"Good enough" moisture control in basement for insulation

Question

Summary: I would like to insulate the upper half of our basement walls with rigid foam. I know that our basement walls show some signs of excess moisture, but I'm unclear on the sources of that moisture, whether it is an ongoing concern, and what would constitute "good enough" remediation (good enough to install the rigid foam as described).

Details: We're in Massachusetts in a 1930 house, the soil in our area is classified as "fine sandy loam". We have gutters on all sides with downspouts that are extended a couple of feet away from the house. They're cleaned regularly. The surrounding landscape is reasonably well graded away from the house. The basement walls are built from cinder block units and there is a concrete slab. At some point they must have been painted by an old version of drylock-type paint.

Symptoms: About 50% of the wall area looks ok, with only some chipping paint . There is some minor spalling in the bottom two rows of concrete blocks in about 40% of the walls . One wall area, about 10%, has more significant spalling up to about the 7th out of 10 rows of concrete blocks.

I have to run a small dehumidifier to keep down relative humidity. I have used the “plastic sheet test” (taped a small piece of foil to the wall and checked for wetness on inside and outside surfaces), which came out dry on both sides.

I’m assuming that the spalling in the bottom rows is a kind of “rising damp” from below. Given the age of the house, I guess there is not much in terms of a capillary break. Confusingly, the area with the more significant spalling higher up on the wall actually abuts our mudroom, which has a vented crawl space on a dirt floor. I would have assumed that this would keep the foundation dry, since there is no way for rain to fall right next to that wall.

Proposed solutions: I’m not sure what else I can do on the exterior to reduce the moisture in the wall. My plan right now is to clean and patch the spalling concrete and then use drylock or a similar product. After this, I will glue rigid foam to the upper half of the wall, to only cover the above-grade portion of the concrete and the top 2’ of below-grade area. My hope is that this will allow the wall to dry in the “wetter” bottom rows.

Questions: Will the proposed solution keep the walls dry enough to install rigid foam? How will I know that I have succeeded in managing the moisture?

Answer 1

You’ve done the “plastic sheet test”, your gutters are not to blame, the lands is properly pitched and your soil has good drainage, so it’s unlikely that the water is coming from the ground. I say that the source of moisture is the warm, humid summer air condensing on the cold basement walls. Follow my explanation.

Massachusetts has cold winters and warm, humid summers. (I live in southern New Hampshire with a similar climate.) Your basement walls are uninsulated and cold. In winter, the upper part of the basement wall is coldest because the outdoor air is cold and the ground is frozen at least a foot deep. The lowest part of the basement wall is not quite as cold because the soil below the frost line is at a temperature of roughly 55F. So insulating the upper walls would keep the basement warmer in winter. But no matter: you don’t get condensation in the winter months because the indoor air is relatively dry; the dew point (the temperature at which condensation will occur) is lower than the temperature of the basement wall.

Things get more interesting in the summer. The upper part of the wall gets much warmer because the outdoor temperature is higher and the ground warms up. But the soil below the frost line is still around 55F, so the lower part of the wall is cold to the touch, and it stays cold all summer.

As outdoor air enters your house in the summer through the myriad air leaks of 1930s construction, it brings humidity with it. Summer temperatures in this region, especially in the mornings, are just a few degrees above the dew point. Any of that air that makes its way into your basement will ignore the warm upper walls, but upon contact with the lower wall, the moisture in that humid air will condense. You will have a microscopic film of moisture on the lower parts of the walls, and that moisture will be absorbed by the concrete. On especially humid days, there may be droplets and drips.

This is what has caused the damage to the wall surface primarily below the outdoor frost line: the constant wetness during your humid summers.

Solutions

If you can seal any obvious air leaks, then do so.

One small dehumidifier may help a little, but you would have to run multiple dehumidifiers to keep the basement air below 50% relative humidity or so in the summer. But this would cost a fortune in electricity.

You need to insulate your walls, and ideally, your floor too. Insulating just the upper part of the walls may keep you warmer in winter, but it won’t stop the condensation in the summer, which occurs down low. And you need a vapor barrier to keep humid air away from the concrete.

I think the ideal solution would be a layer of rigid foam on the walls to act as a thermal break, then a stud wall from floor to ceiling with glass fiber batts, followed by a continuous vapor barrier, then covered with gypsum board. Between that and the existing small dehumidifier, you won’t get any more condensation from humid summer air.