Basement Concrete Walls - Insulation Options

 
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Old 02-28-2011, 09:30 AM   #1
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Basement Concrete Walls - Insulation Options


Concrete basement walls for a basement finish in a cold-weather climate. The issues are two-fold: 1) insulation/warmth; 2) air-quality/mold-prevention.

Keeping in mind that much of the winter condensation in a basement is caused by the warm air of the space touching the cold concrete of the foundation. There are many opinions on the best approach. I have chosen one basic method, but have a specific question about people's preference, and why. Here is the main method:

Glue rigid-foam to the concrete walls. This is meant to keep air from touching the foundation. Regular insulation (i.e., fiberglass) still allows air to pass through to the wall.

The question is, is it best to glue the foam to the foundation first, and then frame the walls against it, or is it best to frame the walls first against the foundation, and then glue rigid foam to the concrete BETWEEN the studs.

I see some pros and cons to each:

Rigid foam first:
PROs:
1. no wood touches the wall, so you can use regular lumber (lumber used in direct contact with concrete wall would have to be pressure-treated).
2. wood will not transmit temperature from warm room directly to the concrete.
3. This leaves a full 3.5" of space to use R-13 fiberglass between studs.

CONs: lose 2" of living space, side-to-side.

Opinions on the best method? Or perhaps there is an alternative method altogether?

Thanks -Drew
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Old 02-28-2011, 05:55 PM   #2
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Re: Basement Concrete Walls - Insulation Options


having completed dozens of basements - all in homes with poured concrete foundation walls and 90% of the homes were less than 10 yrs old, I can say without question, that your option for building stud walls with the studs in contact with the foundation wall won't fly - unless you want wavy, out of plumb walls when you are done. I always give myself an inch or two out from the wall to make up for the fact that no foundation wall will be perfectly plumb and square to all the other walls. Having said that, I personally have yet to utilize the 'foam-on-wall' method; have always done what the code (in my area anyway) called for - R13 paper faced FG insulation. I wouldn't say the paper faced batts are ideal, but have never, ever had a mold or moisture problem.

In the future, I am going to push the HO to flash & batt with spray foam & FG


Last edited by Sweebs; 02-28-2011 at 06:03 PM. Reason: added comment
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Old 02-28-2011, 06:35 PM   #3
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Re: Basement Concrete Walls - Insulation Options


If it were my place, I'd frame the walls and have it spray foamed.
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Old 02-28-2011, 06:48 PM   #4
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Re: Basement Concrete Walls - Insulation Options


In my area, (Saskatchewan, Canada), basements are a different beast than any other part of the house, given the potential for flooding and the guarantee of condensation (4degree concrete slab and walls meeting 20degree room air). We highly, highly recommend using the rigid foamboard glued to the walls approach. This is the ONLY way to guartantee against moisture issues (read: mold) from condensing on the interior walls. The unique thing about foamboard is unlike poly, it actually eliminates condensation as the dewpoint is met gradually and thus there doesn't leave any condensing on either side of the product.

It also acts as a great protection against water penetration (remember that anything organic will result in mold, including paper-backed batts), and will prevent any sponging effect of the batt insulation from soaking in moisture from eventual future seepage from cracks etc. which in turn wicks into the studs, and begins a mould problem.

I also recommend the foamboard on the floor at 1" to lift everything off the ground for all of the same above reasons.
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Old 02-28-2011, 11:57 PM   #5
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Re: Basement Concrete Walls - Insulation Options


Why is erybody talking about the foamboard on the interior and not the exterior? I put the foamboard on the exterior and r-11 or 13 on the studded interior walls. I'm really not sure how much you gain by insulating the studded walls with 2" of foam on the outside and everything buried. If the studded walls just had airspace, it would still help.

On the other hand, $300 of insulation in the basement walls isn't a lot to spend.
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Old 03-01-2011, 06:15 AM   #6
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Re: Basement Concrete Walls - Insulation Options


Quote:
Originally Posted by cleveman View Post
Why is erybody talking about the foamboard on the interior and not the exterior?
Because typically for a project like this, the house is long since completed and backfilled, and the builder didn't insulate with future basement finishing in mind. Besides, I have my doubts that exterior foam would insulate enough to preclude the possibility of condensation on the interior of the wall.
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Old 03-01-2011, 06:32 AM   #7
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Re: Basement Concrete Walls - Insulation Options


Ten years ago I put Foam on the outside of my basement (before backfill.) I couldn't find very many experts who thought this was a great idea, but I did it anyway.

I found it to not only help insulate, but I used T G sheets that have really assisted in preventing moisture from pushing through the wall. I know this because the few places I didn't get the foam in place are the places I see more moisture concerns.

