Welcome to the Energy Efficiency Myths series from Direct Energy! As many myths arise from incomplete knowledge, they can create seemingly possible answers that many people accept as fact. Each month, we will examine common misunderstandings about energy efficiency — whether it’s in your home or about the energy industry — and deliver real facts behind the myth (and how they they might be costing you money).
Summer Cooling Myths — Part One
Some myths are really just over simplifications or generalizations that work for some homes but not others. Others come from assumptions made by the building industry that in turn became part of the building code in spite of the fact that there was never any research to back up the original assumption. Here are two of those ilk that could have you fooled and are actually costing you money.
Powered attic ventilation reduces your cooling costs.
Not really. In fact, powered venting could actually raise cooling costs under the right conditions.
Ideally, ventilated attics not only help cool the roof in summer but vent excess moisture during the winter to keep the interior dry and free of mold. As far as cooling is concerned, though, there’s some contention over details. Basically, if you have an average amount of insulation in your attic (R25), keeping your attic cool will reduce your air conditioning use. The building code requirement is one square foot of net free vent area per 150 square feet of attic area has to be vented. According to one expert, attic venting seems most effective when there is even more air flowing by convection through the attic. For example, in homes with soffit and ridge venting, there should be more soffit venting space than ridge venting (a 60/40 split).
However, if you push that ratio the other way (more ridge vent space), there is the possibility for the ridge vents to create enough negative pressure (or suck) that it could pull cooled air up from the living space because most attics are not air sealed. Using powered gable or roof top fans can have a similar effect —again, due to inadequate air sealing of the attic. In both instances, homeowners wind up using more energy to cool their home.
However, in homes that are well insulated, research has suggested that cooling savings from attic ventilation were fairly modest. Plus, to add even more confusion into the mix, wind has a greater effect on cooling a ventilated attic than the convection process.
So, to summarize, if you have an average amount of attic insulation, a powered attic ventilation fan can help cool your home, BUT:
- Be sure to generously match the fan’s cubic foot per minute (CFM) requirement with plenty of soffit supply space
- Properly air seal your attic.
Open crawl space vents in summer to keep the crawl space dry.
False. It could actually make the moisture levels worse and increase the humidity in your home.
Crawl spaces are usually dark, cool places prone to being damp because moisture can’t escape. The assumption was that by installing venting in the sides of the crawlspace, dry outside air would enter and evaporate the moist inside air. The real world facts show otherwise. Summertime in most parts of the country, especially the southern states, tend to be oppressively hot and humid. So instead of moist air being carried out from the crawl space, the humid outside air enters the crawlspace and the water condenses out onto the cooler surfaces. Running the air conditioner adds to the problem because it cools the home’s framing and provides additional surface space in the crawl space for humid air to condense on. From a study back in 1962 , it has been known that homes with rotting structural members “was usually linked to wet (uncovered) soil in crawlspaces, continuous air- conditioning, or low indoor temperatures (≤70°F /21°C).”
How much moisture is that? Crawl spaces with bare earth floors can add roughly 7 gallons of water into the air of a home each day.
By 1985, it was known that while a plastic vapor retarder/barrier could reduce moisture in a vented crawl space, moisture could still penetrate into the home above if the home was not air sealed where wiring and plumbing penetrate the floor. Building experts realized that in order to protect the home’s structure and to improve a home’s energy efficiency, the old assumption about keeping the crawlspace dry was totally wrong. The only way to dry out a crawl space is to eliminate sources of moisture and prevent it from getting into the crawl space altogether.
How to eliminate Exterior Moisture Sources:
- Channel rain run-off away from your home.
- Clear blocked rain gutters and downspouts.
- Grade soil around your home to slope away from the foundation at a minimum of 5% for at least ten feet.
- Knock out and seal existing air vents. Outside air brings in more moisture than it takes away.
- Seal any cracks or holes in the foundation.
- Install a vapor barrier. Measure your crawlspace so you can put down at least a 6 mil plastic vapor barrier over the entire bare earth floor of the crawlspace. Secure the vapor barrier with either a good elastomeric sealant or a pressure treated nailing strips fastened on the foundation wall.
- Insulate and seal the walls. Install fire-rated rigid foam panels (use either blue [Dow] or pink [Owens-Corning]) around the entire crawlspace).
- Seal and insulate the joist bays. Joist bays are where floor joists meet the band joist just above the foundation. While these might be tightly nailed together, they’re not air-tight. One or two might not let in a lot of air but 20 of them definitely would.
How to Eliminate Interior Moisture Sources from Your Crawl Space:
Eliminate water sources coming down from the living area that may contribute to moisture levels in the crawl space.
- Leaky plumbing— repair all cracks and bad connections.
- Water softener discharge lines should empty into a drain using an appropriate 2” air gap.
- Air conditioner condensate lines and dryer exhausts should be routed to vent outside.
- Sump pump crock pits should be kept closed and sealed if possible.
By eliminating moisture and humidity from your crawl space, your air conditioning will use less energy removing moisture from the air in your home.
Do you know of any Energy Efficiency Myths you’d like us to dispel? Share with us in the comments!