Wet-Spray Cellulose Quandary
Are you aware of any moisture problems from someone using wet spray cellulose directly on basement cement walls? How about on the sill and band joists in basements without any retaining device such as Sheetrock? Will it stay put when dry?
A local homeowner wanted to wet-spray cellulose everywhere, including the foundation walls and sill area. But the contractor was not comfortable insulating this way, because he was concerned about moisture on the cement walls below grade. He was also worried that the cellulose might shift after it dried if there was no retaining system at the sill area.
Paul Schroeder, Technical Director
Hampden County Energy Office
Rick Ariagno, owner of the Illinois insulation contracting firm Home Energy Control, Incorporated, responds: You asked a couple of tricky questions that could warrant some concern if not handled carefully. We spray rim joists all the time and find the material stays put without a problem. Since the rim joist between the first and second floor is always covered with drywall, we don't have to worry about anyone disturbing the insulation, or about loose, dried material flaking off. The exposed rim joist in the basement is a different story. We spray this area wetter to get a higher density and a harder crust on the surface. The installers have to thoroughly clean overspray off the plywood and floor joist to prevent dried fuzzies from flaking onto the basement floor. If the insulation is disturbed after installation--by an HVAC contractor, for example -- it is easiest to replace disturbed insulation with a batt and careful sealing.
Whenever you go below grade, you have to be cautious of moisture problems. We have installed cellulose behind finished basement walls without special protection and haven't had any complaints. Cellulose installed below grade must be completely dry before it is covered with drywall. Since the material can only dry to the inside and there may be less air movement in the basement, drying below grade may take longer than usual. Fans can decrease drying time. To be safe, we create a drainage plane behind the insulation. This can be done by tacking 4-mil poly to the mud sill and letting it drape down the wall before the studs go up. Quarter-inch or half-inch foam sheathing behind the studs is even better for creating the drainage plane. If water does eventually seep in, it will drain down the wall and onto the floor rather than leaching into the spray-applied cellulose. If the installation and budget permit, err on the side of caution rather than spraying directly on the basement walls. Finally, if I knew a seepage problem existed, I would use an alternate insulation system.
Making Sense of Heat Ratios
On page 15 of John Proctor's If Bigger Isn't Better, What Is? (Sept/Oct '96), you recommend choosing air conditioners with low sensible heat ratios (SHRs) in hot, wet climates.
I called my contact at the local Carrier/Bryant distributor and he said this was not a criterion consumers could use to shop for air conditioners. It sounded like the ratio was more or less constant throughout the product line. SHRs aren't even given in the Air Conditioning and Refrigeration Institute (ARI) guide.
On a related topic, the Rocky Mountain Institute's book Homemade Money recommends that readers close off air conditioning registers in unused rooms. I would expect this to aggravate problems with low air flow across the indoor coil.
Madison Gas and Electric
Author John Proctor responds: The relationship between the latent capacity (moisture removal) and the sensible capacity is a critical design consideration in selecting the correct air conditioner for the local climate. All the major manufacturers (including Carrier) and most of the minor manufacturers provide this information in extensive performance tables. Most companies provide sufficient information to calculate the SHR for their equipment under many different conditions.
Contrary to your distributor's comment, not all equipment is alike. Different compromises are made in the design process. True, the ARI directory does not provide SHR. But in general, the ARl directory does not provide sufficient data to select equipment.
Your local distributor has the performance tables for all the equipment it sells. Any installing contractor can obtain this performance data. If the distributor refuses to share that information with you, you should ask yourself if you want to deal with someone who either does not know how the equipment performs or does not want you to know.
From the performance tables, you will see that the SHR decreases significantly when the airflow across the inside coil is reduced. You are certainly correct about the problems associated with closing registers on a heat pump or air conditioning system. Starving the coil for air reduces its efficiency and may cause failure if liquid refrigerant makes it back to a reciprocating compressor. Gas furnaces, incidentally, are far less sensitive to closed registers, but the efficiency is reduced and the heat exchanger can be stressed by repeated overheating, especially if the burner is often being turned off by the limit switch.
How Heat Pumps Compare
If Bigger Isn't Better, What Is? was the best article I've seen on A/C sizing. I still have two questions. How does a 24,000 Btu/h, 240V wall unit compare to a modern heat pump? Also, what is the effect of using a dehumidifier to help with humidity reduction?
Lake Placid, Florida
Author John Proctor responds: If the wall unit in question is an electric resistance heater then a 1997 through-the-wall heat pump will provide the same heat at less than half the cost. If it is an old (circa 1970) through-the-wall heat pump, and there is no problem with the old unit (are the coils clean?), then a 1997 whole-house heat pump will probably use more energy. A through-the-wall heat pump heats and cools the immediate area rather than the whole house and it does not use a leaky and poorly insulated duct system to deliver the heating or cooling.
Dehumidifiers remove moisture and deliver heat to your home. If you are trying to cool and remove moisture, then dehumidifiers may be a necessary evil. Your first choice would be to use an air conditioner that effectively removes both sensible heat and moisture. A properly sized air conditioner selected and installed to meet your specific needs is the best choice.