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Old 01-17-2019, 10:44 AM   #1
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Are there any conditions fiberglass insulation acceptable in basement?


I have a walkout basement, and I am finishing the walkout side. Meaning, one interior wall, one wall where concrete block is fully exposed to air on the outside, and two side walls partially exposed and partially underground.

My basement is very dry, no water, no leaks, low humidity. House was built in mid 1990s. Plastic taped to floor and tin foil taped on wall "tests" did not collect any water. I have dryloc on the walls I will be finishing. French drain with sump pump.

My contractor wants to use fiberglass insulation with a 1-2" air-gap between the concrete block and insulation which will rest inside framed 2x4. No rigid foam, no plastic vapor barrier. Is this an acceptable use of fiberglass insulation in the basement?
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Old 01-17-2019, 11:10 AM   #2
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Re: Are there any conditions fiberglass insulation acceptable in basement?


It's acceptable only if the relative humidity is maintained low enough the exposed walls never reach dew point temperature on the interior surface. If those walls do reach DP temperature condensation or even ice in extreme conditions may form. If you know the most extreme Lo temperature of your area, the RH necessary to prevent that can be calculated.
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Old 01-17-2019, 11:10 AM   #3
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Re: Are there any conditions fiberglass insulation acceptable in basement?


That's the "Kia" version for cost and labor. Some posters will soon chime in with the "Cadillac" versions, but will perform better.

FWIW in my area many contractors install 24"oc framing with batt to meet the newer code insulation requirements, cheaper than encapsulated or messing with rigid.

Last edited by 3onthetree; 01-17-2019 at 11:14 AM.
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Old 01-17-2019, 12:06 PM   #4
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Re: Are there any conditions fiberglass insulation acceptable in basement?


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Originally Posted by SeniorSitizen View Post
It's acceptable only if the relative humidity is maintained low enough the exposed walls never reach dew point temperature on the interior surface. If those walls do reach DP temperature condensation or even ice in extreme conditions may form. If you know the most extreme Lo temperature of your area, the RH necessary to prevent that can be calculated.
It rarely gets below 15 degrees here in the winter. Doesn't this calculation depend on the temperature inside v. outside too?
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Old 01-17-2019, 12:56 PM   #5
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Re: Are there any conditions fiberglass insulation acceptable in basement?


The gap and fiberglass are both old school even though they may work in 90+% of the time.

A gap allows moisture from the bottom of the wall (floor and foundation) to be transported up to the more exposed and colder areas of that concrete.

Fiberglass insulation is very air permeable so any moist air entering the wall cavities will pass right through to the foundation. And FG is mold food.

Now, not sure if you have excluded rigid foam insulation but it provides an ideal interior thermal break to separate the warm moist air from inside the home away from the cold foundation, thus the first inside surface and air can reach will always be above the dew point.

1" is often enough and is actually very easy to install. Butt the framing up against the rigid and fill the cavities with insulation. I prefer Roxul and you can do some searching to see how it performs in a basement environment.

The plastic or foil test is misleading as the results only mean today there was no issue. Concrete is a unique material and is constantly passing some amount of moisture from the soil to the inside. Often we see a nice dry basement but store some cardboard boxes down there and you see a different side of the concrete's personality.

We don't see your location so code requirements need to be determined.

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Old 01-17-2019, 04:15 PM   #6
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Re: Are there any conditions fiberglass insulation acceptable in basement?


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The gap and fiberglass are both old school even though they may work in 90+% of the time.

A gap allows moisture from the bottom of the wall (floor and foundation) to be transported up to the more exposed and colder areas of that concrete.

Fiberglass insulation is very air permeable so any moist air entering the wall cavities will pass right through to the foundation. And FG is mold food.

Now, not sure if you have excluded rigid foam insulation but it provides an ideal interior thermal break to separate the warm moist air from inside the home away from the cold foundation, thus the first inside surface and air can reach will always be above the dew point.

1" is often enough and is actually very easy to install. Butt the framing up against the rigid and fill the cavities with insulation. I prefer Roxul and you can do some searching to see how it performs in a basement environment.

The plastic or foil test is misleading as the results only mean today there was no issue. Concrete is a unique material and is constantly passing some amount of moisture from the soil to the inside. Often we see a nice dry basement but store some cardboard boxes down there and you see a different side of the concrete's personality.

