Help on Insulation, foam board as backing?

[fastfalcon94]

I just finished building and siding my garage. I live in Vermont so it can get pretty cold up here. It's 64x40x12.5 with a 64x18x8 storage area above that I am going to use as a wood shop/storage area. I plan on using a Modine 200,000 or 250,000 btu heater downstairs. I want to insulate the walls and both ceilings so that I effectively have 2 zones. For now I'll just use some vents in the upstairs floor that I can open/close if I want to have heat escape into the woodshop/storage area.


I wish I could go back in time and have the trusses engineered with 6" wall studs instead of 4". Because I want to add 6" of insulation to the walls upstairs, but I'm stuck with a 2x4 wall. I was thinking that it may be easier if I were to use 2x4 insulation and then run foam board on the back of the 2x4 walls. This will give me the 6" (assuming 2" thick foam board) of insulation I want and it will provide a solid backing to keep the insulation from falling down. Will this work? I'm worried about moisture barriers. I would use faced insulation though and foam board that isn't sealed on one side. The alternative would be to use 2x6 insulation, and to add 2" of board to the back side of each stud, then make my own backing.


My other concern was with the 18' feet of floor between the two levels. Would I use non-faced insulation there? Hopefully this make sense. I created a picture in paint that may help better explain it.

 

[Gerald O]

You're going to need a rigid backing of some sort anyway behind the fiberglass insulation, so using foam board as the backing can serve that purpose. Be aware that fiberglass is now available in R15 for 3-1/2" walls. You can also fur the walls out with 2x2 if you really need to make them thicker.

 

[Voi]

I wish I could go back in time and have the trusses engineered with 6" wall studs instead of 4". Because I want to add 6" of insulation to the walls upstairs, but I'm stuck with a 2x4 wall. I was thinking that it may be easier if I were to use 2x4 insulation and then run foam board on the back of the 2x4 walls. This will give me the 6" (assuming 2" thick foam board) of insulation I want and it will provide a solid backing to keep the insulation from falling down. Will this work? I'm worried about moisture barriers. I would use faced insulation though and foam board that isn't sealed on one side. The alternative would be to use 2x6 insulation, and to add 2" of board to the back side of each stud, then make my own backing.

There has been some discussion about using rigid foam on the interior side of on Green Building Advisor and other sites. It's an excellent way to reduce thermal bridging and ends up being a better insulated wall than "just" a 2x6 wall with batts.

The downside is that is can be expensive and the wall is better protected from condensation on the backside of the sheathing if it's on the exterior. As far as that latter concern, some have said it's not as big of an issue in a building like a garage that won't be heated 24/7 and/or having the same moisture producing appliances like a home.

Before you take my word on that I'd suggest doing some searching over there and even posting your own thread.

Before you fir out your studs I'd look into a Mooney wall. It utilizes horizontal strapping and then blown in insulation. Or sometimes rigid foam between the strapping. Either way the horizontal strapping reduces thermal bridging compared to adding fir strips to the back of the studs.

Mooney wall might be better for hanging stuff from the wall -- which is often necessary in woodshops.

 

[fastfalcon94]

Thanks for the replies so far.

I did some research on the mooney wall. I really don't want to use blow in insulation anywhere. The garage will be heated in the winter. The downstairs will be set around 50 degrees and I'll turn it up when I'm out there. The upstairs won't be heated unless I open the vents for now, but maybe one day it will be.

I'm not quite sure what you mean by this "the wall is better protected from condensation on the backside of the sheathing if it's on the exterior". Are you saying I could get condensation on the back of the foam and it would run down into the ceiling insulation? I definitely would not want that.

I had to go to lowes at lunch to get some nails. I took a look at the foam they offer. They had some 2x4x8 sheets for $34. I'm sure I could get it a little cheaper through contractor pricing from the local place near my house. They didn't have a full sheet of 1" thick present, just some little 2x2 squares. The 1" thick stuff still seemed heavy duty enough to me as well and assuming I can get full sheets would be a lot cheaper.

 

[Voi]

I did some research on the mooney wall. I really don't want to use blow in insulation anywhere. I'm not quite sure what you mean by this "the wall is better protected from condensation on the backside of the sheathing if it's on the exterior". Are you saying I could get condensation on the back of the foam and it would run down into the ceiling insulation? I definitely would not want that.

I had to go to lowes at lunch to get some nails. I took a look at the foam they offer. They had some 2x4x8 sheets for $34.

I didn't write all that very well. When rigid foam is on the exterior it helps keep the building sheathing warm and prevents condensation between the sheathing and the batts. The ratio of the R value of the foam and the R value of the batts needs to be correct for your climate zone.

So if you have 2x6 walls full of fiberglass, for example, you actually need more exterior foam than if you only have 2x4 walls full of fiberglass.

Now when the foam is on the interior it's not helping keep the sheathing warm. I try to stay up on these things but it seems like building scientists aren't in complete agreement about what happens when interior rigid foam is used. So I would recommend posting your own thread over on GBA.

My guess is that they're going to say it's no big deal in a woodshop and will likely have some suggestions on smart vapor retarders and ratios of foam to batts.

They'll also get into actual R values of various types of foams and will likely have suggestions for where to get reclaimed foam in your area at much less than $34 per sheet.

 

[fastfalcon94]

Circling back to this issue as I'm starting to work more on the garage now that spring is coming. GBA said I would be fine using 2" unfaced as a backing. Just assuming it's $34 a sheet this would $1088 worth of foam I would need to buy and install.

If I went with 1.5"x2" furring I'd have around $150 in wood to make the 2x4 walls a 2x6 wall. This would also allow me to add some horizontal bracing afterwards which I think would help tie the trusses together even more. This would provide more support than foam. If I went with this approach I would probably use a paper faced R19 insulation on the walls. This way I could staple the insulation in place. I would still need some kind of breathable backing though to keep the insulation from peeling off the paper and falling backwards.

Any ideas on what is commonly used? I've thought of a few things...

- 1x3 strapping (similar to what i did on the downstairs ceiling to make it 16" OC)

- galvanized fence wire (I could zigzag it back and forth down the walls using metal staples)

- plastic rolls (assuming they make some 4-6" wide thick plastic rolls I could staple to the back of the walls)

- Snow fence (something like this: https://www.lowes.com/pd/Hanes-Geo-...-in-Orange-Contractor-Sand-Snow-Fence/3522630)

I'm not sure if there is a right/normal way to go about this. The furring would save me time and money. Plus I already have wire stapled to the back side of the walls so it would be easier for me to notch a furring strip than it would to try and dig out foam.

 

[yeldogt]

Why do you want to use such a large heater .. do you want to leave the doors open.

I can get my 1700 sf shop to 70+ degrees with 35k BTU's net?

How about flash with foam and batt -- what's the wall thickness on the lower level?

 

[Voi]

Any ideas on what is commonly used? I've thought of a few things....

I would rip down sheets of foam or nail-base to reduce the thermal bridging.

Update your GBA thread with this thought if you're unsure.