Home repair: Cabin may benefit from crawl-space insulation

Q. I have a cabin that is built on a slope. The crawl space tapers down from about 2 feet to about 4 feet in the front. The foundation is 8-inch concrete block, with a dirt floor. The floor joists appear to be 2x8s on 16-inch centers. I have acquired some 3-1/2-by-3-1/2-foot rigid-foam-insulation panels about 2 inches thick. There is a foil backing on one side. I was thinking of fastening these panels on the bottom of the joists. I don’t want to use roll insulation, as it will create a mouse hotel. I’m sure that they will find accommodations between the rigid insulation and the floor, but at least it won’t be pulled out all over. I am also planning to put plastic over the dirt floor. Do you think fastening this insulation to the bottom of the floor joists will help keep the above floor space a little warmer? Thank you. Your book is great!

A. Thank you, I am glad you find the book helpful.

If you put the foil backing facing the floor sheathing above, it should reflect any heat lost from above, but only if your cabin is heated. Otherwise, it should not do much. I am assuming that you use the cabin only seasonally, and that during the winter little moisture is generated that could be trapped in the floor-joist cavities. Laying plastic over any crawl-space dirt is always highly recommended.

Q. We have a 2-year-old home that has a two-car garage with a poured concrete floor. The outer edge, under the overhead door, is a little higher, so that melting snow puddles inside the garage. The low point is about 3 feet back from the overhead door. Do you have a suggestion for draining this water? Could we cut a hole in the low spot and let the water drain into the fill under the floor?

A. If there is a stone bed or if the soil beneath the concrete slab is absorbent, it should work. You can rent a 2-inch concrete core bit to drill a hole in the low spot.

Q. We have a home built around 1950. An interior wall in our bedroom showed efflorescence, i.e., paint lifting up and bubbling on the wall surrounding the chimney. We think the interior plaster was applied right to the bricks. We had the chimney repaired with new flashing and a new cap. We waited almost two years before repainting to let things dry out, but now the problem has reappeared. Do you think there is still a source of moisture that should be addressed? Is there any solution short of sheetrocking over the problem? I did not find interior above-grade efflorescence addressed in your book. Thanks for any help.

A. If the repair work was properly done, it is possible that the chimney bricks or the mortar joints between them are absorbing water and that the moisture is wicking through and affecting the plaster. You may want to apply a clear siloxane sealant to the chimney. You can get it in masonry-supply stores.

Q. I have a Cape that is 50-plus years old with a 19-by-15-foot addition on the back of the house. I have all new windows and have had a lot of insulation blown into the crawl spaces and over the addition. In the winter, I have a bad ice buildup in the back of the house, by the valley of the addition and around onto the addition. Eight years ago, I had a new roof put on with a roof vent at the top and metal at the edge of the roof. I also have gutters around the house. What can I do to get rid of this ice problem? Thank you for any suggestions.

A. It sounds as if a serious insulation omission was made when the addition was built. One possibility is that insulation was overlooked — a common mistake — at the joint between the main roof and the addition roof. At this point, if there is no access to the attic of the addition to check for missing insulation and to make necessary repairs — or if there is no attic — the best way to determine where the heat loss is coming from is infrared thermography. Once the source of the problem is determined, a solution should be available.

Q. Years ago, I added a second floor with a second furnace in the new attic. The other furnace is in the basement. Since then, I notice that the snow on my roof melts days in advance of my neighbors. During the winter of 2010-2011, this resulted in large icicles hanging from the gutter. So far I have avoided any damage, but I am concerned about potential damage as well as the heat loss. The floor in the attic is filled with paperbacked pink insulation, but there is no insulation on the inside of the roof. We do have a ridge vent. There is a ceiling fan on the second floor and a wooden trapdoor access to the attic. Both of these openings allow some warm air to move from the second floor to the attic, but I do not think there’s much because the fan has louvers that close and the door in the ceiling is the type with the drop-down stairs and a plywood panel that covers the opening fairly well. What do you recommend to prevent the icicles and heat loss? Respectfully.

A. Both the whole-house fan opening and the folding stairway contribute to the ice-dam problem and should be insulated and weatherstripped. Adding a furnace and any other type of heat-producing appliance in an attic in climates where snow is common is a serious mistake. The best way to correct the situation is to remove the furnace, as any other solution is unlikely to be successful.

Years ago, I was involved in a similar situation in a large house in New Hampshire. We built a “tent” over the furnace and ducting system with framing lumber and insulated it heavily with rigid polyisocyanurate reflective insulation. We installed a fan in one gable and a louver in the opposite one in insulated “tubes” into and out of the tent. It lessened but did not solve the problem. The only thing I can think of, if moving the furnace to the basement is not an option, is to fill the rafter bays and cover the rafters with sprayed, closed-cell polyurethane insulation. It might work, or at least minimize the problem.

Q. I have gutted our condo bathroom looking for the source of black mold. I finally discovered a 3-inch copper water main for our condo that runs along two walls between the Sheetrock ceiling and the concrete floor above. Its white covered 1/2-inch fiberglass insulation is dripping condensation and is covered with black mold, causing big patches of mold on top of the Sheetrock that have bled through. My issues: 1) How to abate the mold. Our condo property manager said the black mold is not the nasty stuff. All old buildings have it. 2) How to prevent the condensation. It seems that good insulation with a perfect vapor barrier will do the trick. Our bathroom is unusable. I have carpenters and a plumber waiting for our manager to fix the mold problem.

A. Mold should never be ignored. Did the condo manager have it tested to be able to tell you that it is not the “nasty” stuff? He may be right, but it could turn into nasty stuff.

If the mold is not too extensive, a common treatment is to wash or spray the areas with fresh Clorox bleach. It is effective on hard surfaces but questionable on porous materials like drywall, wood, etc. Regardless, the existing pipe insulation should be removed.

The next question is, what is causing this problem? Why is it happening only in the bathroom? Has it been determined that no pinhole leak exists in the copper pipe? Water with a low pH eats at copper pipes. Or is it due to excessive moisture generation in the bathroom that has overwhelmed the 1/2-inch pipe insulation? If so, there should be a more effective moisture control to prevent it from convecting into the chase. Is it also possible that the existing pipe insulation has some “holidays” — areas not fully covering the pipe — allowing condensation to occur, which transfers to insulated areas? If it is determined that the existing pipe insulation is not effective enough, it should be replaced with a more effective one.

Q. My Illinois home was built with short aluminum ducts rising from the bathroom exhaust fans and aimed in the general direction of the nearest roof vent. There was, of course, the usual winter condensation and resulting damage. A gable vent system would have required a 30-foot run of ducting over heavy blown-in insulation. The insulation would have had to have been channeled to lay the new ducting atop the joist, and then would need to be mounded to insulate the duct. There would have been risks of stepping through the ceiling as well, as the space is not easily accessed. And the closest gable was on the windward side of the house.

I elected to install an eaves vent. The eave was about 5 feet from the fan. The 4-inch ducting was installed flat on the ceiling after it was sealed with tape, and the generous batt and loose insulation was replaced over it. I don’t have the name of the product I used, but the outside hardware looks identical to the Fantech UEF4 dual vent. It has been there for five years now without a single problem. The moist exhaust remains warm enough while it is running so as to prevent freezing of the dampers in winter. There have been no more leaks or damage.

If I had to do this again, I would have used something more like the Lambro Undereave 143. That vent can be oriented to direct the exhaust flow forcibly out away from the eaves, which I would much prefer over the Fantech. However, the Fantech design employs two dampers, so if one does lock up or freeze, the other would still vent.

The important point with eaves installations is that you use the Lambro and do not install an exhaust near any intake screen of the attic ventilation system.

Direct outside venting should be code where weather makes it necessary. A gable system is much better if installed during construction.

A. You have approached the condensation and leakage in an interesting way, and it has obviously worked for you. You are right in advising never venting a bathroom or kitchen fan out through a screened soffit vent in eaves. The exhaust will be sucked back into the attic — the last thing we want.

However, it is a mystery to me why builders don’t vent these appliances down and terminate them through the rim joists of the first floor. This is something that I have done, and recommended, for more than 40 years. It seems so obvious to me because it follows the laws of physics: The stack effect of the house helps seal the penetration of cold air when the fan is not on, and there is no condensate as the air passes through conditioned spaces. In the early 1970s, at the time of the energy crisis, I responded to the federal and state requests for submissions of energy-saving ideas and submitted this system. It won me awards from both governments, but it does not seem to have taken off with builders. It is so easy to do in new construction that one would think they would jump on it.

DEAR READERS: A new law in Illinois requires a dialogue between landlord and tenant if high levels of radon, a cancer-causing odorless and colorless radioactive gas, have been found in rental housing. In Illinois, 35 percent of homes have levels of radon above the level of concern. All homes should be tested for radon, and relatively inexpensive measures can be taken to remediate excessive concentrations.

Two tests are readily available to homeowners: the charcoal and the alpha track. Charcoal kits give only a 24- to 48-hour reading, whereas the alpha track gives the highest reading over several months, which is why I favor it. Radon testing is best done during the heating season because in the summer, when the windows are open and people go in and out often during the day, the indoor readings are probably similar to those outdoors, and not much help. Even in air-conditioned houses, summer comings and goings will affect accurate readings.

Ÿ Henri de Marne was a remodeling contractor in Washington, D.C., for many years, and is now a consultant. Write to him in care of the Daily Herald, P.O. Box 280, Arlington Heights, IL 60006, or via email at henridemarne@gmavt.net. His book, “About the House,” is available at www.upperaccess.com and in bookstores.

© 2011, United Feature Syndicate Inc.