My House is Too Hot
My House is Too Hot
Solutions for homes that just won’t cool off
During the summer, your house is too hot. What’s the solution?
The simplest thing to do, of course, is to get a bigger air conditioner. That crude solution certainly works: if you blast enough cold air into a building — even a leaky, poorly insulated building — you can lower the indoor air temperature. (Of course, adopting this approach is no guarantee of success, since central air conditioning systems are often poorly designed and haphazardly installed.)
If you care about efficiency (or your pocketbook), and your house is too hot, you’ll probably prefer a more intelligent and nuanced approach than “I need a bigger air conditioner.”
How do homes get hot?
So before you install a powerful new air conditioner, you should first investigate whether you can address the factors that are making your house hot in the first place.
There are five basic ways that homes get hot:
- Heat is transmitted through the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. whenever the outdoor air temperature is higher than the indoor temperature, or whenever solar radiation raises the temperature of the siding or roofing above the indoor temperature. To reduce the rate of heat transmission through walls and roofs, you’ll need to improve your home’s insulation levels.
- Heat enters the building when the sun shines through windows; this is solar heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss.. To reduce the rate of heat transmission through windows, you’ll need to shade your windows or install windows with low-solar-gain glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill..
- Heat enters the building when warm infiltrating air enters through cracks in the building envelope. To reduce the rate of heat transmission from infiltration, you’ll need to perform air sealing work.
- Heat enters the building when warm exterior air is introduced (deliberately or accidentally) by the HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. system. To address this, you should reduce the ventilation rate to the minimum that is consistent with comfort and health, and (if possible) eliminate any ductwork outside of your home’s thermal envelope.
- Internal heat gains are generated by pets and people (who emit body heat and moisture), light fixtures, electrical appliances, and combustion appliances (like kitchen ranges and water heaters) located inside the building’s thermal envelope. To address these heat sources, choose energy-efficient appliances, cook outdoors, and put your dog in the doghouse.
Deciding which mechanism is responsible for your hot-house problem takes judgment. In most hot homes, the three biggest factors are solar heat gain through windows, thin ceiling insulation, and ducts located in an unconditioned attic.
Solar heat gain through windows
In general, the first line of attack should probably be to address any unshaded windows — especially east-facing windows and west-facing windows. Solutions include:
What about interior shades or blinds? They can help a little bit — but nowhere near as much as exterior shades or blinds. Interior blinds don’t stop solar heat from entering your house. The radiating heat enters through the glass and hits the blinds. The blinds heat up, and then the blinds radiate that heat into the room. So the heat ends up indoors.
For more information on solar-control measures for windows, see these three articles:
Thin ceiling insulation
In most cases, the second line of attack should be to investigate the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the existing ceiling insulation. In an older house with insufficient insulation, the ceiling on the top floor of a house may get warm to the touch on a summer afternoon. If that’s the case in your house, you probably need more ceiling insulation. (An infrared thermometerA digital thermometer capable of measuring the temperature of a surface from a distance ranging from a few inches to a few feet. Most hand-held infrared thermometers include a laser to help aim the device; the laser plays no role in temperature measurement. Used as an inexpensive substitute for a thermal imaging camera, an infrared thermometer can detect hot or cold spots on walls, ceilings, and duct systems. can be a handy way to check the temperature of your ceiling.)
Minimum code requirements for ceiling insulation range from R-38 (in Climate Zones 2 and 3) to R-49 (in Climate Zones 4, 5, 6, 7, and 8). Of course, installing more insulation than minimum code requirements never hurts.
If your house has a vented unconditioned attic, adding more insulation to the attic floor is usually fairly straightforward. However, if you live in a Cape, or if your house has insulated cathedral ceilings, insulation improvements aren’t easy. For more information on improving ceiling insulation, see these three articles:
- Borrowing a Cellulose Blower From a Big Box Store
- Insulating a Cape Cod House
- How to Build an Insulated Cathedral Ceiling
What about wall insulation?
Wall insulation helps slow down heat flow through walls, and that’s a good thing.
But heat flow through walls is rarely the most important factor in the type of hot-house problem we’re discussing here, for several reasons. Attics get hotter that walls, because attics are more directly exposed to the sun’s rays; and adding insulation to walls is generally more complicated and expensive than adding insulation to a ceiling. These factors push improvements to wall insulation farther down the priority list than improvements to attic insulation.
In some areas of the country, HVAC contractors routinely install ducts in vented unconditioned attics. Needless to say, attics are hot during the summer — often hotter than the outdoor air temperature.
