Latest Discussions : Basement & Foundation


11:35AM | 12/27/04
Member Since: 02/26/01
35 lifetime posts
I am finishing the basement of my 1937 Northern Virginia house and have read about the insulation issues until I’m blind, particularly the discussion on this thread,

I have one question and here are the background facts:

My partially below grade basement is relatively dry in that there is no water. Also, no incoming moisture from the floor (did the plastic-taped-to-the-floor test and no condensation). The wall that is below grade has a small band of mold (approx. 6 inches high) that has been there for years without getting smaller or larger. Also, initially, it was humid in the summer as the rear walkout door and door to the garage didn’t seal well. I solved that last year and bought a de-humidifier that kept the humidity at about 50% throughout the summer. I don’t know if the mold was caused by the humidity or moisture permeating from the ground, but I am assuming it is both. Drylok on the wall is not an option as the original owner painted the wall and I understand Drylok must be applied to unpainted block. I cleaned the mold with bleach and it has not come but I suspect that it would take a while to reappear even if it is from moisture permeating the wall (I suspect the dry winter air retarded its growth and kept it in check all these years).

When I frame the wall, I intend to offset the frame such that there is about a 2 or 3 inch gap between the foundation wall and stud wall. I intend to install faced batt insulation between the studs, install a sheet of extruded polystyrene over the stud wall, the hang the drywall over the extruded polystyrene sheet. (The ceiling will be insulated with unfaced batt insulation in the joist bay in the finished section.) I don’t plan on installing a sheet of polyurethane over the block foundation wall since I’m assuming some moisture is penetrating the wall and would become trapped between the polyurethane and the foundation wall. The envelope between the foundation wall and stud wall should allow for any moisture to evaporate, particularly since I intend to continue using the dehumidifier in the part of the basement that is not going to be finished (i.e. where the furnace and water heater are). Although not required (since it has a window to the outside) the bathroom will have an exhaust fan that will run when the light to the shower area is on, thus eliminate the shower as a moisture source. Also, due to the age of the house, the basement does not have A/C ducted to it (the new A/C air handler resides in the attic with no way to duct into the basement), thus a window unit will cool in the summer. Heat will be from either electric baseboard or a branch from the hot water radiator system (probably baseboard type).

The question, is there anything seriously wrong with this arrangement? I think that given my particular geographic location (humid summer, dry winter) I have addressed the moisture/mold source problems. All comments are appreciated, particularly since that in order to comment, you will have read this rather long post!


07:16PM | 12/27/04
Member Since: 11/27/04
172 lifetime posts
just a comment on the air gap behind the stud wall. if there is no insulation behind your studs then there sets up a convection loop that can cause the cold and moisture to settle to the bottom of the wall. cold air falls, warm air rises. i would put styrofoam(1 inch) against the wall then strap the wall with flat 2 by 4's to fit 1 and a half inch styrofoam in between the flat studs)drill concrete screws in to hold .then a 6 mil vapor barrier to truly seal the wall from inside moisture(note: on the unfinished ends,you need to seal the opening with accoustical caulk,plus vapor barrier on the rim joist with r20 insulation). this will create no organic matter for mold to get a hold on.

and put the styrofoam up to the top of the concrete.moisture will escape on the outside if it's not burried above the typical 8 inches below the sills. and the usual of the ground sloping away from the foundation.

oh and reading that post is that there is a system of plastic against the foundation up to the soil level of outside(to allow moisture to escape out of the foundation at the top). then the platic wraps around the bottom of the stud wall and up on the inside up to the top of the stud wall/rim joist.

and never use the black foundation tar on the inside. this is an exterior product.


06:03AM | 12/28/04
Member Since: 02/26/01
35 lifetime posts
theeagle, thanks for the response. I have a couple of follow up questions. It seems as you are describing a system that eliminates an air gap between the foundation wall and the warm side of the stud wall, which in turn, elimates organic matter for mold to grow on. But, if the 6 mil vapor barrier is on the warm side of the stud wall, doesn't that provide mold with the wood to grow on?

