How best to handle moisture inside concrete block basement walls?
I have read most of your basement moisture articles but don’t see moisture in basement walls address directly. I have already re-graded around the house, fixed the gutters, installed an interior french drain, and run a dehumidifier. I keep the basement humidity very low due to mold concerns, no higher than 40%. When I drop the humidity probe into the walls it reads in the 70s.
Recently reading that Joe Lstiburek says its not necessary for basement walls to dry to the inside I assume insulating my rim joists with spray foam (so that the openings on the top of the block are covered) and coating my basement walls with waterproof covering such as Thoroseal, UGL Drylok, and Xypex (rather than insulating the entire basement walls considering I live in NJ which is in climate zone 6 or higher) is my best bet. Is that correct?
Just an additional point of information, since I installed the french drain no water enters via the walls or basement floor.
Thank you – Joe
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Replies
Joe,
First of all, it's normal for a below-grade concrete block wall in contact with the soil to have a high moisture content. After all, the wall is in contact with damp soil. Soil is almost always damp.
You didn't tell us whether your house has a layer of sill-seal between the concrete block wall and the mudsill. (This sill seal is a code requirement, but it is sometimes missing from older homes.) If your foundation lacks sill seal -- which acts as a capillary break -- you should be aware that there is a small but real risk that installing spray foam on the interior of the rim joist will increase the rim joist moisture level. In almost all cases, the benefits of interior spray foam outwiegh the disadvantages, but in homes with a high moisture level, some building scientists recommend jacking up the house 1/2 inch so you can slide metal flashing or rubber roofing between the top of the foundation and the mudsill.
If the outside of the rim joist is well above grade, and sunshine has access to the house, and there are no bushes near the foundation, I wouldn't bother jacking up the house to insert the capillary break.
In all cases, sealing the holes at the top of the block wall is essential. Stuff some fiberglass or mineral wool in each hole before you spray the top of the wall with closed-cell spray foam.
It's also a good idea to install a continuous layer of rigid foam to the interior side of the block wall. This will reduce your energy bills and reduce the amount of water vapor that enters your house. For more information on this work, see How to Insulate a Basement Wall.
Martin,
Thank you for your prompt response.
There is no sill-seal to act as a capillary break. The rim joist and floor joist sit right on the block. There is over three foot of block above grade around the entire house. The front of the house (about 30 feet) has a porch blocking light from reaching the above grade portion of the block wall. In the back there is a mudroom over a very small crawlspace that blocks a little under 10 feet of the block wall from sunlight. No bushes near the house.
Do you think I can insulate the rim joist?
I could go in a step-wise fashion and cover the holes at the top of the block wall first, leaving the rim joist open and checking for moisture build up at the bottom of the joist?
Re: rigid foam on the walls. I live in NJ (climate zone 6 or higher) and the referenced article states "if you live in Climate Zone 3 or anywhere colder, installing basement wall insulation is almost always cost-effective" so I am wondering if just coating my basement walls with waterproof covering such as Thoroseal, UGL Drylok, and Xypex is a cheaper and easier option. Moisture is my major concern.
Thank you again for your time.
Joe
Joe,
The advantages of insulating the interior side of your rim joist outweigh the small risk.
You quoted my article about why anyone in a cold climate (like New Jersey, where you live) should insulate their basement wall. (Climate Zone 6 is colder than Climate Zone 3.) So you know the answer to your question: yes, you should insulate your basement wall, because the work is cost-effective (meaning that you'll save enough energy to justify the cost of the work) and because basement insulation is code-required in your climate zone.
Q. "I am wondering if just coating my basement walls with waterproof covering such as Thoroseal, UGL Drylok, and Xypex is a cheaper and easier option."
A. That option may be cheaper and easier, but it won't be as effective at limiting moisture entry through your block wall.
Martin-
Thank you. That makes sense. I was thinking zones for plants, not housing codes.
That's the plan then - seal the gaps in the top of the block wall as you suggested, insulate the rim joists as outlined in your article, and then insulate the block wall as outline in your other article.
Thank you for the guidance. It is very much appreciated.
Joe