We don't see your location so code requirements need to be determined.

Bud
NJ. Good to know. It sounds like I might be part of that 90+%.
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Old 01-17-2019, 04:31 PM   #7
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Re: Are there any conditions fiberglass insulation acceptable in basement?


Quote:
Originally Posted by mferrigno1950 View Post
I have a walkout basement, and I am finishing the walkout side. Meaning, one interior wall, one wall where concrete block is fully exposed to air on the outside, and two side walls partially exposed and partially underground.

My basement is very dry, no water, no leaks, low humidity. House was built in mid 1990s. Plastic taped to floor and tin foil taped on wall "tests" did not collect any water. I have dryloc on the walls I will be finishing. French drain with sump pump.

My contractor wants to use fiberglass insulation with a 1-2" air-gap between the concrete block and insulation which will rest inside framed 2x4. No rigid foam, no plastic vapor barrier. Is this an acceptable use of fiberglass insulation in the basement?
We build like that up here 1" away from concrete because the inside of the concrete can be all over the place, with block I would make that space smaller. We use vapour barrier and seal outlets and everything because it is important that moisture does not get into the cavity from the basement. I live in a basement suite like that.
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Old 01-17-2019, 04:49 PM   #8
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Re: Are there any conditions fiberglass insulation acceptable in basement?


Here's some reading for you.

https://www.energyvanguard.com/blog/...rrier-Probably

https://buildingscience.com/document...vapor-barriers


https://buildingscience.com/document...ts?full_view=1


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Old 01-17-2019, 06:41 PM   #9
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Re: Are there any conditions fiberglass insulation acceptable in basement?


Thanks, Bud. I'm getting a clearer sense of the issue. Unfortunately, every contractor seems to use the insulation method I described originally: concrete block wall, 1-2" air gap, frame with fiberglass, then gypsum. I actually cannot find someone where that isn't that standard insulation technique.... so I'm not sure what to do...
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Old 01-17-2019, 07:29 PM   #10
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Re: Are there any conditions fiberglass insulation acceptable in basement?


Quote:
Originally Posted by mferrigno1950 View Post
It rarely gets below 15 degrees here in the winter. Doesn't this calculation depend on the temperature inside v. outside too?
Moisture forming on interior walls because of dew point is dependent on indoor temperature, indoor relative humidity and the surface temperature of the interior walls. If the interior relative humidity can be controlled to 15 percent or lower when the room temperature is 70F then there would be very little concern.

Where the problems begin is depicted in the pic example if the indoor RH reaches 45% with the indoor temp. 70 degrees, then the interior wall surface has to me maintained above 48 to prevent condensation. The problem is getting a 100 percent seal so that 45% RH air never reaches the interior wall surface. The next hurdle is getting anyone to guarantee the 100 percent seal. That probably isn't going to happen even if a scientific expert was to attempt it.


Here is a discussion of a similar situation when warmer moist air contacts cold basement walls.
https://www.diychatroom.com/f17/cold...ptions-553969/
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Last edited by SeniorSitizen; 01-17-2019 at 07:37 PM.
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Old 01-18-2019, 10:36 AM   #11
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Re: Are there any conditions fiberglass insulation acceptable in basement?


SeniorSitizen, thank you. This is helpful and makes sense. Based on these calculations I don't have to worry about moisture. But just to be cautious, I will monitor these three conditions after the basement is complete and adjust the indoor temperature or RH if necessary.
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Old 01-18-2019, 11:25 AM   #12
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Re: Are there any conditions fiberglass insulation acceptable in basement?


I don't recall what state you are in, if mentioned, but you must be far enough south that insulation contractors don't feel the need to read. Techniques for encapsulating a basement foundation are well discussed in many trade articles.

Old school used to be the only source of "best practices" so new people in the trades had to learn from those with more experience. With the introduction of building sciences some old methods are being replaced with more reliable guidelines. As for the 90+% that is just a wild guess but there is a difference between modern best practices and old school. The vapor barrier articles are a good example. Even the experts can't get Canada on board for when to and when not to use them.

Foundations are a bit difficult because below grade there will be very little drying to the outside, thus allowing some drying to the inside is good and easily managed. Products like rigid foam board (without a foil or plastic covering) provide a small amount of permeability to accomplish that small amount of drying.

Bud
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