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Old 01-31-2016, 04:50 PM   #1
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Using fiberglass batts in basement


So I've been reading a lot of articles saying NOT to use fiberglass batts in basements as the moisture from the foundation walls will transfer into the batts and cause mold. The overwhelming majority say that fibergalss batts cant touch the foundation walls. However, if I frame my studs a few inches off the foundation walls, then am I safe to use the fiberglass batts within the studs as long as they don't come in contact with the foundation walls? The gap should leave enough room for the moisture to evaporate off the foundation walls, correct?

Thanks for the help.
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Old 01-31-2016, 05:06 PM   #2
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There has to be several thousand post on this one subject if you check the search funtion.
#1, Is this basement 100% dry?
#2, In your area there needs to be at least 2" of foam insulation glued to the foundation before the walls are built, if not your going to have a mold nightmare.
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Old 01-31-2016, 06:59 PM   #3
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http://buildingscience.com/documents...ith-fiberglass
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Old 01-31-2016, 07:16 PM   #4
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Basement foundations have to deal with both the moisture vapor passing through the concrete as well as the humidity in the heated basement air. From the outside the moisture vapor is always passing through and if you leave a gap it is circulated to the top area which is colder and forms condensation. On the inside, warm basement air passes through the fiber and reaches a colder foundation wall. Either way, you get moisture where you don't want it.

The rigid foam insulation solves both problems by maintaining an inside surface warm enough to not form condensation from the inside basement air and it slows the migration of moisture through the concrete. Whatever gets through is easily handled with a conditioned basement.

The link WoW provided shows the rigid foam and depending upon your local building codes you can determine the required thickness. 2" as Joes suggest would probably be sufficient, but you might also be able to use 1" rigid with r-13 batts in the stud walls.

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Old 02-01-2016, 08:58 AM   #5
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Quote:
Originally Posted by joecaption View Post
There has to be several thousand post on this one subject if you check the search funtion.
#1, Is this basement 100% dry?
#2, In your area there needs to be at least 2" of foam insulation glued to the foundation before the walls are built, if not your going to have a mold nightmare.
I've never had issues with water in the basement other than one time where my humidifier's drain hose came undone and I had a leak. However, that was not water coming from the outside in.

The area I'm in drains pretty well and most basements around here are relatively dry. However, that doesn't mean that moisture is not coming in through the foundation walls.

Have people had mold issues by not using foam insulation boards on their walls?
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Old 02-01-2016, 09:11 AM   #6
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Quote:
Originally Posted by joecaption View Post
There has to be several thousand post on this one subject if you check the search funtion.
Is this member to read several thousand posts and or replies to not find the answer he needs as opposed asking a direct question?
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Old 02-01-2016, 09:33 AM   #7
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PSK insulation does have the potential to allow warm moist air to get through the poly blanket and condense on the cold cement wall. The fact is that it probably happens much more than people know about because it is concealed within the insulation. The most effective way to insulate a concrete wall is as indicated in that link that details out how to use rigid foam and standard batt insulation for a high-performance wall and airtight wall.

Be sure to use the airtight drywall approach.
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Old 02-01-2016, 10:48 AM   #8
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Thanks for the help thus far. Does there need to be a gap between the foam boards and the studs/batts?
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Old 02-01-2016, 11:14 AM   #9
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The only reason for a gap would be to keep the wall straight and then the batt insulation would be used to fill any extra space to limit air circulation.

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Old 02-01-2016, 11:27 AM   #10
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Quote:
Originally Posted by Bud9051 View Post
The only reason for a gap would be to keep the wall straight and then the batt insulation would be used to fill any extra space to limit air circulation.

Bud
Do I want to limit air circulation? Some moisture will still get through the foam boards, correct?
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Old 02-01-2016, 11:39 AM   #11
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Air circulation increases the heat transfer effectively reducing your insulation value. The amount of moisture vapor that passes through the rigid insulation is very small and has no problem continuing through the fiber insulation and drywall, effectively drying to the inside.

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Old 02-01-2016, 11:53 AM   #12
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Thanks, Bud. Would rigid insulation on it's own (with a higher R value) be good enough to use without batts, or do I need both?
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Old 02-06-2016, 02:26 PM   #13
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In the pic below you'll see that in the corner of my basement I have my breaker box and gas meter. The alcove has the washer and dryer and I plan on building a wall a few feet off this wall (basically where the window starts) and coming straight across so that there is a separate laundry room and "utility corner." How would I insulate this corner area?? Would I just put the rigid foam boards along the foundation walls in the corner and then put the batts in the studs on the new wall that would be a few feet off of the foundation walls? I would put drywall around the laundry area but can I leave the foam boards in the utility corner "exposed" within the laundry room/utility closet?
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Old 02-07-2016, 11:41 AM   #14
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anyone??
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Old 02-07-2016, 02:11 PM   #15
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Quote:
Originally Posted by harpua728 View Post
So I've been reading a lot of articles saying NOT to use fiberglass batts in basements as the moisture from the foundation walls will transfer into the batts and cause mold. The overwhelming majority say that fibergalss batts cant touch the foundation walls. However, if I frame my studs a few inches off the foundation walls, then am I safe to use the fiberglass batts within the studs as long as they don't come in contact with the foundation walls? The gap should leave enough room for the moisture to evaporate off the foundation walls, correct?

Thanks for the help.
joecaption and Bud9051 provided good information.

Water does what it wants and the the constraints Bud9051 notes apply not only to the insulation, but all surfaces in a basement. Basements are heat sinks.

That said, we hung twelve foot wide 6 mil plastic in a continuous sheet on the basement walls by wrapping furring strips a few times and screwing them to the top of the wall with concrete fasteners. The remaining width draped to and over the floor. We then framed on 24" centers (non-bearing) on the plastic (pressure treated studs for bottom sill plate), ran our electrical and insulated with paper faced four inch R13 thick batts. We than ran more 6 mil on the concrete floor and seamed with 3M™ Polyethylene Tape 483. On the floor we used a felt under layment and floating floor. This provided some insulation and give. Solid concrete will eventually do your leg joints a disservice. All walls, ceilings and sofits covered with standard drywall. No issues since install three years ago.

We used two 100 ft rolls of 6 mil at about $70 each. The tape was about fifteen bucks. The 6 mil is more robust than 4 and IMHO easier to work.

This configuration not only limits the moisture penetration from the surrounding earth, but also inhibits the effects described by Bud9051. We went one more and installed a 70 qt. dehumidifier on a shelf with drain to safe waste. This makes for one less thing to do and keeps the humidity down in summer months. We notice a marked difference between a 50% relative humidity setting and 60%.

I configured for a mini-split ac unit, but since in the dead of summer, the basement never gets above 78, I doubt it will ever be installed. For heat I ran fin tube off our 75 gallon water heater with a domestic water rated circulator and across the line digital thermostat. In spite of many naysayers, the basement is the coziest place in the house.

Water flows downhill and basements are the lowest level. You will want to manage sources like water heaters, drains, humidifiers, sumps, pipe penetrations and similar to avoid collateral damage. We have battery powered moisture alarms purchased in a three pack that helped during a long rainy week where many had flooding.

Hope this helps and good luck.

We have a similar area as you show in your picture. I left as is. Since the near all the walls and floor is covered, that little bit is easily captured by the dehumidifier.

Last edited by throrope; 02-07-2016 at 02:14 PM. Reason: Picture
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