Back then it wasn't very expensive, now the cost has changed somewhat, but if you figure any energy savings, then the benefits balance the increased cost.
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Old 03-01-2011, 02:11 PM   #8
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Re: Basement Concrete Walls - Insulation Options


I will definitely agree that the best way to insulate a concrete wall (and floor) is ALWAYS from the inside in, because u can then put a waterproofing membrane wrapped completely around the outside (behind the rigid foamboard), and now you would have a thermally well-insulated and completely waterproof basement, free of seepage issues as well as condensation issues.

If your not building new, then insulating the outside, as someone else pointed out, is extremely costly (last job we did of this sort for a customer, including a new weeping tile system) was in the range of $40k all said and done. So, the next best option is rigid foam board on the inside walls, as long as we all understand that there really is no true way to waterproof concrete walls in the basement from the inside out (only from the outside can this truly be done).

This is what Holmes would also argue, and my engineers stand by the same (above) points.
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Old 03-01-2011, 02:27 PM   #9
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Re: Basement Concrete Walls - Insulation Options


Foam against wall tape joints
Spray foam rim down to foam sheet
Build wall in front

Insulating on the outside is ideal, but very difficult to do a nice tight job
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Old 03-01-2011, 03:57 PM   #10
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Re: Basement Concrete Walls - Insulation Options


Now what do you do with the floor that may have as much area as the walls and is the same temperature as the lower part of the wall, since the frost level is just a fictitious defined number and is not based on insulation needs.

If you build properly, it is far cheaper in the end.
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Old 03-01-2011, 09:32 PM   #11
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Re: Basement Concrete Walls - Insulation Options


Here in this County they have made it code to either foam board the outside of the foundation or the interior wall has to be studded and insulated. They started this about 3yrs ago, PITA IMO. Any foundation with radiant heat has to have foam board under the foundation/slab too.
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Old 03-02-2011, 09:50 AM   #12
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Re: Basement Concrete Walls - Insulation Options


Insulation on the walls is meaningless when considering floors, especially because of the area involved and the natural temperature of the soil, which is about the same as the average annual temperature. The cool floor can be an advantage if the basement is conditioned and you have low returns to collect the cool air. This can actually lessen the load and provide more uniform temperatures all year.

If I was to build new, I would definitely put 2" of XPS under the basement slab even if not heated, just for easy uniformity/comfort in a climate with our extreme swings (-30F to +100F).
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Old 03-02-2011, 10:28 AM   #13
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Re: Basement Concrete Walls - Insulation Options


My 2 cents? I have seen molding studs in basements. Why do work that will be temporary and that could potentially be hazardous to fix or create concern by inspectors? Nothing is perfect but you know a stud against a basement wall will be wet a good portion of it's life.

I'd at minimum use foam...pinkboard....continuously against the wall. Because I can I would parge it first. Extra credit for a gutter caulked down to collect any water, so that water leaking out will have a known selected exit point.

A studwall; you pick; 2x w/ more foam or fiberglass. The thickness of this determined by the type of insulation and R factor required. I'm not as keen on treated lumber inside. How about the synthetic/plastic wood used on decking for a sill plate?

If the basement was somewhat wet I would consider a cement board base instead of running drywall at the floor.

I have a mostly insulated basement in my 100 year ol house and it has;
1) Dropped the heating cost of the house
2) kept the basement cooler in the summer
3) reduced the amount of humidity condensing in the basement in the summer
4) raised the temperature and comfort level of the first floor in the winter; a few degrees makes a lot of difference!

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Old 03-02-2011, 01:00 PM   #14
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Re: Basement Concrete Walls - Insulation Options


Quote:
Originally Posted by equinoxhomes.ca View Post
I will definitely agree that the best way to insulate a concrete wall (and floor) is ALWAYS from the inside in, because u can then put a waterproofing membrane wrapped completely around the outside (behind the rigid foamboard), and now you would have a thermally well-insulated and completely waterproof basement, free of seepage issues as well as condensation issues.

If your not building new, then insulating the outside, as someone else pointed out, is extremely costly (last job we did of this sort for a customer, including a new weeping tile system) was in the range of $40k all said and done. So, the next best option is rigid foam board on the inside walls, as long as we all understand that there really is no true way to waterproof concrete walls in the basement from the inside out (only from the outside can this truly be done).

This is what Holmes would also argue, and my engineers stand by the same (above) points.
Good points except unless you can waterproof the slab and footings how can you completely waterproof a foundation on the outside to never have seepage? If your talking about the foundation wall well yes but that leaves the slab to be susceptible.
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Old 03-02-2011, 02:09 PM   #15
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Re: Basement Concrete Walls - Insulation Options


If you are concerned with water/moisture in abasement, drain tile installed during construction has been proven to be very effective and economical.

A friend of mine built several thousand homes and every home automatically got both interior and exterior drain tile linked irregardless of the site conditions. It was very cheap since it was scheduled and planned and he never had a complaint about a wet basement. - He was a builder that also did the basement/foundations and did not sub them because of the schedule and lack of control.

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