Plastic sheeting should be enough for dirt crawl space
Q. We own a Dutch colonial home (circa 1929) and are wondering what the best solution would be for insulating a crawlspace in the basement. It is underneath two identical 9-by-20-foot sun rooms (one on first floor, one on second floor), and the rooms are freezing. The crawlspace has a dirt floor and no insulation at all. It is just shy of 3½ feet high. We were thinking of putting down plastic sheeting on the floor to contain the dirt and spreading rocks on top, but would that create a possible mold problem over time? Also, we were thinking that we should insulate the overhead bays of the crawlspace and tacking up something over that. Are any of these ideas OK? What would you recommend? We want to try to address this problem ASAP because it is significantly colder in these two sunrooms than the rest of the house. We do know that eight (original) windows per room also contribute to the problem but feel that a majority of the cold is due to no insulation.
Via e-mail
A. You should definitely put 6-mil plastic on the bare soil. Make sure that the soil is smooth so the plastic won't be punctured as you crawl to install the insulation. Cover the soil thoroughly to contain any moisture. There is no need to spread rocks over the plastic. In fact, doing so may puncture it, reducing its effectiveness.
A mold problem on the joists and subfloor of the lower sun space is more likely to develop, if it hasn't already, by leaving the soil bare. I am assuming that the crawlspace does not get flooded. If it does, you need to correct this problem by repairing any grade deficiencies around the building to prevent water from percolating down to the bottom of the foundation. You should also install the plastic up on the walls to a point that's above the outside grade. Tape it to clean areas and make provisions for any water that may accumulate over the plastic to drain under it. You can insulate the overhead bays by pushing fiberglass batts with integral vapor retarder so the vapor retarder is tight against the subfloor. Hold the batts in place with tiger's teeth (thin metal wire with sharp teeth). Do not install anything below the insulation.
Another way, since you have enough headroom to work with, is to insulate the walls with 2-inch-thick XPS rigid insulation. There are several options for fastening the insulation boards to the foundation: spikes on steel plates that are glued to the walls after cleaning the chosen spots. Or if the walls are clean, it is simpler to apply polyurethane caulking in a serpentine fashion then push the rigid insulation tightly against the walls. Assuming that the sunroom's walls are fully insulated, you will not significantly improve the warmth of either of the sunrooms by insulating the floor of the lower one; the windows are the greatest source of the cold. The only remedy is to heat them if you want to use them year around. Gas fireplaces are a good option.
Q. I have questions regarding the home that we purchased in 1992. I am not the handiest of homeowners, but read your column regularly and believe I can describe the issues and questions.
One of the previous owners constructed a 20-by-30-foot family-room addition to the original 1940s ranch. The addition has a southeast exposure and is constructed over a crawlspace with a poured concrete floor. The "ceiling" of the crawlspace has pink fiberglass insulation between the joists, which are covered with plastic sheathing. (I do not know the R-value of the insulation or the weight of the plastic.) The issue is that the floor -- hardwood, which we installed over the subfloor -- is very cold. I wonder if installing Sheetrock or rigid foam insulation over the plastic sheathing would help keep the room warmer during the coldest months of winter. The room is heated by forced hot air, and the duct travels some 40 feet from the furnace to the register. The heat is supplemented by a gas fireplace, which we use predominately in the mornings and later in the evening to get rid of a chill.
In 2001, we added a 750-square-foot second-floor addition to our home. The addition has three bedrooms and a full bath, with an attic and pull-down stairs for access. The attic is 80 percent covered with plywood flooring so we can use the area for storage. The attic stores our 90-percent-efficient furnace, which is direct-vented through the roof. The second-floor bathroom's fan is vented to the outside by way of the attic. I assume the attic was insulated to code; it has two gable vents, soffit vents and a "mushroom" attic fan. The issue is that water droplets condense on the nails that protrude from the roof. This appears to be more a winter than summer issue, and I assume it has more to do with heat loss and/or moisture getting into the attic space than being affected by the colder attic temps. What can I do to resolve this issue?
Scotch Plains, N.J.
A. The insulation between the joists on the crawlspace ceiling should not have plastic covering it from below. It should be removed and dropped onto the concrete to act as a vapor retarder. (Concrete is notoriously porous.) The insulation should be tight against the subfloor to prevent heat loss at the band joists. If it is not, push it in and hold it in place with "tiger's teeth." (Buy them in a building-supply house.)
The problem is the distribution of the heat. Not only do you have an addition with three walls exposed to the outside, but you also have two other surfaces so exposed: the ceiling of the room and its floor. (I am assuming that the original ranch has a basement, which mitigates the heat loss through its floor.) If there are vents in the crawlspace, they should be closed during the summer and winter -- they are not needed if the crawlspace floor is covered properly. A single duct supplying heat across 40 feet loses most of its heat by the time it reaches the register, especially if the duct is not insulated. This situation is aggravated if there is no return duct in that room. It is also possible that the furnace cannot handle the additional load. Consider having an HVAC contractor survey the situation and calculate the total output of the furnace versus the heat loss of the house and addition. If necessary, insulate the supply duct, install a return duct and balance the system to send more heat into that room. The alternative is to keep the gas fireplace going all the time.
If the mushroom fan is running during the winter, it is likely responsible for the excess humidity in the attic, as it sucks warm, moist air from the living quarters since the gable and soffit vents are unlikely to provide the net free ventilation area equal to the CFM of the fan. The warm, moist air is drawn through cracks around the folding stairs' opening and any others found around the bath fan, electrical boxes on the ceiling and in the walls below as well as at the joints in the ductwork. Turn the fan off and have the HVAC contractor check and seal all joints in the ductwork, which I assume is insulated. (Despite the insulation, ducts can and do leak, as proven by blow-door testing.) It will be difficult and expensive to locate and seal all other perforations into the attic because of the plywood flooring.
Q. Our house was built in 1999 in a new development. A few years ago, we checked our radon level using the kit provided by our township. It was high (around 12). All our neighbors' houses have normal radon levels. We bought an electronic radon meter to constantly monitor the radon level. We patched the cracks in our basement and open the basement windows from time to time. The radon level decreased. When it's warm and dry outside the level is within the norm (2 to 4), but after heavy rain and during the winter, we still get readings of 6 to 8. We have a pipe in our basement with "radon" written on it. We assume that it's a preinstalled radon-reduction system. There is also a sump hole near the pipe. (We don't have a sump pump.)
Our house is lower than our neighbor's house, and we have a drainage easement on the lowest level of our property. How we can permanently decrease the radon level without big investments? We wanted to install a vent fan, but the only place for it is in the attic just behind the wall of our master bedroom, and it could be too noisy to use. Could we install a vent fan in the basement? We wanted to finish our basement but are afraid to do so because of the high radon level. It also will create a problem if we sell our house.
Hackettstown, N.J.
A. The fan can be installed anywhere in the length of the pipe as long as all joints are tight enough so that no radon will leak through them and into the living quarters when the fan pressurizes the pipe. You may find the fan so quiet that it won't bother you if installed in the attic. Put a weatherstripped cover over the sump to keep any radon from there confined. Remember that radon levels vary greatly throughout the day and seasons, and its level is directly related to a differential in temperature between inside and outside. In summer, when windows are open, the radon level inside is very close to the level outside, and there is little we can do about it.
© 2008, United Feature Syndicate, Inc.