Ask the Building Scientists

Question:

What would be the most efficient and effective building practices that would minimize window condensation?

-Perry Cola, Arvida

Answer:

Building in a desert would take care of the problem in a snap! But in less extreme climates, condensation can occur all too easily. The key to preventing condensation is to keep humidity levels inside the house at a normal level. Basically, condensation happens when warm moist air meets a cold surface (e.g. when water droplets gather on a cold window inside a heated room).

To avoid condensation, ensure each room will receive proper airflow by correctly sizing the HVAC system according to the Air Conditioning Contractors of America’s (ACCA) Manual J calculations. Equipment that’s too small or too big won’t be able to maintain a proper level of humidity. Refer to the ACCA website, http://www.acca.org.

Also use windows with a low U-value (below 0.35) and ensure walls are well-insulated and airsealed. Remember that building codes specify a minimum of insulation, not necessarily the amount that’s most efficient. The North American Insulation Manufacturers Association provides more information and resources on its website, http://www.naima.org, about proper insulation.

It’s important to control excess moisture to prevent durability issues, as well as mold and mildew growth. Start by properly sizing the HVAC systems and back it up with efficient windows and insulation.

-Anthony Grisolia, IBACOS
Building Performance Specialist

Question:

We recently observed lint accumulating on the inlet screen of the power vent on a domestic hot water tank. Evidently the room was starving for the required dilution air and actually pulled air from outside the room. Not a desirable condition. How is the need for dilution air calculated?

-Jeff Anderson, Mistick Homes

Answer:

Requirements for combustion and dilution air follow calculations set by the International Mechanical Code (IMC) and the International Building Code (Chapter 17). As you’ve observed, power-vented water heaters and furnaces require combustion air in order to function properly. In the case you mentioned, the power vent must be drawing air from outside into the mechanical closet through the dryer vent, thus accumulating lint on the inlet screen. To avoid situations like this, the IMC outlines several options for providing adequate combustion air for power-vented water heaters and furnaces.

The first option is to locate the equipment in a sufficiently large space, so that natural air infiltration into the large space provides adequate combustion air. In this scenario, every 1,000 Btuh of input capacity requires 50 ft3 of space. If the space isn’t large enough, it must have ducts or openings to provide the necessary combustion air.

The second option is to provide outdoor air for combustion air. The IMC allows for only one opening or duct to provide combustion air. This opening must be within 1 ft of the ceiling, and have a free area of at least 1 in2 per 3,000 Btuh of input capacity of the water heater.

Another solution is to use a sealed combustion hot water tank that would eliminate the problem and the need for combustion air ducts. Also, for your particular situation, the clothes dryer may need a bit of maintenance. Clean out the lint screen, and check the outside dryer vent for clogs. Finally, the dryer vent pipe should be no longer than 25 ft because a pipe that’s too long will inhibit airflow.

-Eric Newhouse, IBACOS
Building Performance Specialist