I'll try and keep this short (not likely): I have a cmu-walled house that has interior moisture problems. The northeast wall of the house gets condensation quite bad during the winter, usually on the lower 1/3 of the wall. I have not had a blower door test done. I know the home isn't incredibly tight, but all windows are caulked, the door weatherstripping is in good shape, all corners/joints/etc. on the ceiling drywall panels are caulked... so it's somewhat better than swiss-cheese.

I am on a very tight budget, and I need to reduce my heating costs, but more importantly, I want to reduce or eliminate the condensation on the northeast wall interior if possible. There's been so much in the past it sometimes drips down and pools on the floor.

The northeast wall does happen to be the uphill side of the home, but the soil level is at or below the slab level. Also, I installed a french drain system around that side of the house last year thinking that I had a moisture wicking problem, but even this spring, there's was only a trickle at the most coming out of the drain pipe on the downhill side. There may still be some moisture wicking, but I've come to realize that damp interior air is a much more likely culprit. I'm in a relatively dry climate- 6000'+ in northern CA mountains. We get snow of course, but it's not humid here at all relatively speaking.

So... I think I need to ventilate the interior air more, and I have ideas on how to do that, but this post is mainly about insulating the one wall (for now). I'm thinking about getting some 2" polyisocyanurate panels, and fastening them with some sort of anchors to the exterior of the northeast wall, to take advantage of the thermal mass of the cmu blocks for interior warmth. I'm planning to put something like Home Slicker or similar between the panels and the cmu wall so there is some ability for moisture to escape (I don't really need a drainage path/rain screen... but it would work like that as well I guess). I have some metal roofing panels I'd like to put over the outside to finish it off. Will look crappy but will cost me nothing for now.

Any thoughts on this? Anyone with similar experience?

Cold blocks cause condensation (and waste energy). I would add foam to the exterior and then cover it with stucco.

Where is the moisture coming from? this is the basic issue. I wouldn't be spending money on fixes until you have a good idea about that.

What is the floor and wall and roof construction? list all the components. Are there vapor barriers and/or insulation under the slab? VB and/or insulation on the walls? Sealant on the exterior of the wall below grade?

Floor is slab on grade. May be a vapor barrier, but probably not a very effective one if any at all. Probably no insulation there either. Flooring is old hard composite glue down stuff with peel-n-stick vinyl flooring over that. Walls are cmu only, with a few layers of paint on either surface. I doubt there is any fill in the cmu cavities, although some areas are solidly grouted (around windows, doors). Windows are dual pane vinyl frame, low-end models. They get condensation on the lower few inches as well when it's 30F or so and below. Exterior of walls are not below grade, but the footings are... and are not sealed. Roof is flat w/ torch-down surface. There is no eave ventilation currently (needs to be fixed eventually of course). I don't think the roof is leaking- I've inspected that closely.

While stopping the source is best, you may still need to run a HRV to get humidity low enough to stop window condensation.

If you are up for a larger project, you can line the interior side of the walls and the floor with plastic sheet and foam. Then cover with drywall and new flooring respectively. This will address both heat loss and most exterior moisture gain issues.

Venting the space between iso & CMU to the exterior doesn't buy you anything from a condensation point of view, with or without a mesh like RainSlicker in there. The vented gap would just lower the temperature of the wall below what it would be without the gap.


The fact that it's above grade and gets no mid-day sun on the northeast corner means that it runs colder on average than sunnier or below grade parts of the wall. The colder the wall is, thehigher propensity for condensation, and it doesn't take a super-high interior moisture content or ultra-cold outdoor temps for that to occur on an uninsulated CMU wall. Once it's insulated and air-tight on the exterior side, the insulation alone should take care of it, if it's a condensation issue.

Unless you have 2' over hangs there's a high likelihood of there being rainwater/spash-back issues keeping the wall wet too, which should also go away if you insulate on the exterior and put up some sort of siding over it. If it's a wood or fiber-cement siding there's a good rationale for including a rainscreen gap (or using RainSlicker to achieve back ventilation), but if it's vinyl it's inherently back ventilated.

Hollow core CMUs have very poor wicking power compared to poured concrete, and moisture problems on above grade sections are (almost) never from groundwater drainage. Below grade portions is where groundwater & drainage issues become the prime suspect.

The peel'n'stick vinyl flooring IS a vapor barrier, and a POWERFUL vapor barrier at that. If there's high interior moisture (TBD- in order to tell, monitor it with a cheapie $10 AcuRite, available at box stores or even Wal-Mart), it's not coming through the vinyl clad slab.

Dana, thanks for your input on the mesh layer behind the iso. That's one thing I'm a bit unsure of: how much moisture can actually migrate through the painted cmus (if any) and if I'm significantly reducing the insulation properties by providing a path for air to flow between the insulation and exterior of the walls. That's actually why I was thinking of the home slicker product- I figured maybe it would slow down airflow enough so that the wall wouldn't be cooled too much, yet still allow for any moisture that does happen to get in there to drain and/or dry out. Oh, and I'm 99% sure there's no rain or splash-back issues at play here. For one, I've never even seen the exterior walls get wet during storms, but also, the interior moisture problem was quite bad this last winter, even after over a month with no precipitation at all... but the temps were in the single digits.

Thanks for the info on the wicking characteristics of cmus- that's good to know. Also, I'm going to get a moisture meter as soon as possible. I'm very curious to see what I'm actually dealing with. There is no bathroom fan nor vent hood over the range, so I've actually started running the clothes dryer on the "air fluff" setting as a simple means of ventilating the house. Seems to be effective so far.

I just need to figure out what layers to do on the exterior... maybe I'll use Tyvek drainwrap rather than Home Slicker.. It appears to provide less room for airflow.

"...temps were in the single digits...""

In an ~R1.5 wall like uninsulated CMU even in a heated 68F room single-digit exterior temps put the interior face of the CME near the freezing point, (if not below). "Healthy" 35% RH/68F air has a dew point of about 39F, so it's not surprising to see copious condensation on above grade exterior walls (and windows) when it's in single-digits outside. If the interiorface of the CMU was not painted much of that moisture would be adsorbed into the concrete rather than condense, but with a painted interior (even with cheap latex) the diffusion rate of water vapor is slowed significantly and it'll condense on the paint. But with even an inch of foam on the exterior the surface temps would be well above the dew point in a heated house, and it'll stay dry.

I've actually started running the clothes dryer on the "air fluff" setting as a simple means of ventilating the house. Seems to be effective so far.

A HRV will be much more energy efficient.

jonr- thanks for mentioning an HRV- I had never even heard of those before. I think one of those would probably help my situation considerably.

Dana1- that is great info on the dew point and effect of insulation on the wall temperature. I have a lot of work to do on the house to make it more comfortable/efficient, but it certainly sounds to me like insulating the exterior will get me the most bang for the buck right now. Unfortunately, I'll probably only be able to do the worst wall this summer, but assuming that noticeably improves the situation, I'll tackle the rest soon after.

Thank you both for the help.