The Case of the Leaky Casings

Posted Wednesday, February 22, 2012 in Sustainable Maine

The Case of the Leaky Casings

by Paul Kando

You weather-stripped the door and weather-stripped the windows, yet there is still a cold draft around the darned things when the wind blows outside? Don’t be surprised, you are not alone. Older frame houses all have this problem and so do many newer ones. The root of the problem is the conventional way windows are installed.

Figure 1 shows the corner of a window before the casing has been added. Observe the gap between the rough opening (the framing around the “hole” for the window) and the window frame. Into this gap someone is inserting a shim: a wedge-shaped piece of wood. It is with such shims that windows are leveled in the rough opening before they are nailed or screwed into place. After this, in older houses, the finish carpenter installed the inner and outer casings, and the job was done. The problem is that this left nothing between your heated room and the chilly outdoors but two pieces of wood trim and ample room for air leaks around the casing.

More recently, to reduce this heat loss, builders would stuff the gap full of fiberglass insulation. This helps; however, when the fiberglass is stuffed too tightly, all the insulating air is squeezed out, leaving the equivalent of a layer of glass, which has an R value of only 0.14/inch. If, on the other hand, the fiberglass is left loose to trap plenty of insulating air, there will be air leaks through it and around the window or door, because a mass of glass fibers does not block air flow. Therefore stuffing with fiberglass is not the best solution.

Some newer windows come with a plastic flange, a piece of plastic skirting that extends out from the window frame in all directions. This flange is nailed or stapled in place from the outside after the window or door has been shimmed in place. The flange is designed to cover the gap between rough opening and frame, but the gap will still leak, unless the flange is tightly sealed to the outside wall sheathing with an appropriate (strong and durable) sealing tape. Alas, this taping is a “newfangled idea” so it probably was not done when your house was built, and even if it was, it does not stop warm, humid indoor air from entering the gap and the moisture condensing there.  

What to do?

The gap between window or door frame and rough opening is best filled with spray foam, which both seals and insulates. Moderately expanding foam, made for windows and doors, must be used. High-expansion foams can force the frame to bend, binding the window or door. If you can remove either the inner or the outer casing without damaging it, remove one or the other all around the opening. Fill the gap with spray foam, reinstall the casing, caulk it in place, touch up the paint and you are done.

If you can’t remove the trim without damaging it, my friends at Green Building Supply in Portland came up with the alternative shown in Figure 2. Drill small, 3/16-inch diameter holes into the gap about 10 inches apart all around the window or door. This can be done either through the frame where the door leaf or window sashes fit (as shown, red arrows) or through the inside or outside casing. Insert the 1/8-inch nozzle of a foam gun into one of the holes, squeeze the trigger and spray for four seconds. Remove the nozzle and repeat the process for each of the remaining holes. When the foam has set, cut off any excess protruding from the drill holes with a razor blade, fill with wood filler as needed and touch up with paint.   

In applying spray foam I recommend the use of a foaming gun and the appropriate canisters of foam, instead of a  spray can. This will save you money. Once you start to use an ordinary spray can of foam, you must use all of its contents or the foam will cake up, preventing you from using what remains in the can at a later time. This is not the case when you use a foaming gun with a screw-on canister: as long as the canister stays attached to the gun, you can stop the flow of foam and restart it at a later time. If you invest in a foam-gun kit you can also change nozzles, sizing them to the job. Therefore you will  have much better control over the amount of foam you use. As a result you will waste less foam and do a neater job. Clean the gun’s innards with the cleaning fluid recommended.

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