April 1, 2004

Abstract: 

Top ten blunders that rot your house, waste your money, and make you sick. Reprinted with permission from Fine Homebuilding Magazine, April/May 2004, pages 52-56. 

Top 10 Blunders that rot your house, waste your money, and make you sick

I've been a building scientist for more than 20 years. Before that, I was a builder. Even though I followed standard building methods, some of my houses were turning into drafty wrecks that were rotting on their foundations. What went wrong?

In 1983, I went back to college to get my Ph.D. in building science, the study of how buildings work and why they fail. Houses are more complex than they used to be. They have more insulation, they have elaborate heating and cooling systems, and they are built to be nearly airtight. These components can make comfortable, energy-efficient houses that serve their owners well. But if they're put together wrong, a house and its occupants can be in for some serious problems.

I've turned the basis of the mistakes I made as a builder into one of the lectures that I give at building conferences across North America. While I deliver these seminars in a somewhat lighthearted manner (sometimes to standing-room-only crowds), I can assure you that these blunders are not funny.

One: Vapor barriers on basement insulation will rot your walls

We expect a house’s walls to get wet during construction, either from the weather or from the materials (that’s why joint compound is called mud). But they also can get wet once the house is finished and occupied. Wet walls need to dry.

There are two types of wall that can get wet: regular walls above the ground and basement walls below the ground.

Additionally, there are two sides to a wall: an inside and an outside. It’s smart to design walls to dry to both sides, but drying to at least one side is a pretty important objective. A vapor barrier on the inside of a wall means that the wall can dry only to the outside. This is OK for regular walls in cold climates (like Canada, where there are only two seasons: last winter and this winter), but it’s not OK for basement walls, not in any climate. Basement walls can’t dry into the ground because—you guessed it—the ground is wet. Because basement walls can dry only to the inside, wrapping the inside of a finished basement wall with a sheet of plastic is a bad idea. Wet basement walls wrapped in plastic can’t dry. We must build walls so that they can dry during wet seasons (drawing facing page). By using extruded polystyrene (XPS) foam for insulation, you can eliminate the inside condensing surface and prevent water intrusion from the outside, too (see FHB #160, pp. 50-55).

Two: Vented crawlspaces are moist enough to grow mushrooms

In the old days, we didn’t insulate crawlspace floors, and we didn’t air-condition houses. Crawlspaces (especially the floor framing) were warmed by the houses themselves. Now that we insulate floors, crawlspaces are within a degree or two of ground temperature. During most of the summer, this temperature is below the dew point of the outside air, even up north.

Ventilating a crawlspace allows moist outside air to condense on cool crawlspace surfaces. Consequently, the ventilation air is wetting the crawlspace rather than drying it. It’s like opening a basement window in July: The walls sweat. And wet walls become moldy walls quickly.

The whole point of venting a crawlspace is to remove moisture. If we could import hot, dry air from Tucson to vent moist crawlspaces in Tupelo, venting crawlspaces would be a great idea. But for Tupelo air to vent Tupelo crawlspaces, the air needs to be dry enough to pick up moisture, and it needs energy (heat) to evaporate the moisture. This isn’t going to happen, and here’s why: Tupelo air isn’t hot and dry. Neither is Toledo air, Tallahassee air, nor Toronto air.

A crawlspace is just a mini-basement and should be treated as such. It’s like a basement for a troll. You should condition the air in your mini-basement. Make it part of the house because, despite what you may think, it already is. Heat it in the winter and cool it in the summer with a supply duct or grille (but ask your fire inspector about this). Don’t insulate the floor; insulate the perimeter and install a continuous ground cover to keep out moisture (see FHB #153, pp. 94-99).

Three: Ducts and air handlers in attics waste money and suck in bad stuff

Why put the heating and cooling system outside the space that it needs to heat and cool? We insulate walls to R-19 and ceilings to R-50, yet we slip an R-4 sleeve over ducts and call it good. Where were the adults when this decision was made?

What’s more, ducts leak. Let me rephrase that. Ducts leak a lot—about 15%. It’s like installing a large exhaust fan in the attic to suck conditioned air out of the house and pull unconditioned air in through the cracks. If radon were a valuable commodity, we could mine it this way.

And think about winter. Heating bills go through the roof, and escaping air heats the attic, melting snow and forming ice dams. Goodbye, heated air; hello, water damage.

You can’t make ducts and air handlers tight enough so that they won’t leak (don’t even think about duct tape). The least you can do is put them in a conditioned zone such as the basement or a conditioned crawlspace, or you can move the attic insulation up to the roof.

Four: Using joists or studs for ductwork rots framing

A panned stud or joist cavity is an air return that uses framing members, sheet metal, and drywall as ductwork (photo above). Why is this bad? Because the air is pulled through a leaky framing cavity instead of a duct, contaminants come along with it. The result is that air will be sucked from any leak available to equalize the pressure (drawing below). In a humid climate, the house will suck wet air into the wall cavities. If the house has a vapor barrier inside the face of the drywall, the water will stay inside the wall (air will make it through the cracks). Wall cavities are a bad place to store water.

If the stud-bay or joist-bay return is in the garage, pollutants such as carbon monoxide or vapors from gasoline or solvents can be sucked into the living space. If the air return is in a furnace room, combustion gases can be sucked out of the flue (backdrafting).

Here’s the test: Turn on the HVAC unit and spend a romantic candlelit evening with your significant other. Burn those cloying perfumed candles; the fine soot particles in the candles act wonderfully as tracers for airflow. Next day, look for stains on the carpet at the baseboards near a stud cavity air return. Air is being sucked into the wall under the baseboard, filtered by the carpet, and marked by the soot.

If we did plumbing this way, we’d flush our toilets into the floor framing. . .

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