GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon
Q&A Spotlight

Solving a Basement Odor Problem

Does this basement wall lack a vital moisture control barrier?

A foundation renovation included exterior damp-proofing and interior closed-cell foam. So what is the source of an odor of damp masonry in one room of the house? Illustration courtesy of NgCanuck.

Henry has completed a basement renovation at his 1950s Toronto bungalow and is now grappling with an unexpected problem: One room of the house smells strongly of old, damp brick.

From all appearances, the renovation was done carefully. The foundation wall, consisting of concrete block with brick above, now includes exterior waterproofing, damp-proofing membranes, and a layer of closed-cell spray polyurethane foam in an interior 2×4 wall.

And yet, he writes in a Q&A post, all is not well.

The odor is coming from the floor and/or wall of one of the rooms in the northeast part of the house, which is located directly over the basement laundry room.

“I am not getting the smell anywhere else on the first floor,” Henry writes. “It seems to be strongest when the temperature changes drastically between inside and outside, i.e. the weather has been cooler the last few days and it’s less noticeable, although I haven’t correlated a specific trigger for the smell.”

There’s no smell in the laundry room itself, and he’s found no evidence of either a leaky dryer vent or water leak. Nor has an extensive check of the attic and basement walls uncovered any visible water. Henry is beginning to suspect the cause is an uninsulated air gap at the perimeter of the floor framing, as suggested by the drawing at the top of this column. He’s thinking of adding more spray foam to close the gap.

Is he right? And if so, what should he do about it? That’s where we start this Q&A Spotlight.

Insulate and add a capillary break

Zephyr7 suspects the odor is indeed coming from the damp block via the gap shown in the drawing. Because the block is sealed on the inside by a layer of closed-cell foam, the wall is staying wetter longer. So, he suggests, go ahead and seal the gap with spray foam, but make sure to add a capillary break between the top of the masonry wall and the rim joist.

“What you don’t want to do is treat things with spray foam in a way that keeps your rim joist wet with no possibility to dry out,” he says.

Zephyr7 explains that a capillary break is a piece of water impermeable material that prevents water from wicking out of the masonry wall and into the wood. He’s had good luck with using pieces of high-density polyethylene, which slides easily into small gaps.

Others have recommended pieces of EPDM, a rubber-like roofing membrane. It’s more flexible, so a little more difficult to stuff into small gaps, he adds, but it’s readily available.

The moisture can only go up

Concrete block has a powerful capillary draw, adds Dana Dorsett, and without a break between the footing and the walls there’s only one way for the moisture to go—up.

“If there isn’t a capillary break between the footing and the walls, with closed-cell polyurethane limiting the drying toward the basement, the only way for the moisture to go is up,” Dorsett writes. “With sufficient unpainted above-grade exterior exposure (at least a foot, two feet would be better) it may be able to dry toward the exterior.”

Moisture in the brick veneer also could be contributing to the problem. Henry should be able to see weep holes every third brick or so near the bottom of the wall, and vents every third brick or so at the top of the wall, to create a convective drying loop. If not, adding them could help to mitigate the problem, Dorsett says.

Will adding weep holes create a problem?

Henry can’t find any evidence of weep holes or vents in the wall, and he wonders whether adding them now would court unexpected problems with moisture.

“I’ve had a conversation around weeps and vents,” he says. “The only catch is if they are installed during construction my understanding is that there should be a weep sill or something to allow any moisture/water to run out behind the brick. In the current brick, there [are] no weep holes so I’m pretty confident no weep sill. As part of a retrofit, would one need to be installed?”

That’s a question will have to leave for our resident technical expert, Peter Yost.

Does the block really need to be dry?

To Jon R, Henry’s situation is a good example of why a more vapor permeable foam, such as unfaced expanded polystyrene (EPS), would be a better choice. The reason is that it would allow more inward drying than closed-cell polyurethane foam and result in drier block.

What, asks Michael Maines, “so you can dump moisture directly into the basement in perpetuity? No thanks. The block doesn’t care if it’s damp.”

Well, Jon R says, the block may not care if it’s wet or dry, but the homeowner certainly will.

“I’ve been in plenty of damp basements,” Maines says, “and find that owners prefer them to be dry.”

Dorsett agrees with Jon R, noting that the rate at which moisture comes through the wall, even with 1 inch of Type II EPS, is comparable to what comes through a coat of interior latex paint—that is to say, not much.

“Jon R is right,” he says. “It’s insignificant in terms of what happens to the indoor [relative humidity] in the basement, but makes a real difference in the moisture content of the wood in contact with the top of that wall. Even a tiny amount of ventilation with dry conditioned air from upstairs keeps the basement humidity in check, if diffusion through the foam is the primary source of moisture.

“In the ‘-A’ climates [that is, the moist, eastern climates on a standard climate zone map] leakage of high dew-point outdoor air in summer would typically deliver an order of magnitude or more moisture than a whole foundation wall would through 1-inch Type II EPS.”

