Air Infiltration in Walls

Even if you wear a thick, $500 LL Bean sweater, you’ll freeze to death if a cold wind blows through it. The same is true for walls. You can insulate the heck out of a structure, yet if you don’t stop air infiltration, the insulation’s R-value will be severely diminished. So, before you start looking at high-tech thermally responsive batt encapsulations and drainable housewraps, remember one thing. The first step to designing and building a wall system is the same today as it was in 1988 or 1788: Control air infiltration while managing moisture and water vapor.

This semester, I have been teaching a graduate seminar at Norwich University School of Architecture & Art. In our wall-design section, most of the students came in with proposed wall systems that had one of two features: 1) They were entirely sealed with vapor barriers on the warm and cold sides of the wall “to keep all moisture out of the wall,” or; 2) They had no vapor barrier or weather-resistant barrier (housewrap) “so the wall can dry out with air rushing through.”

What these designs lack is balance. One probably would have rotted the wall out over time, and the other would have been a very cold building to live in, with the wind whistling through. Indeed, a wall has to be able to dry out and dry in, and there are some great products out there to accomplish this. Rather than discuss brand names, we’ll look at principles of air infiltration and weather-resistant barriers.

The ideal strategy will block air infiltration, yet let water vapor escape. But note that there is a difference between a vapor barrier and a weather-resistant barrier, also called housewrap. Housewrap allows water vapor to pass through it yet blocks liquid from entering a structure. On the other hand, a vapor barrier blocks moisture from passing through yet it can actually trap water unless it’s used appropriately.

Let’s dive a little deeper. Housewrap typically is made of polyethylene or polypropylene. Exactly how the housewrap allows water vapor to pass through depends on how the product is manufactured. Some brands are manufactured by layering the fabric, some by spinning the fabric, and some housewrap is manufactured as a waterproof product that is then perforated at the factory with a designated number of holes per square foot. Note that the combined performance of water holdout and breathability is key. If breathability is too low, moisture vapor won’t escape. If breathability is too high, the housewrap can’t block water.

I recently drove by a remodeling project where the contractor had applied housewrap to the sheathing, overlapped the edges by 12 in. or so, and stapled it in place. Looked pretty neat, but of course it was completely wrong. If the structure ever had mold or rot behind the housewrap, the housewrap warranty would almost certainly have been voided because housewrap is a sealing system. Whether you want to incur the expense or not, you have to buy the roll of housewrap along with the seam-sealing tape, as well as the flashing systems for sealing rough openings. By properly combining these items into a sealing system, you can achieve a good air infiltration and moisture control strategy.

These days, when you walk into a convenience store to get a cup of coffee, you, like me, are amazed at the proliferation of the types of coffee available. The same thing is happening in housewraps (and roof underlayments, too).

Your choices are expanding and increasingly exotic. These new housewraps include innovative textured substrates and drainable housewraps with crinkled or channeled surfaces that allow water to quickly drain away from the back of the siding before it can seep into the wall. I fully expect that drainable housewraps will be the norm within a few years.

Finally, if you’re using stucco or manufactured stone, there is another level of air infiltration and moisture control you need to master beyond what is used in traditional walls, and some codes require two moisture infiltration layers.

Also, if you’re using cedar or redwood siding, note that these contain tannins, which are water-soluble chemicals that can leach out when the wood gets wet. These also allow water to easily seep through a permeable housewrap, whereas a drainable housewrap may allow any leachates to drain away.

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