4 turned sideways" as meaning applied horizontally... rereading it apears he ment on the face.
Probably what he would be better doing is screwing down OSB or plywood over the 2" foam. No air gap, just OSB, Foam, Drywall.
He can run a 1.5" thin wall conduit at 48" above the floor the entire perimeter. Adding a new junction box is easy as you just need to pull off one OSB panel, cut into the existing conduit, add the box, connectors, and wire, then replace the OSB panel again. If you use screws to attach everything you can reuse the same holes.
I'm not sure that OSB is an acceptable fireproof covering for the foam.
Good points and BTW I assumed the furring would be vertical so we are both equal.
Its is something I have thought about myself as I have a sheathed wall in my garage that attaches to the kitchen. While I would love to take down the siding and do my whole house, I'm not sure I am up to all that work of moving windows and everything. This one wall would only have one door and one stove vent to detail out so I am thinking like the OP that it might be a place to start. Wouldn't even have to worry about flashing in a garage!
I currently only have one outlet on the 2 walls that I am discussing. I want to add more outlets before adding the rigid insulation. There should be not reason why I cannot cut the current drywall enough to allow me to run the wire through the existing walls. So now I would have the wire run to outlet locations and could put on the rigid foam board. Now my 2 issues are how best to attach sheetrock and secondly how how to support the outlet boxes. When I mentioned laying 2x4 sideways, I would put these up like a normal wall with top and bottom plates. Then the other 2x4s would be on 16" centers going vertically. I only mention doing these sideways to save space. I obviously could build a regular wall after putting up the rigid insulation, but this would take up space and seems like overkill.
sny
The 2x4's would be overkill.
Rip your own or find 2x2's-admittedly these can be all over the place-not straight etc. But, they move around ok, especially if you get one run right on, then use a block to persuade the rest in line at the nailing locations.