Our expert’s view

Let’s wrap this up with some thoughts from GBA Technical Director Peter Yost:

I spent a bit of time emailing with Henry (original poster of this Q&A Spotlight) to make sure we had the right assembly and to get more information about the building.

Henry’s brick is structural, not veneer. Henry checked in with more than one local mason who said it was not brick veneer. I asked Henry about this after he said there were no weeps in the brick. While we don’t know the exact configuration of the above-grade exterior wall, it’s probably two or three wythes.

The house has a concrete block foundation wall topped by structural brick. The window sill and the well below the window are potential problem areas.

This makes his exterior wall a mass or load-bearing masonry wall. Brick veneer is moisture-managed with a dedicated air space between the brick and the rest of the exterior wall and weeps to let any water that gets behind the brick a way to get out. Mass walls rely on a combination of deflection (overhangs, for example), hygric capacity (the masonry safely holds quite a bit of water), and finally in Toronto, lots of heat flow that protects the masonry wall from freeze-thaw damage.

Building Science Corporation has a number of information resources on interior insulation retrofits of load-bearing masonry walls. In one (BSD-114: Interior Insulation Retrofits of Load-bearing Masonry Walls in Cold Climates), John Straube says this about the moisture balance in these types of exterior wall assemblies in cold climates:

“For a moisture-related problem to occur, at least five conditions must be satisfied:
1. A moisture source must be available.
2. There must be a route or means for this moisture to travel.
3. There must be some driving force to cause moisture movement.
4. The material(s) involved must be susceptible to moisture damage.
5. The moisture content must exceed the material’s safe moisture content for a sufficient length of time.”

Henry’s home is single-story with substantial overhangs and gutters. Given the list above, I am pretty sure that Henry’s building configuration—one-story with big overhangs—means that his exterior load-bearing walls don’t get wet enough for freeze-thaw to be a problem. And Henry’s problem to date is very localized; it’s a point-load moisture issue rather than a more generalized situation.

Having said that, if Henry can easily decrease the wetting of the least protected portions of his mass walls—the three courses of concrete block above grade—he should go ahead and do just that.

He is a bit lucky. You can readily paint concrete block to good effect to reduce bulk water wetting (and also select a paint that is vapor permeable for some outward drying potential). With brick, paint does not work very well because the hairline cracks formed with time between the brick and the mortar can’t be spanned by exterior paint. Even silanes and siloxanes—typically recommended for increasing the water resistance of exterior brick—have very limited crack-spanning ability.

I think the damp smell is driven by air leakage. I can’t prove this but the fact that it is local, and the picture Henry posted, point to air leakage to me. To prove this, Henry could get a blower door test done on his home.

In our ongoing email exchange, Henry and I also discussed the several utility penetrations right at this corner of the building (Henry says the smell is more toward the corner than right at the windows in the basement and first floor). Going after exterior and interior air sealing of these penetrations is a good idea.

But his block foundation wall is wet, at least it’s wet close to where the odor is. Again, localized. So, if air leakage is the source of the odor, what is the source of wetting? In the photo Henry posted of the corner with the downspout and compressor, you can see the dryer vent exterior exhaust; the window well that extends out just about as far as the gutter and overhang; and a double-window with an exterior sill of “rowlock” brick.

The dryer vent located this for me as the area with the odor problem. The window well and rowlock brick are two potential sources of bulk wetting. The window well simply extends out to and maybe beyond the deflecting overhang and gutter (Recently, Henry said it was raining really hard in Toronto and indeed the outer rocks in the window well were wet).

The rowlock is a classic problem: you can get away with a leaky-as-a-sieve sill made up of porous bricks and even more porous mortar at the base of a window with brick veneer but not for load-bearing masonry. All the water that the glass sheds can super-load the exterior sill with water. If you look at correctly detailed mass wall sills, the sill is typically impervious masonry like granite, metamorphic stone, and sometimes it is even metal.

This window sill is impervious to water. Photo courtesy of Contractors Masonry Ltd.

My bet is that if the rowlock proves to be the problem, it’s on the windward side of Henry’s home (circumventing the deflection provided by the overhang and gutter). But frankly, I am betting on the window well being the source of moisture for the damp smell.

In our ongoing email exchange, Henry states that indeed the rocks in the well do get wet when it rains, but there are drains for the window well. A simple test would be to purchase a clear plastic window well shield and see what difference that makes.

Michael Maine’s cool use of the psychrometric chart in this Q&A: I called Michael to talk this through with him. I think in addition to the psychrometric chart—which relates the vapor content and temperature of air volumes to vapor pressure—we need to look at the equivalent moisture content of materials and how that relates to vapor pressure of adjacent air volumes. But since we think the odor is an air leakage problem, we are going to leave this super-interesting topic and Michael’s numbers for another day.

-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.

0 Comments

Log in or create an account to post a comment.

Related

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |