Q. During the last below-zero freeze, my second-floor tub drain all of a sudden was clogged in the morning. I called a drain company, which came and declogged it. They said the clog was in the trap. I was wondering whether the trap water froze, due to the cold air coming into the pipe from the pipe vent. The trap is not directly under the tub drain; it’s about 3 feet behind it in an insulated wall space near an exterior wall. Could the trap water freeze because of the vent pipe, which goes out the roof one story up in the attic?
GRIFF
A. Given the facts you just presented, your drain trap was absolutely frozen. I’m hoping your supply lines are not running through there as well. Freezing is a particular problem when pipes run through outside walls, uninsulated or under-insulated attics, or crawl spaces.
Open the space and look for uninsulated areas and air leaks. Wind is a major contributor to freezing pipes, and even a tiny opening can let in enough cold air to do the job. I would remove all of the old insulation for a proper visual inspection. An infrared camera is useful in these situations. One thing people do not realize is that fiberglass insulation does not stop air infiltration; air leaks must be stopped in order for it to work.
I had a similar problem. Air was leaking through insulation and into the insulated wall space through the soffit vent, causing the shower pipes in that wall space to freeze. The shower was up against a sloped ceiling and had a small insulated cavity behind it, 24 inches from the exterior wall.
“Proper vents’’ ensure the right amount of ventilation from the eaves up to the ridge vent. They are usually made of Styrofoam and come in different shapes and sizes. Insulation is then applied over them. Proper vents are designed to allow fresh air to travel through the soffit vent up the underside of the roof and out the ridge vent. A soffit vent allows air to flow into the attic or into the proper vent. This helps keep the attic cool in the summer and allows moisture up there to evaporate. Proper ventilation also helps prolong shingle life and prevent ice dams.
Another problem I often see is insulation jammed into spaces. Fiberglass insulation needs to stay fluffy and loose. Compressed fiberglass loses its R-value because its dead-air space is reduced. When properly installed, fiberglass insulation will not settle or deteriorate.
To resolve my problem, I added Styrofoam “proper vents’’ all the way down to the soffit vent. I then reinstalled high-density fiberglass insulation. Spray foam is better, but, depending on the size of the project, it may not be cost-effective. The insulation batts need to be secured in place. That is why they have the folds along the sides. These folds can be opened and stapled to the wood framing.
You should also inspect and insulate the floor rim joist (the exterior-wall section of the floor joist); look for air leaks around electrical wiring, dryer vents, and pipes; and use caulk or foam insulation.
After insulating all of these areas, I covered the entire exterior wall and rim-joist areas with 1½-inch Styrofoam rigid-board insulation to seal air gaps and increase the R-value. These boards fit together with a tongue-and-groove connection. I glued the insulation board to the studs and one another at the tongues with construction adhesive. I then secured them with 2½-inch screws and No. 12 finish washers to hold it them in place and tight to the studs while the glue dries.
I also used foil HVAC tape to seal all of the rigid-board seams, and filled any large gaps with expanding foam. This not only will make your unheated wall space warm, it will stop the air flow that is freezing your pipes and trap. I should note that I installed a grill on a side wall into this unheated cavity to allow warmth from the room to penetrate this space.
Rob Robillard is a general contractor, carpenter, editor of AConcordCarpenter.com, and principal of a carpentry and renovation business. Send your questions to homerepair@globe.com.
