Attic Ventilation in Hot Homes
Question: Can you tell me how best I can ventilate my home, which is modeled on the Cape Cod style with the upper level having a short knee? I got answers from three contractors all of which were different from each other. With the onset of summer, the second floor and the attics become oppressively hot. I intend to install new shingles. I think that this is the right time for making changes.
Answer: Even if your home was a Victorian, tri-level, colonial or whatever, I will have the same answer for you. The attic in any house that is traditionally built gets heated up with direct sunrays. I have found it difficult to breathe in attics during mid-summer. The attic resembles a blaze that within seconds my skin burst out in sweat. The temperature, I guess, should have been around 130F.
Countering infrared heat and high temperatures is indeed a challenge in Cape Cod style home. The roof rafters are steep and its bottom usually rest atop the walls of the first floor. A lesser livable space is created on the second floor as the roof rises to meet at the centre of your house. Knee walls or small vertical half walls meet beneath the rafters. The ceiling area of a good number of Cape Cod style homes have a ceiling area that is small and flat in the midst of the living area of the second floor. The underside of the roof rafters creates the remaining ceiling space.
A major problematic area is the slanted ceiling. Several Cape Cod roofs are bordered with 2 x 6s and on odd instances, 2 x 8s. Carpenters can use rough lumber of smaller dimensions because of the small vertical knee wall. Space for insulation and the free space required above the insulation are hardly there with the small lumber. It is necessary to have free air space above insulation. It is the channel through which cool air can flow through the attic space.
Hot attic air should be expelled continuously to keep your attic sufficiently cool so that this makes sure that the completed living space is cool. The air gets hot in the attic resembling the heating in a furnace. The noon temperature on a summer day with direct sunlight can heat up the roof surface to as much as 190F. This severe heat then passes on to the wood rafters and wood roof sheathing. The amount of heat radiated is large though it does not glow like a cooking element of an oven. The heat is collected by the air in the attic which then is transferred to the insulation which in turn gets hot and the heat is transferred straight to the completed ceiling. The transfer of heat is known as conductive heat transfer.
I prefer to rely on Mother Nature and would rather use wind power to expel hot air accumulated in the attic. Large quantity of hot air is pulled out from the attic spaces b turbine vents that are wind-powered. It hardly takes 30 minutes by your roofers to install these cheap devices. Installing them on the rear of a roof will also make it hardly visible when one looks from the front yard. Do not purchase turbine vents with internal braces. You should get one with external bracing instead.
Large volume of air is also moved by electric-powered attic ventilation fans or what is called as PAV fans. The problem with these fans is that they may cause great suction that they then draw air even from the interior of your home. In case you are operating air conditioning then the cool air can be drawn to your attic from the finished living space. Low-roof static pot vents or soffit ventilations required in abundance by both turbine vents and PAVs. This assists in the cooler air from the outside easily enters and replaces the hot air that is expelled in your attic.
I installed radiant barrier chips to increase the comfort at my home. The heat is reflected back to its origin by these thin plastic pieces that functions like mirrors and are highly reflective. In another sense, this works similarly like the aluminum foil that is used to cover a casserole dish. However, any radiant barrier that is of single layer would lose its potential to reflect heat if it is covered with dust. The chips offer this advantage as it does not create this kind of problem. On installation of chips, thousands of thee drift down against the insulation. You will find that there are six or seven layers of radiant barrier lying one on top of the other. Even if the top layer gets dirty, it does not matter as the layer below acts as though they are new.
The house will not become necessarily cooler with more of the insulation. When the sun sets, these can increase the heat load. The heat transfer is slowed by the insulation as it is designed for this function. If your home is warm and you do not want the cold attic to receive the heat, then this is the thing for you. The insulation should be as cool as is possible, especially the ones that reach up and contacts with the ceilings. The heat finds its way back to the attic if the insulation is hot.
You need to move large volumes of air through the attic space for keeping the insulation cool. The outside temperature of 90-98F is definitely much cooler than 140-160F. the extreme temperatures if your attic can be thwarted by your air conditioner in your home. If you are able to make sure that the attic temperature is lowered by even 15-20F you will not only be cooler but also saving money.
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August 21, 2008 | Ventilation
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