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purpasadu 10-26-2011 03:32 PM

Basement Exterior Above Grade Insulation
 
Hi everyone. I'm a longtime reader, but first time poster.

One room in my basement had issues which allowed water to penetrate the foundation in an area above-grade, which eventually led to a severe mould problem. I have since dealt with the water issue by completely replacing the portion of the above-grade foundation wall which was letting water in. It was so bad that when I took the sledge hammer to that portion of the wall, water started pouring out. Once it was opened, I saw that the empty cavity in the cinder blocks were full of water, just sitting there.

My question now has to do with insulating that wall. In its previous form, it had bats of fiber glass insulation, with a moisture barrier between the insulation and the drywall. That certainly caused a lot of the issues as it trapped moisture in its place. I have been doing a lot of research, and have read through just about every topic on this board dealing with basement insulation. What I have concluded is that exterior insulation is best and fiber glass insulation is always dangerous in a basement. I have also seen conflicting opinions on interior insulation. Some say that interior insulation is a bad idea, others say that interior insulation should only be above-grade, some say that walls and floors need to be insulated tightly to create a thermal break and other say that a gap should exist between the insulation and the drywall.

Here's what I hope to do and would like your comments. The room I'm redoing was a basement addition. The roof of this room is the concrete deck to the house. Ceilings are only about 7 feet high and less than 2 feet of the walls are above-grade. I was planning on gluing rigid board insulation to the ceiling and not insulating the floor or interior walls. I wanted to dig about 12 to 18 inches around the exterior and insulate the exterior walls above-grade only. By doing that, I hope that my foundation walls will be kept warm, by both the exterior above-grade insulation and the ground, but that if there is any condensation of moisture that comes in, there will be an air cavity between the foundation wall and the mould and moisture resistant drywall I install which should allow it to dry. I also plan to cut out a few holes in the drywall and place grills over them to allow airflow. By gluing rigid insulation to the ceiling, I will prevent a lot of heat loss (not so worried about cool air loss as I am in Canada).

To protect the above-grade exterior insulation, I thought about covering it with pressure treated plywood and perhaps stucco or parging.

Does this seem reasonable, or am I way off?

I appreciate anyone taking the time to read and respond.

Thanks.

Ron6519 10-26-2011 04:22 PM

Not the way I would do it. No reason to add insulation both in and out. Just add it inside.
Adding to the outside could have issues.
What is the relationship of the foundation wall to the side wall of the house? Most houses you cannot add that much thickness to the foundation without it sticking out past the house wall.
Post some photos of the wall(s) you want to do this with.
Adding insulation to the foundation, underground is done all the time. It usually protects the waterproofing membrane from back fill penetrations, so you don't see it.

AllanJ 10-26-2011 04:30 PM

Did you install a perimeter drain system, either on the inside or on the outside (with the drain pipe about at foundation footing level)?

Also it is very common to paint a waterproofing layer (similar to roofing cement) on the outside of the foundation. When ythiw ois this is done you do not put a moisture batrrier on the inside wall surface corresponding to below grade. You do put a moisture barrier on the isnide surface corrdesponding to above grade. So that the outside waterproffing is not visible, it usually stops a few inches below grade.

Any kind of insulation can be put against the ouside surface after the waterproofing layer is painted on.

purpasadu 10-27-2011 08:59 AM

thanks for those comments. I don't plan to insulate both on the inside and the outisde, I want to just insuulate on the exterior and only the portion that is above grade. No perimeter drain was installed.

purpasadu 10-31-2011 09:20 PM

I'm worried because I've got contractors telling me that I have to use rigid all along the interior to create a thermal break to prevent the warm air from contacting the cold foundation, causing condensation. That makes sense to me, but I'm also being told by a mason who specializes in foundations that you should never insulate the interior of the basement. He told me that insulating the above-grade portion from the exterior would keep the foundation wall at a good temperature and by leaving gap between the drywall and the cinder block foundation, I would allow airflow which would dry off any moisture. That also makes sense to me. Does anyone have any thoughts?

jomama45 11-01-2011 08:30 AM

I tend to agree with your mason, and would use rigid insulation outside, down to the frost level at least, if that's feasible. You really should cover it above grade with something like "ground break", aluminum, etc... to keep it from degrading rapidly form UV rays.

fix it guy 02-28-2012 04:42 PM

air circulation behind finished basement walls
 
Quote:

Originally Posted by purpasadu (Post 761132)
I'm worried because I've got contractors telling me that I have to use rigid all along the interior to create a thermal break to prevent the warm air from contacting the cold foundation, causing condensation. That makes sense to me, but I'm also being told by a mason who specializes in foundations that you should never insulate the interior of the basement. He told me that insulating the above-grade portion from the exterior would keep the foundation wall at a good temperature and by leaving gap between the drywall and the cinder block foundation, I would allow airflow which would dry off any moisture. That also makes sense to me. Does anyone have any thoughts?

If you sheerock your wall onto studs and don't seal space between top 2x4 of wall you will lose at lot of the benefit of your insulation. Also air circulation in basically an enclosed space will be little or none.


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