Ventilating Attics in Hot Homes
DEAR TIM: What is the best way to ventilate a Cape Cod style home that has a short knee wall in the upper level? I have asked three different contractors and received three different answers. The attic and second floor areas get stifling hot in the summer months. I am getting new shingles installed and thought this might be the time to make changes. Joan S., Royal Oak, MI
DEAR JOAN: You could have told me your house was a ranch, a colonial, tri-level, Victorian, etc. and my answer would be the same. High attic temperatures are a reality in just about any traditionally built home that gets direct solar radiation. I have been in attics in the middle of the summer where it was nearly impossible to breathe. Sweat burst from my skin within seconds of entering the inferno. My best guess put the temperature somewhere near or above 140F.
Your Cape Cod home offers distinct challenges with respect to combating high temperatures and infrared heat. The bottom of the steep roof rafters typically rest on top of the first floor walls. As they rise and meet at the center of the house, they create a smaller liveable space on the second floor. Small vertical half or knee walls intersect the underside of the rafters. Most Cape Cod homes have a small flat ceiling area in the center of the second floor living area. The rest of the ceiling space is created by the underside of the roof rafters.
The slanted ceiling is a huge problem area. Many Cape Cod roofs are framed with 2 x 6s and on rare occasions, 2 x 8s. The small vertical knee wall allows carpenters to use smaller dimensional rough lumber. But this small lumber leaves very little space for both insulation and the required free air space above the insulation. Free air space above all insulation is a necessity. It is the conduit that permits the free flow of cool air through an attic space.
To effectively cool your attic, which in turn helps to keep the finished living space cool, you must constantly exhaust hot attic air. The air in an attic gets hot not unlike air that gets heated in a furnace. The actual roof surface temperature can soar to nearly 190F in direct sunlight on a hot summer day at noon. This intense heat passes through to the wood roof sheathing and wood rafters. Even though they do not glow as would a cooking element in an oven, they are radiating enormous amounts of heat. The air in the attic collects this heat and in turn transfers it to the insulation. The actual insulation gets hot and transfers its heat directly to the finished ceiling. This is called conductive heat transfer.
I love using Mother Nature's wind power to exhaust hot air from attics. Wind-powered turbine vents do a magnificent job of pulling vast amounts of hot air from attic spaces. These inexpensive devices can be installed by your roofers in less than 30 minutes. A typical home needs three turbine vents. I like to put them on the backside of a roof so they are barely visible from the front yard. Be sure to buy turbine vents that have external bracing instead of internal braces.
Electric-powered attic ventilation (PAV) fans will also move lots of air. But these fans can develop so much suction they can actually draw air from inside your home. If you use an air conditioner, this can be bad as you draw expensive cool air from your finished living space up into your attic. PAVs, as well as turbine vents, require abundant soffit ventilation or low-roof static pot vents that allow cooler outside air to readily enter the attic as hot air is exhausted.
I increased the comfort of my own home by installing radiant barrier chips. These highly reflective thin pieces of plastic act like mirrors and reflect heat back to its source. They work exactly like a piece of aluminum foil that covers a casserole dish. But a single layer of any radiant barrier can lose its ability to reflect heat if it gets covered with dust. This is why the chips are so much better. When the chips are installed tens of thousands of them float down onto the insulation. Look closely and you will discover that you might have six or seven layers of radiant barrier on top of one another. The top layer may get dusty over time, but the chips just below work like they are brand new.
Adding more insulation will not necessarily make a house cooler. In fact, it can add to the heat load after the sun goes down. Insulation is designed to slow the rate of heat transfer. That is great if you have a warm home and don't want heat to seep into a cold attic. But you want your insulation, especially that which touches up against your ceilings, to be as cool as possible. If the insulation is hot, it slowly liberates this heat back towards the attic.
The best way to keep insulation cool as well as attic air is to move lots of air through the attic space. Even though the outside temperature may be 90 - 98F, it is much cooler than 140 - 160F. Your home air conditioner works very hard to combat extreme attic temperatures. Lower the attic temperature even by 15 - 20F and you will save money and be cooler.