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Basement insulation/vapor barrier with cement block foundation

Basement insulation/vapor barrier with cement block foundation


  #1  
Old 10-16-15, 01:52 PM
R
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Basement insulation/vapor barrier with cement block foundation

I'm looking for some advice and clarification as I start finishing my basement, beginning with the insulation and framing of the walls. Some background: I live in Minnesota, the house was built in 1979, and the basement is fully enclosed (no walk-out; a few small 36"x12"-ish windows above grade) with cement block foundation.

My first question is regarding insulating the block foundation before framing. I've read and heard some conflicting advice from various people (the guy at my local Menards) and online forums. My plan is to use either 1.5" (R7.5) or 2" (R10) rigid foam board (polystyrene) insulation, and securing it directly to the foundation with foam board adhesive. Now, the guy at Menards told me to NOT "seal" the gaps, in order to let the walls breath, but a lot of what I've read online in forums says that I should tape the seams between the boards with tuck(?) tape, and seal ALL edges (ceiling, floor, corners, etc.) with an airtight insulating foam spray in order to create a vapor barrier between the cold foundation and the warm inside air. Which one is correct? Are there other variables (such as climate) that would determine which route to go?

After insulating the foundation and stud-framing the walls, I'm also wondering if it would be necessary/recommended/worth the extra cost to then add fiberglass insulation in the stud cavities before hanging drywall? Or would the foam board be enough? I've also heard varying arguments on this, and would ultimately determine how thick of foam board I end up using.

The last question is will it be necessary for me to put up 4 or 6 mil poly plastic sheeting directly behind the drywall, acting as a vapor barrier? (This may depend on the answers to the previous two questions).

My biggest concern is moisture and mold growth; so is there anything to be concerned about with potential mold when using 2 layers of insulation (the rigid foam board and the fiberglass)? My second concern is cost; I'm on a pretty tight budget and don't want to spend money where I don't have to, but I'm definitely willing to spend money where necessary to do it properly the first (and hopefully only) time. The third concern is the insulating factor; un-insulated and un-finished, the basement gets pretty cold in the winter (lower 50s F) and I won't be running HVAC to that particular area when finished (I may do electric baseboard heating though).

Any guidance would be greatly appreciated.
 
  #2  
Old 10-16-15, 02:09 PM
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  #3  
Old 10-16-15, 02:42 PM
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Those were somewhat helpful, but they don't seem to directly address the questions I have. Or it could be that I'm not understanding them correctly (which is quite likely, since I'm new to this). Maybe this is hopeful thinking, but my goal in posting here was that someone could answer these questions specific to my situation. Maybe I should simplify my questions:

1) Should I seal the seams and edge gaps on the foam board insulation, or just glue the boards to the foundation and let it "breath?"
2) Should I use fiberglass insulation in addition to the foam board insulation, or is it not worth added cost? Or will that cause a moisture/mold problem?
3) By sealing the seams and gaps on the foam board insulation, does that create a sufficient vapor barrier, or do I need to add plastic sheeting directly behind the drywall? By doing both, does that create a "double vapor barrier" (as I've heard referenced?) which could cause moisture and mold growth?
 
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Old 10-16-15, 03:01 PM
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1 There should be very little gap. What you're trying to create here is a condensation barrier and the foam needs to be very complete for this.
2. Fiberglass between the studs in your framing is a good idea, so is Roxul.
3. The foam is a vapor retarder and sufficient, you do not want a vapor barrier on a below ground wall as it will need to dry to the inside.

Hang tight to see if Bud contradicts or augments anything I've said here.
 
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Old 10-16-15, 03:17 PM
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That was more helpful, thank you.

Regarding my first question, this process is what I was referring to when I said "seal" the gaps:

https://www.youtube.com/watch?v=zDNyjR1SHUQ

The "gaps" being between each sheet of foam board, and between the foam board and the floor/ceiling/corners.

Would this be a good instructional for me to follow?
 
  #6  
Old 10-16-15, 07:49 PM
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No objection ss you are right on.
rl, confusion is common with the variety of information available over the internet. Is my opinion and collection of information going to be any better that others? Unfortunately the decision ultimately falls back to you, and of course your local building official. I'll try to be shot and let you follow up where you need more information.

1. No gaps, especially the closer to the top you get, the foundation is colder up there.
2. There is a ratio of rigid to fiberglass which maintains the inside surface of the rigid above the dew point. 1.5" with r-13 fiberglass should be fine, just be sure to seal all air leaks around electrical penetrations so as to not invite air circulation into the wall.
3. No extra vapor barrier behind the drywall should be necessary as the rigid foam will allow very little moisture to pass through. Once through it needs a place (direction) to dry.
4. Your total r-value should meet current codes. Mn looks like r-15 to r-19 depending upon the insulation you select, so 1.5" rigid plus r-13 should be good.
https://energycode.pnl.gov/EnergyCodeReqs/
5. Continue your insulation efforts above the foundation and into each joist cavity with attention to air sealing the house to foundation and other cracks and penetrations.

Bud
 
  #7  
Old 10-21-15, 08:40 AM
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I'm currently finishing up a similar project. The sum of all my research led me to this: Rigid foam insulation attached directly to block wall with adhesive and a 1/4" capillary break at the floor, taped seams between the sheets of foam insulation, spray foam the top gap between rigid foam and rim joinst, stud framed wall against the foam board, unfaced fiberglass insulation between the studs (and at the rim joist), drywall, latex paint.

Using this method the rigid foam board is your vapor barrier, but the seams must be sealed. Do not use plastic behind the dry wall or you will create a double vapor barrier; an area that cannot breath and release moisture.

Use unfaced fiberglass in the basement. The paper facing is what facilitates mold growith. Moisture will breath through the unfaced bats, and through the drywall and latex paint, which is what you want.

I'm using Dricore on the floor in my basement. The stud walls actually sit on top of the Dricore, eliminating any contact with the concrete floor and allowing moisture to breath below the floor. It is pricey, but I thought it was worth it.

Good luck!
 
 

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