Basement Insulation

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  #1  
Old 01-18-14, 07:34 PM
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Basement Insulation

My buddy is finishing his basement, which is a poured foundation about 3/4 of which is below grade. He already installed pink foam board directly against the interior foundation walls and framed the walls. I believe the foam board has an R-value of 3. Question is, does the foam boars have an integral vapor barrier? And if so, should he install unfaced batts to avoid moisture getting trapped sideinside the wall?
 
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  #2  
Old 01-18-14, 07:49 PM
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Most foam products are vapor permeabile, except those with a foil or plastic facing. R-3 sounds like the fan-fold which may have a plastic facing. In any case it is rather low on r-value. The purpose of using a rigid foam product directly against the concrete is to insulate sufficiently so that the inside surface is always above the dew point. I'm not sure what your VA climate would require, but one in at r-5 would probably be a minimum.

As for a vapor barrier, NONE is the correct choice. The rigid foam will allow a small amount of moisture to pass, assuming no plastic of foil, and that is necessary to prevent moisture accumulation in the below grade areas. Below grade it cannot dry to the outside.

Verify what he already has installed and we will go from there.

Bud
 
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Old 01-18-14, 08:31 PM
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I'm going over there Monday to run the electrical so I'll have to post back with the details. I believe this is what he has: Shop Pactiv 1/2-in x 8-ft x 4-ft Extruded Polystyrene Insulated Sheet at Lowes.com
 
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Old 01-19-14, 04:16 AM
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The specifications for Pactiv do not indicate a film covering and list 1" as having a permeance less than 1. Couldn't find a rating for 1/2". The link below will discuss specifically drying issues. Under walls the last statement is "No interior vapor barriers should be installed in order to permit inward drying." The amount of moisture vapor that will pass through the rigid insulation is very small, but that drying to the inside prevents an accumulation of moisture behind the foam. If a vapor barrier is installed, everything behind the foam will become as wet as the soil outside and that can grow more than grass.
Bud

Info-511: Basement Insulation — Building Science Information
 
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Old 01-19-14, 05:35 AM
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That is very useful information. Thank you. It clearly says no vapor barrier on the supplemental insulation (batts in this case) so my assumption was correct. I'm still a bit confused about letting moisture dry to the inside. Is this referring to the moisture from the ground saturating the foundation? If so, this makes sense. The foundation can't dry outward when it is in constant contact with moist soil, so it has to dry inward. Is this correct?
 
  #6  
Old 01-19-14, 01:14 PM
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If I remember correctly a study was done where they omitted the vb and measured the amount of moisture that entered. It was something like 1 pint per 4'x 8' sheet per year. I don't have the specifics, but it was extremely low. One person adds more moisture than that per day and it is surely less than what enters with nothing covering the concrete.

The irony is, despite the new research that says to forget the vb below grade and in many cases everywhere, some building codes still require it. One has to make their own argument and try to convince the AHJ to leave it out.

Another important point for your friend is to be sure there were no water problems before he started. You can't cover up a once a year leak and hope it will not be a problem.

Bud
 
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Old 01-19-14, 06:13 PM
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Another important point for your friend is to be sure there were no water problems before he started. You can't cover up a once a year leak and hope it will not be a problem.
No moisture problems. It rained pretty good for two days straight last week and his sump pump didn't even turn on, so that's a good sign. Due to the cost of the foam board, he decided to only install the board in the media room and plans on doing the remaining walls with batt insulation. Should he first put some sort of vapor retarder directly on the concrete then install unfaced batts between the studs just like the other (foam-boarded) area?
 
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Old 01-19-14, 06:58 PM
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The problem with batts against a cooler surface is moist air will find a way in and ultimately condense forming moisture inside the batt insulation. It is very hard to do any job right when the budget isn't there.

Here is a quote from the link below when asked about using fiberglass insulation. "Can I insulate on the interior with fiberglass batts or mineral wool batts?
No. Fiberglass batts and mineral wool batts are air-permeable. When this type of insulation is installed in contact with concrete, the moisture in the interior air condenses against the cold concrete surface, leading to mold and rot." Note, he said interior air, not just the moisture that migrates through the concrete.

How to Insulate a Basement Wall | GreenBuildingAdvisor.com

Be sure he is using pressure treated wood in any place it contacts the concrete and the special fasteners that the wood requires?

Just what you wanted, more reading.
Bud
 
  #9  
Old 01-20-14, 05:11 AM
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What if he leaves a gap between the studs and concrete wall? This would prevent the batts from contacting the wall and moisture from collecting on the insulation. There is one long wall (about 25 ft) where he left a 10" dead space in order to conceal a couple pipes, so the batts in this area would not be touching concrete.

Yes, he is using pressure treated wood for the sill plate. Not sure about the fasteners he is using to attach the sill plate to the foundation floor. Considering they are made for fastening wood to concrete, hopefully they are treated. I'll check today when I'm over there.
 
  #10  
Old 01-20-14, 07:27 AM
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Being in VA rather than my harsher climate of Maine I suspect he will not have my problems, but the guidance these technical articles provides is good. When a gap is left behind a wall, it allows for air circulation that will transport the moisture from below grade up to the colder above grade areas and condense as moisture. Where he has left a larger gap he needs to detail the floor above to insulate it from the cold gap. But not with fiberglass as it will further reduce the rim joist temperature but still allow moisture to seep through.

Now, there are still contractors out there building walls directly against the foundations and filling them with batt insulation covered with a vapor barrier and not having problems. But there are examples of similar installations turning into a mold farm. It is difficult to do everything perfect and when you do it seems that a few years later the experts change their mind. Best I can do is educate and make people aware of current guidelines, but the ultimate decision is theirs based upon their home and their location.

I checked the energy code requirements for VA and it looks like 2" of rigid insulation from top to floor plus detailing over the top and into the rim joist. Don't you love government regulations and how easily they spend your money.

Bud
 
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Old 01-20-14, 05:38 PM
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What is meant by "detailing"? And I was just over there today and he doesn't have the thick pink foam board like I initially thought. He has 1/2" foil-faced foam board. . Should he tear this stuff out? It's glued, so it would be a PITA. Is it possible to peel off the foil facing?
 
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Old 01-21-14, 01:51 AM
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Your PM folder is full.

Bud
 
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Old 01-22-14, 07:08 AM
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Your PM folder is full.

Bud
Not anymore. Not anymore.
 
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