A few days ago there was a topic of insulating basement walls. There
was discussion of what was best. Someone pointed out using 1" XPS foam
board directly on the concrete walls, then using R13. . So now as I am
gutting out 1 room of my basement, I have a poured concrete wall, and
i'm trying to figure out what is best.
I had old wall paneling and old insulation from when the house was
built in 1960. When I removed the paneling, it did have some mold, but
I think that this was due to the fact that the wall was 2X3 and the
insulation and studs was tight against the walls, not allowing the
walls to "breathe". The concrete walls are in good shape. No signs of
real water problems.
My plan was to use build a new 2X4 wall, but leave about an 1" or so
away from the walls to allow the walls to breath and also so the r13
insulation would not touch the walls. I was also planning on using
"Thoroseal" on the walls to seal it good. Was wondering if its better
to leave a gap behind the walls and besides losing an inch of real
estate, any downside to doing it like this? Also am I still better
using 1"XPS on the walls?
Suggest you read this:
The method you describe can work under the right conditions, but the
foam method works better over a wider range of the conditions often
seen in basements. You don't say where you are located; the discussion
below assumes cold winters.
Just leaving an air space between the insulation and the walls only
1. you have a very good vapor barrier on the warm side of the walls so
no warm inside air reaches the colder wall. This is hard to do,
especially with wiring penetrations, etc.
2. You have no water vapor coming in from the outside. The original
thoroseal (not the latex base) is pretty good, but not perfect. OTOH,
poured concrete walls are usually less porous than block walls, so
that's in your favor.
The air space just doesn't allow enough circulation to allow the space
to dry if there is any source of moisture, and since you have to worry
about moisture from both inside and out, it very hard to achieve that.
The foam method eliminates the concern about warm inside air reaching
the cold wall, and also blocks vapor transmission from the outside
wall to the inside.
No system will work if you have liquid water coming through the wall.
That you have to fix first.
Yes, that is the link I read a few days ago. Very interesting. I live
in Long Island,NY. Hot summers, cold winters. My basement is about 5
feet below grade. Is the XPS foamboard resistant to moisture/mold?
Also you mentioned a few days ago using 1 or 2" XPS with unfaced
insulation. Is there any harm using Kraft paper insulation with the
On Wed, 9 Sep 2009 06:12:27 -0700 (PDT), Mike rock
XPS is not harmed by moisture and does not support mold growth.
Best not to use kraft faced insulation for two reasons. First, it is
a second vapor barrier and you can get get moisture trapped between
the two barriers. Second, the paper can support mold growth if it
If you can't find unfaced insulation, or you already have kraft faced,
it is pretty easy to strip the kraft paper off the fiberglass.
Cheap method, that can be easily reversed by next owner if they so
desire. My idea was to hang a foamboard curtain from sill plate (taping
the vertical seams), using lath and screws, down to maybe a couple
inches above the slab. I want bottom loose and open so any water leaks
are quickly obvious, and the panels quickly removable if needed.
However, nobody around here carries the fire-rated panels (Thermax?)
that you don't have to put drywall over. I can't find any online sources
that include a price list. I mainly want this in the unheated original
garage bay, now buried under 2 bedrooms, that has a lot of foundation
showing above grade. The rest of the basement, I can get warm enough by
just redoing the band joist insulation above the sill plate, caulking
cracks, and making better foam plugs for the leaky uninsulated windows.
On Wed, 9 Sep 2009 05:27:53 -0700 (PDT), Mikepier
I doubt if the "Thoroseal" is going to help at all. It would
appear that existing poured concrete is about as waterproof you are
going get with cement based products. A little ventilation might
help, but a sealed air space between the outer wall and the new wall
is not going to do anything.
Any real water problem needs to be stoped from the outside.
My suggestion is to be sure not to use any materials that will
feed mold behind the finshed surface and venitilate the basement area.
I suggest that you may want to add additional insulation on
the upper half of the wall. The bottom is generally your friend, warm
in winter and cool in the summer.
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