You also mention "if there is no insulation behind your studs then there sets up a convection loop that can cause the cold and moisture to settle to the bottom of the wall." I agree with that statement but there my post relied on two other facts I omitted that might impact teh convection/condensation issues. First, the house sits on a slope so regardless of how I grade the yard, there will be moisture potential from uphill. Also, the house is almost 70 years old and although I have sealed outlets, etc. it is anything but a tight house. As a result, it seems there will always be some amount of air circulation in the basement, even in the air gap (or envelope as I have seen it referred to), particularly since the dehumidifier will be operated in the summer.

I greatly appreciate your input and additional comments you can add to my issues. I want to make sure this project lasts as long as the rest of the house will.


05:02PM | 12/28/04
Member Since: 01/28/03
693 lifetime posts
Buildimg codes prohibit studs from being any where closer than 1-3 inches from a masonry basment wall and also prohibit 6 mil poly sheets from being placed in any framed wall (United States) and against any masonry basment wall unless the sheet terminate into a peripheral foundation drain or the sheets terminate over the footer and under the basement concrete slab.


06:41PM | 12/28/04
the vapor barrier is on the "warm in winter side" to keep the moisture from the house getting into the wall. the moisture from the foundation side is another matter, which can be tricky if the foundation leaks from time to time. but not having fibreglass insulation in the wall to collect moisture is always a bonus.

some people will put metal studs in the basement ,but these studs can rust over time.

the thing always is if you do or don't leave a gap, then once you drywall then any moisture in the wall will never be known unless you get a trickle of water out the bottom. some people with new homes will wait a year to finish the basement in order to make sure there are no leaks. another alternative(even though more cost) is to dig up the outside and put water proofing membrane on the outside(rubber or a heavy dimpled plastic membrane).but most people are reluctant to tear up a 70 year old yard and any porches in the way.

if you are concerned about a water problem then you can do as part of what "homebild" suggests with the heavy dimpled plastic on the foundation inside(plantin is one name)starting 8 inches down on the foundation and then go with an internal drain tile(open the concrete edge ,put in drain tile and gravel,tuck the plastic down into the trench and re concrete, and have the tile go to a possible floor drain). then no future worries about water leaking in through the foundation and causing water problems in the framed wall (the gap at the top will allow the inside moisture that gets in the wall to evaporate out the top). if your concrete is old and thin then it is not too hard of a job.

there are lots of methods out there. and all depend on the foundation and any leakage from it.

good luck...


06:54PM | 12/28/04
Member Since: 11/27/04
172 lifetime posts
great, the bbs ate my post and logged me off .should of copied before i posted.

you could put in an internal drain tile with the heavy dimpled platic on the inside of the foundation starting 8 inches down from the top and tuck it into the trench that terminates into a floor drain. .dig up the outside edge of the floor and put in the 4 inch perforated tile and fill trench with rock and re-concrete. this will allow all foundation leaks to go into the drain tile instead of building up behind your finished wall. pretty easy job if the concrete is old and thin.

the 6 mil poly on the finished side helps stop the house moisture from collecting in the wall.

but all the systems you read about depend on the foundation and if or when it leaks.there are a lot of finished walls out there that are damp or wet behind that never show until water comes out from under the framed wall. and fibreglass insulation is a great sponge until it gets full.

good luck .


07:43PM | 12/28/04
Member Since: 11/27/04
172 lifetime posts
i know the plastic one is not the best alternative, but reading the post

shows that alot of people have basement problems and all sorts of methods to cure it when finishing the basement.and some not so good.

even new houses are not sealed properly due to town planners/inspectors not knowing that foundations will eventually leak with damp-proofing. the foundations should be sealed with rubber water proofing or the heavy dimpled plastic membrane to stop any water penetration. i have seen some block foundations ,that are not filled with concrete for strength, have small drain pipes in the bottom of each block cavity going into an internal drain tile in an effort to catch the leaking water.


