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Insulating/drywall over basement concrete wall-outside wall exposed to elements


dvarga's Avatar
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NJ

09-10-14, 02:50 AM   #1  
Insulating/drywall over basement concrete wall-outside wall exposed to elements

I need to have a warmer basement this winter. I plan on putting up studs and hard foam against the wall, then cover over with drywall. The outside of this particular wall is completely above ground, that is - it is exposed to the outside air. And, this outside wall rarely if ever gets wet from rain or snow.

Question: is a snug fit between the foam and the studs sufficient, or should I also caulk the joints? I am thinking that there might not be a moisture issue and caulked joints would give a better seal?

Best,
D. Varga

 
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chandler's Avatar
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09-10-14, 03:13 AM   #2  
Leave 1" between your studs and the foam. You will find it helpful in attaining a straight wall should there be any aberrations in your concrete wall. It also allows air circulation between stud bays. Adding Roxul or fiberglas insulation in the bays will complete the envelope.

 
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09-12-14, 03:13 AM   #3  
QUOTE: The outside of this particular wall is completely above ground, that is - it is exposed to the outside air.
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Don't be surprised, at some point after the first cold weather, to find condensation on the inner surface of the concrete wall.

All it will take is for the wall surface to reach 52F with a room temperature of 75 and 45 percent relative humidity.

 
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09-12-14, 05:00 AM   #4  
Hi Chandler,

So you mean leave 1" of space between the edge of the foam and the stud? Or the foam is snug against the stud and there is 1" of space between the foam and the drywall? Thx in advance.

D Varga

 
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09-12-14, 05:23 AM   #5  
In a perfect world no gap is necessary, but getting a straight wall is usually impossible. The trick to leaving a gap is addressing SeniorCitizen's concerns, which is condensation on any cold surfaces where inside warm air has access.

The solution is to seal the drywall plane so no inside air can reach that cavity, top plate, bottom (pressure treated) plate, and all penetrations like electrical and plumbing.

Since your entire wall is exposed to the cold, that helps with ground moisture but hurts with condensation. There are guidelines for how much rigid insulation is necessary to keep the inside surface above that dew point.

Also, be sure to tape and seal the rigid insulation as some moist air will always get back there.

Bud

 
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09-13-14, 03:38 AM   #6  
Finally, another question. I have a section of this basement wall (the outside of which is completely above ground) that has steam return pipes and drain pipes right next to it. The space between the pipes is such that I have 1.5" to work with, between the pipes and the basement wall. The basement wall by the way is very straight.

So I intend to put 1" wood stripping on the wall and put 1" hard insulation between the strips. Then put drywall onto the strips. The room will not get above 70 degrees in the winter. The humidity in NJ winters are low. With this scenario in mind, do you think this will work without generating mold?

Best,
D Varga

 
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09-13-14, 04:53 AM   #7  
Very poor way to finish a basement. Strips won't allow for good insulation, nor will it allow for you to easily run your electrical. Build a wall with studs.

 
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09-13-14, 05:38 AM   #8  
Mr Chandler has built a few more of these than I have (certainly hundreds more) so double ditto on his comments .

In photo #4 in the link below you will see where they use continuous rigid insulation against the exterior wall, all-be-these walls are warmer because they are mostly below grade. Then they build the stud wall against the rigid and fill between the studs with insulation and no vapor barrier.

Block or concrete walls provide very little insulation value and when above grade they become a major heat loss issue. One inch of rigid foam is about r-5. 8 inches of concrete is less than r-2. An above grade wall in NJ (I'm guessing) needs to be in the R-20 range and higher is better. By using the continuous rigid insulation against the concrete you reduce the thermal bridging of the studs, thus improving the overall wall performance.

As for the steam return pipes you need to insulate between the wall/floor and those pipes. And, not insulate between the pipes and the warmer basement. We can't see what you are looking at, but stopping the drywall above the pipes and finishing below with ventilation is what I'm thinking.

Bud

BSD-103: Understanding Basements — Building Science Information

 
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