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drjenkins 09-30-2012 07:09 PM

Insulation and moisture in basement
Hello all. First post here. I'm thinking of finishing my basement. It's about 25'x48'. Walls are concrete block. There are no interior walls or structures in the basement at the moment, other than the main beam with some posts under it and the stairs into the basement. There are 3 tiny windows at the top of the walls around the edges.

I have a french drain with a 2" gap around the edge of the floor at the walls. This connects to the sump pit where the pump is located. I have a single dehumidifier going 24/7 that keeps the humidity below 35% even in the rainiest of whether. There has NEVER been any water on the floor in the basement, although the sump pump does kick in every so often.

I've done a moisture test by taping film to 5 different areas of the walls and the floor and observing through a number of soaking rains (1-2 days of rain). No moisture on either side of the film. Not a drop.

Now, the issue that I have is that during heavy rains, meaning 5 days of soaking rain, I do get moisture coming through the concrete block at the bottom of the wall in a few areas. The wet spots are in the first or second row up from the floor. Any moisture goes into the french drain, so again, there's never any water on the floor. These areas have gotten wet only once in the past 4 months.

What are my options for finishing a basement that has these wet spots? I'm not going to dig around the entire foundation to improve the french drain, as that is way too expensive. I spent some time this weekend to see if there's anything I can do with the grading outside, which I may be able to improve. But I'm not expecting it to solve the issue outright. I could seal the concrete block, which may force the water to drain downward through the block into the french drain (hopefully?). I had thought leaving an air gap to allow it to dry might solve it, but I read elsewhere on this board that that's not a good idea. What other options are there?

Note, aside from this moisture issue, I wanted to use 2" foam boards all the way around the wall for insulation. But, obviously, I want to make sure that this moisture issue is properly addressed before I do anything.

Any suggestions are greatly appreciated.



Windows on Wash 10-01-2012 08:50 AM

If you have an internal french drain system, you need to provide a drainage gap to divert moisture to it. Putting rigid board tight to the foundation walls can be a problem.

Most bulk moisture issues through walls are a result of poor moisture management (i.e. grading and drainage) outside. Most people assume that they have a high moisture content and it cannot be helped, however, I have yet to see a home that could not eliminate 95% of the problem with some gutter re-routing and re-grading.

If you are going to drain to the inside, you should leave and air gap/drainage plane between the wall and the insulation. You could rip down strips of the foam and apply the larger sheets to the strips and air seal them. This would allow for drainage and management of the moisture while still insulating and air sealing the interior space.

concretemasonry 10-01-2012 10:12 AM

You do not have a french drain, but just a trench around the basement waal to collect the moisture that has already leaked. - It was common in many eastern areas years ago.

The good is that it is cheap, collects the water and directes it somewhere to be removed. The bad is that is a source for moisture and dirt and debris can plug it if it cannot be observed or kept clean. I had a friend that got mice into the rim joints area and droppings/gnawing debris eventually plugged the little trench.

The fact that the walls are leaking moisture, means there is either poor drainage, poor exterior moisture barriers or no exterior drain tile or it has plugged/failed.



Gary in WA 10-01-2012 10:29 PM

If sealing the concrete (Drylok or ?), the moisture (on concrete wall side) will slowly gravitate through the blocks down to the footing/wall joint, wick inside and end in the trough. You need to leave the wall side of the trough unsealed as well as the wall/floor joint to permit the blocks to drain. You would slow the moisture from showing in one small area and spread it to other blocks from wicking and capillary action. If no sealing, the moisture will slowly saturate the foam board wherever it surfaces and if excessive, drain down on the foam board side at the (concrete wall inside face) to pool at the bottom and eventually drain (wetting the concrete slab and wicking to the trough). Either way, you need foam to insulate the concrete to prevent summertime condensation on that interface. The f.b. should be glued to prevent room air from reaching the blocks- imperative. BSC showed (2002) f.b. glued in a few dabs (in red) to hold, fig. 15:

In 2007, they showed full length glue (in red), realizing the greater adhesive pattern, the better to stop air, fig.1: Read the text to the left on “periodically damp”. So, no air gaps at the concrete/foam interface for convective loops and to spread any air leaks/condensation to the whole wall, rather than confine it to a small area. Use close together, vertical glue patterns (no horizontal ones) to allow any excess moisture to move downward to drain; The glue will give enough room for excess water movement yet stop air movement, no other added spacers, Any larger air space and the foam board wouldn’t effectively insulate the colder block wall and result in convective loops. You could substitute poly sheeting for the vapor barrier/retarder or foil-faced polyiso foam board, just so long as; there is foam to insulate, no air interchange, and no moisture-sensitive material in the sandwich. Canned foam under the f.b. also the joints (or mesh tape and mastic), poly sill sealer under the p.t. bottom plate and air seal the rim joists;


drjenkins 10-03-2012 11:47 AM

For years I've been thinking that the grading outside was not going to be a large impact on the moisture. However, it just now occurred to me that the spots in the basement do line up with changes in the grading outside the foundation. I'm going to first work on the grading outside this weekend and see if that makes a difference. I'll extend the gutters, as well. Can't hurt.

I'll then look at ideas for the foam board and making sure that the moisture has a place to go. Yes, it occurred to me that I can't cover over the trench, but it's good to hear confirmation.

Thanks guys. I'll update here in a week or so.

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