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Basic Insulation of 2x4 Garage Wall


Thors Twins's Avatar
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09-25-12, 01:40 AM   #1  
Basic Insulation of 2x4 Garage Wall

Hi folks,
in the near future embarking upon a garage project that will involve insulating & heating the space. would like to get the space as 'tight' as possible

Details:
1. roughly 20'x20' and is currently not insulated, above ceiling and in walls.
2. western wall is 2x4 walls, north wall is 2x6 framed. 1/2" drywall interior (which is beat up and will probably replace when finishing the walls).
3. has a 6'w x 5'-4"h window lowE glass (on west facing wall) and an uninsulated wood garage door about 16' wide (north facing).
4. west facing wall has an ~18" overhang. north facing wall has about 8" overhang

Notes:
1. I replaced the window this past weekend, in doing so, removed some of the shiplap cedar siding. No rotten wood anywhere indicating that the overhang does a good job of keeping the 1/2" sheathing & framing dry.
2. currently, there is no tar paper or vapor barrier, just the cedar siding that will be replaced in the future.
3. concrete slab floor with no basement/crawl space below.
4. the studs are unusually spaced, 24" c-c vs 16", and some studs are doubled and even quadrupled.
5. no air conditioning of the house or space.

Questions:
1. do i need a vapor barrier? if so where would it best be located, interior/exterior?
2. insulation batts or spray foam?
3. EPS or XPS foam board, exterior or interior?

where do i start?

 
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XSleeper's Avatar
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09-25-12, 05:54 AM   #2  
1. do i need a vapor barrier? if so where would it best be located, interior/exterior?
Since you will be heating the space in the future, the answer is yes. Vapor barriers always go on the warm side of the wall, so that would be behind your drywall.

2. insulation batts or spray foam?
Not sure you want to spend the bucks to spray foam a garage, but that would be the best. You could also look into cellulose... not the dry loose stuff, but the kind mixed with an adhesive that allows it to stick in a wall cavity that has no drywall on the face. Since you mention that you will probably replace the drywall since it is beat up, you might as well remove it, then insulate, the put up your VB. (if you have the insulation professionally installed, you should probably follow the instructions of the company as far as whether one is needed or not, for warranty purposes). Once you remove the drywall, you'd be wise to air seal the sill plate to the floor with some construction adhesive or caulking. Also caulk the gap between any studs that are doubled, tripled, etc.

3. EPS or XPS foam board, exterior or interior?
IMO that is not needed. However if you do install it, it would go on the exterior and it's purpose would mainly help with air sealing and as a thermal break between your insulated wall and your sheathing.

Keep in mind that whatever you do in insulating the walls and ceiling is not going to help your cold, uninsulated cement floor (it will radiate cold at the edges, and could possibly sweat in the winter when you crank the heat up.) and your uninsulated garage door. Garage doors are pretty drafty too, so even if you get an insulated garage door and some decent weatherstrip, expect some air infiltration around that door.

 
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09-25-12, 05:55 AM   #3  
Of course closed cell foam insulation would be the best, but $$. Since you don't "live" in your garage like Vic does I would suggest batt insulation with kraft facing the inside of the structure. Don't get it too thick as compressing insulation will reduce its effectiveness. Use R13 in the 2x4 walls and R19 in the 2x6 walls. Ideally I would look for Roxul insulation. You'll never go back to fiberglas if you do.

When you replace the siding, be sure to apply Tyvek or comparable breathable moisture barrier.

 
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09-25-12, 08:03 AM   #4  
let me see if i got this straight...

1. so a tyvek or comparable air barrier on the exterior, right? it would be moot to have both eps/xps and the wrap barrier, yes?
2. sill plate sealing - is that caulking around the seams where studs & sheathing touch the plate?
3. Roxul mineral wool - i've actually used similar before for industrial applications. didn't realize their is residential applications of mineral wool stuff. that's great, easy to work with.

for this project, I just need it where i light a fire and can work on the car/bike/etc without freezing my butt off. and if i win the lotto, build a separate garage & make this a fully liveable space. still need that winning ticket.

thnx for the guidance~

 
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09-25-12, 09:01 AM   #5  
The Tyvek is not an air barrier, but is a breatheable moisture barrier to allow air out, but not water in.

There is a big difference between EPS (usually used for coffee cups) and XPS. The term "vapor barrier" is really not an accurate measure, since several coats of breathing paint can fall under the arbitrary definition of a "vapor barrier" as measured by perms in a lab test.

Roxul is a very good material and superior the FG in many ways. It provides better air sealing, contributes some thermal mass/inertia and does not melt like fiberglass, so that it is used many times in industrial/commercial applications.

Dick

 
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09-25-12, 08:14 PM   #6  
it would be moot to have both eps/xps and the wrap barrier, yes?
It might actually be counterproductive. You need the moisture-wicking material on the outside, to help keep your insulation dry and effective. You need the vapor retarder on the inside, for the same reason.

 
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09-25-12, 08:33 PM   #7  
The Tyvek is not an air barrier
Actually, it is an air barrier. House Wrap ? DuPont

It is also vapor permeable. Just because it is an air barrier does not mean that water vapor is trapped behind it.

Foam might be "counterproductive" if it holds moisture in when it is trying to escape, which I think is what Nashkat was referring to. Tyvek over foam is, IMO a good idea- the only way it might NOT be, would be if liquid water was somehow condensing on the outer surface of the foam, (such as from a massive interior air leak at a wall penetration where warm humid air would condense on a cold foam vapor barrier) the Tyvek would trap that liquid water behind the membrane in the same way that it prevents liquid water from getting INTO the wall.

A wall sheathed with foam does not quality as a WRB in the strictest sense of the word unless it is actually waterproof, meaning all the seams must be taped - or, a WRB like a housewrap has been applied over it. In some cases, the WRB is installed first, then the foam. (Window installers prefer the WRB on the OUTSIDE so that they can seal the nailing flange of the window to it... otherwise there is no guarantee that the window won't leak.)

 
Thors Twins's Avatar
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09-30-12, 09:42 AM   #8  
what is 'wicking material'?

 
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10-01-12, 07:35 PM   #9  
A material that retards air flow but allows water vapor to pass through. Tyvek is the best-known example.

 
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