Roof vent pipe frosts over in the cold

Q. My 3-inch vent pipe for my house has been frosting up and plugging. I have never had this problem in the past. I know it had been subzero for a while. To me that is the worst time to have to go on the roof to unplug it. It is a PVC pipe that goes about 18 inches up. Is there anything I can do to prevent this from happening?

A. A plugged vent pipe will affect the house's plumbing, particularly impeding proper flushing of the toilets.

Your vent pipe is too long over the roof. It should not exceed 12 inches - 8 to 10 inches is OK. Having it so long gives it at least 6 inches more exposure to the frigid temperature. You should cut off a minimum of 6 inches.

And if your attic is cold, the exposed vent pipe should be insulated. Vent pipes seldom freeze that solidly; the frost is more crystallized and can be easily punched through with a broom, which I assume is what you are doing.

Q. Long time reader, first time writer. Please take a look at the attached picture. It shows a corner in our dining room that has frost from the floor to about a foot above the floor. I have the same issue in a room upstairs. The downstairs room is in the northeast corner of the house, and the upstairs room is in the northwest corner. The downstairs corner is above a sealed-for-radon crawl space and the upstairs corner is above the garage door.

We live 30 miles west of Chicago. Our house has vinyl siding put on about 15 years ago. The house was built in 1979.

Can you give me an indication of why we have this issue in the two corners and what type of repair person to contact to fix the issue?

A. The dining room corner over the crawl space and the affected corner of the other room are excessively cold because both the northeast and northwest corners of the foundation are the coldest. These rooms also suffer from exposure to a draft or an insulation problem.

If you have a crawl space vent in the area below the dining room, you should not only close it but also insulate it.

If there is no vent in the vicinity, check the band joists and, if they are not fully insulated, please do so by placing R-19 fiberglass or Roxul insulation tightly against the band joists throughout the crawl space and staple a plastic vapor retarder to the floor sheathing, the sides of the joists and the mud sill in each space to protect the insulation from moisture migration.

If the joists are not insulated, consider insulating the full depth of the joists for about 4-feet from the band joists out in the affected corner. If you use fiberglass, and it has an integral vapor retarder, it should be against the floor above. If the fiberglass is unfaced, there is no need for a vapor retarder.

You may also want to install a 1-inch thick piece of rigid insulation to the bottom of the joists in the area with the fibrous insulation as long as the rigid insulation is in contact with the fibrous insulation.

As for the room over the garage, if the garage ceiling is open, repeat the procedure. But if the garage ceiling is covered with Fire-Code gyp board, you may have to have insulation blown in the affected area.

Hopefully, this will take care of the problems. A skilled contractor should be able to carry out these instructions, but you'll need an insulating contractor to blow in cellulose.

Q. I went into my attic during our recent frigid cold snap. All the rafters and the underside of the roof deck were covered with frost.

Several years ago I had a ventilating fan installed on the roof which runs in the summer when the temperature gets extremely high. Is such frost common during frigid cold days? Is there a way to correct this problem?

A. It sounds as if there is a considerable amount of convection of warm, conditioned air into your attic, and that should be addressed and corrected.

The problem is so substantial that I recommend you have an energy audit performed to detect all the possible avenues of convection.

Some contractors who do energy audits are also certified to perform the needed repairs. Just be sure to check them out through any authority that has control over the process if your state does have such a program. Ask for references and check them out also.

You haven't mentioned any other type of ventilation besides the attic fan, which you are only using in the summer. You should know that attic fans are not the best way to ventilate an attic. Unless they are provided with net free ventilation area (NFVA) equal to or greater than their CFM rating, they draw the needed makeup air from the conditioned space: A/C in the summer and heated air in the winter if they are operated at that time of year.

The most effective attic ventilation system is one where ventilation is provided with continuous soffit and ridge vents in conjunction with an uninterrupted air space between the two.

If you do not have such a system, you should consider it. Although it can be done anytime, a good time to make the necessary improvement is when the roof covering needs replacement.

• Henri de Marne, a former remodeling contractor turned columnist and consultant, is the author of "About the House with Henri de Marne" (Upper Access Publishing). He continues to take questions from readers for this column and his website, www.henridemarne.com. Email questions to aboutthehouse@gmavt.net.