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Brian, Architect
Category: Home Improvement
Satisfied Customers: 3123
Experience:  Licensed Architect- 17 years, L.E.E.D. AP
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I have an Apartment building with Crawl Basement. Underneath

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I have an Apartment building with Crawl Basement. Underneath the floor we have 3.5" insulations which has vapor barrier on both side. The floor is about 4 ft from the soil. We have plastic vapor barrier on the soil. Basement ventilation is through a few small windows.

A lot of insulations had retained moisture and rotted. My questions.

1. Why they used Insulation with vapor barrier on both side 40 years ago. It retains
moisture in the insulation and started the rotting.

2. New 3.5" insulations from Home Depot has insulation on one side. It seems
to be better replacement and avoided moisture trapping.

3. In my house, I don't have insulation in my floor (I have a full basement).
Do people use insulation in half basement?

Hi, my name is XXXXX XXXXX I will be helping you today.

1. You are correct, insulation should never have a vapor barrier on both sides unless it is sealed air tight and never gets punctured. I'm not sure why that was installed because it will trap moisture.

2. The kraft-faced batt insulation is a better choice but rigid insulation is even better, since water doesn't effect it. Read more below about how and where to insulate and use the thickest batt insulation that will properly fit in the joist cavity, to give you the most R-value. Just don't stick with the 3-1/2".

3. As far as insulation placement, read below.

Crawlspaces are either vented or they are not. The insulation placement is different for each.

I live in a cold winter location, but the procedure will work anywhere, and this is how I basically design them:

If a crawlspace is vented, it is ideal to treat it as an outdoor space (no different than an attic space), and insulate the floor between the crawlspace and living space. If you have heating/ cooling and water pipes running thru the crawlspace then you have a conflict with the cold outside air freezing the pipes and then it must be treated as an indoor space; in this case you would not insulate the floor, but insulate the perimeter of the foundation and not vent it.

Back to a vented crawlspace, I would insulate the floor with batt insulation and then a layer of 2" rigid insulation applied to the bottom of the floor joists with all joints taped and sealed with spray foam. This provides an air and thermal barrier between the crawlspace and the house above. The batt insulation by itself, is only acting as a filter since moisture can still migrate thru it; and it will do this all by itself because humidity in the air is always trying to balance itself out; and it will migrate from crawlspace to house with no problem, without the rigid insulation.

If the crawlspace has soil for the floor and is vented then I would lay one layer, on the soil, of 6 mil polyethylene vapor barrier with all joints overlapped 6-12 inches; no need to tape joints. Run poly up concrete sidewalls, but no need to seal, just secure to hold it in place. If it is not vented, the polyethylene joints need to be taped and sealed to the concrete sidewalls.

Please reply if you have any questions.

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Customer: replied 4 years ago.

I am happy with your answer. Will digest it and ask more intelligent follow up questions on how to remedy it.

Great Albert. Reply at your convenience.

Customer: replied 4 years ago.

1. I do not have pipes in the half basement. So that space can be a vented space.


2. The half basement is blocked off (a dead space of 900 sq ft). I have vapor barrier laid on the soil. The basement has a very small venting grill so it is poorly vented. I plan to tear off the old Insulation and put in


Owens Corning Eco Touch Kraft 3 1/2" R-13 Insulation.


Since the ventilation is minimal, I guess I do not need high R-values.





Sounds good. With a vented crawlspace, treat it as if it was an outdoor space. Recommended ventilation is best if you can get multiple openings, located close to corners, and separated from each other. This gets nice cross ventilation to keep the humidity levels down.

If you have mild winters then the R-13 is fine. If you have cold winters, I would install a thicker batt such as R-19, but the thickness depends on the joist size. You can find recommended batt thickness for joist size on the insulation packaging. Even with minimal ventilation, remember it is an "outdoor space" and to save on heating/ cooling costs you want to install as thick of a barrier as possible.

Dirt floor sounds fine with the existing vapor barrier.

Good luck with everything.
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Customer: replied 4 years ago.

Thanks. You answered all my questions


You are welcome Albert and thanks for the positive rating. Have a great day.