04:05AM | 12/29/04
Member Since: 02/26/01
35 lifetime posts
homebuild & theeagle:

Thanks for the input and advice. Homebuild, I didn’t even think about consulting the building code. I have a copy (I believe it’s BOCA for my jurisdiction) so could I trouble you to point me to the section on the stud wall distance and, less important, the prohibition against 6 mil poly sheets. It more than likely will have more information I need to know before finalizing my drawings for county permit review. (The county permit folks here are great to the do-it-yourselfers and will help on not-so-obvious details as long as you have done your homework in advance.)

Again, thanks for all the input. It helps set those of us who insist on doing it ourselves (and doing it right) on the right track.


02:32PM | 12/01/15
I am planning on constructing a wall in my basement by attaching 2x3 studs directly onto concrete and then dry wall on top. They're will be a 1" gap at the bottom between the dry wall and concrete floor and an air gap at the top along the unfinished ceiling joists. Do I need to add any plastic since I'm not sealing anything? There will be no insulation, this is purely aesthetics.


02:46PM | 12/01/15
I should add that we don't get water in our basement so my concern is with attaching wood directly onto concrete. Since there is an air gap at the top and bottom I assume moisture will not collect?
Basement wall


05:25PM | 02/13/16
We have a newer home and drywall in basement. We started seeing mold at bottom of drywall after I closed the a/c vents last summer. If I cut vent holes in drywall to let air circulate, will that help with the mold issue allowing the drywall to breath? I was going to put return vent screens in the wall every 8 feet to let air from home and behind the drywall circulate. Will this cause mold or any other problems?


11:29AM | 05/16/16

We just bought a house in Calgary, AB, Canada and notice that the basement is below grade and is insulated with fiberglass batt. There are no drywalls at the moment; hence, I have access to the vapor barrier and the fiberglass batt. The house is 10 years old and the basement looks dry. I notice that there is no air gap between the foundation wall and the fiberglass batt. Basically, the fiberglass batt is touching the foundation wall and the framing studs are 2x4. I was thinking of putting a 1x6x6 treated wood fence board per cavity box to distance the fiberglass batt from the foundation wall. This is not the best solution but I believe it may be the cheapest and quickest. So one side of the 1x6x6 treated wood would be touching the foundation wall and while the other side will be touching the fiberglass batt. I am hoping that the thickness of the fence board will create the approximately 1" air gap, see attached picture. Please let me know your thoughts and comment.
Basement wall


01:32PM | 01/25/20
That’s a great article.

I Have a question about my insulation approach. I framed at 4 inch away from them wall because of a pipe running horizontally.

Can I put foam boards and close with drywall, leaving air gap between the foam board and the drywall or should I use unfaced fiberglass not touching the concrete wall, leaving an air gap between the concrete wall and the fiberglass?

What approach is more suitable for my case?



12:51PM | 05/08/20
Are you me? Only difference is I have fiberglass bat all around, and no extruded polyurethane.
The only thing extra I'm doing is install a psuedo closet to house a dehumidifier for the in between space. I'm in the middle of my DIY but I'm curious if one large unit is enough or considering the twists and turns of the "space between", should I install a tiny one in every room. Also curious if anyone's had issues with it sucking up any floating fiberglass bits. No answers anywhere thus far.
I also have a tiny access door in my drywall for the most random hose sprocket, but it will also house a hydrometer so I can check on things.

Also I installed my 6mil plastic directly on the cement, from floor to top, and passed inspection. I figure if any mold or mildew grows from the cement moisture, at least it's behind the plastic, right?

Any gaps over, under, or in your drywall will allow the space between to breath. Breathing is good, but also allows sound to penetrate to adjacent rooms.

Don't place the fiberglas/insulation directly on the cement. Not only will it absorb moisture collecting on the wall, it will directly collect and convect the temperature you were supposed to be insulating against.

But no matter what, run dehumidifiers anyway


12:53PM | 05/08/20
^the above post was supposed to have paragraphs for separate ideas. That apparently got lost in html translation

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