If you’re unsure whether there are any ducts in your attic, it’s time to climb up there and look. If there are ducts up there, make sure that the ducts are well insulated, that duct seams are sealed to prevent air leakage, and that the ducts are securely fastened to register boots. Even if the ducts are insulated and in good shape, the fact that they are outside of your home’s thermal envelope means that their location is contributing to your comfort problem.
If the ducts are poorly insulated, it’s time to replace the ducts in your attic with new ducts insulated to at least R-8 (or to wrap the existing ducts with thick duct insulation). In some cases, it makes sense to bury your attic ducts in a thick layer of cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. or spray foam insulation. (Be careful, however: when metal ducts are buried in fluffy insulation like fiberglass or cellulose, condensation may form on the ducts during the summer. This problem is common in hot, humid climates; it’s less likely to occur if the ducts have good duct insulation wrapped in a vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). jacket.)
In all cases, the best (though not the least expensive) solution to attic ductwork is to convert your vented unconditioned attic to an unvented conditioned attic.
For more information on these issues, see:
- Keeping Ducts Indoors
- Creating a Conditioned Attic
- Sealing Duct Seams
- Inaccurate load calculations (leading to oversized equipment)
- Improper refrigerant charge
- Excessive or insufficient air flow over the coil
- Poorly designed return air duct system (with too few return air registers)
- Poorly designed supply air duct system (leading to comfort complaints in rooms that are distant from the air handler)
- Ducts installed outside of the home's thermal envelope (for example, in an unconditioned attic or an unconditioned crawl space).
- Leaky ducts (ducts with unsealed seams)
You may be wondering, "Should I try to cool off my attic with a powered attic ventilator or a radiant barrier?"
The answer is no and no. For more information, see:
Problems with the cooling system
If your east-facing windows and west-facing windows are shaded, and your attic has adequate insulation, and there aren’t any ducts in your attic, your discomfort problems may be related to a poorly designed, poorly installed, or malfunctioning air conditioning system.
For example, the second floor of many two-story homes is often warmer than the first floor during the summer. This problem is most common in homes with a single cooling zone. (For more information on this issue, see Keeping Cool in a Two-Story House.)
While it’s possible to tweak your forced-air system to reduce this type of discomfort, the best solution is either to zone the two floors separately or to install separate air conditioners (for example, ductless minisplits) for the two floors.
Many central air conditioning systems have design and installation errors. Among the most common problems:
For more information on some of these problems, see All About Furnaces and Duct Systems.
Finding a competent HVAC contractor to assess the design and functioning of your air conditioner can be difficult. If you doubt the advice you’ve received from your HVAC contractor, you may want to discuss your HVAC problems on GBA’s Q&A pages.
What about overventilation?
While a little bit of fresh air is important for occupants’ comfort and health, too much ventilation air can tax a home’s heating and cooling equipment.
Most experts advise that a home should be ventilated at the rate recommended by ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. (a residential ventilation standard). The rate recommended by ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. 62.2 is 7.5 cfm per occupant plus 3 cfm for every 100 square feet of occupiable floor area.
A few experts, however, advise that a lower rate makes sense, especially during hot weather.
To learn more about this issue, see these two articles:
So, if your house has a mechanical ventilation system, it’s important to verify that the equipment isn’t introducing too much exterior air to your house.
Note that some homes with forced-air heating and cooling systems have an fresh air duct that introduces outdoor air to the air handler’s return-air plenum. Unless this type of system is equipped with a motorized damper controlled by an AirCycler (also known as a FanCycler), the system can introduce very large volumes of outdoor air into the home. If you suspect that your home may have this type of ventilation system, inspect it carefully to see whether the system includes the equipment needed to prevent overventilation.
For more information on ways to measure air flow rates, see Is Your Ventilation System Working?
Infiltration and exfiltration
Although infiltration and exfiltration issues are rarely the chief cause of hot-house complaints, these air leaks matter. If your house is old and leaky, sealing air leaks will reduce your energy bills and increase your comfort, summer and winter. Start with the biggest holes in your basement and attic floor. Once you’ve sealed those, focus on the small holes in your basement and attic floor.
For more information on this issue, see:
Internal loads include light bulbs, refrigerators, TVs, ovens, pets, and all of the people attending your dinner party.
Internal loads are rarely the most important factor in hot-house complaints. But if you can cook on an outdoor grille instead of a kitchen range, your house will stay cooler. If you want to optimize the performance of your cooling system, you’ll pay attention to internal loads. Efficient light bulbs and efficient appliances will lower your cooling load.
Martin Holladay’s previous blog: “Capillary Breaks Above Foootings.”
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