Is Your Attic Like A Dry Sauna?

Summer time has arrived in Lewiston and Clarkston with it's usual suddenness by bursting upward 20 degrees in just few days to bust the 100 degree barrier. Fortunately, as they say, it's a dry heat.

Pretty much what I expect from attics this time of year.

High attic temperatures are not uncommon and, to a small extent, unavoidable. The average roof acts as a large solar energy collector and, depending on the axis of the house, can accumulate very high heat loads. My home, orientated with a east-west main axis, has extensive roof exposure to the south that matches the arc of the sun through the sky. Ideal when I someday add solar PV to the roof, it now acts to increase my heat loads.

High attic temperatures cause several problems inside and outside the home.

Inside, the heat places an increased load on any cooling systems that you have. I've measure ceiling temperatures as high as 120 degrees from the heat generated in the attic. The less insulation you have, the more pronounced (and expensive in utility costs) this becomes. In high humidity environments, this is even worse due to the nature of water and its ability to act as a heat sink, absorbing the energy. To balance it out, you end up running the air conditioner longer, adding to it's wear.

On the exterior, shingles are subject to thermal cracking. That is, the backing material of the shingles, usually fiberglass, is impregnated with asphalt and over-laid with granules. When the roof temperature climbs, these materials go through a wide range of expansion and contraction, but not at the same rate. The backing material can split or the asphalt cracks, just as you've probably seen it do in a driveway.

Either way, you have shingle damage that will lead to early failure.

Preventative measures include improving the ventilation in the attic. Too little ventilation traps energy as the air becomes superheated. My record for attic temperatures was 154 degrees. I wasn't in there long and it took a good hour to recover. 140's are not uncommon.

Attic ventilation can be improved by ensuring that the appropriate venting is in place. Often I see ridge vents improperly cut. Too narrow a vent does not allow enough hot air to escape, raising the temperature. The same will happen if there is no ridge vent and the static or gable vents don't move enough air.

Another common defect is to have too little air entering the attic space or from the wrong location. Older homes may only have gable vents located of either end of the home. Building science has demonstrated that this creates an air passage way flowing directly from gable to gable without ventilating the lower reaches of the roof assembly.

Improving this in the short term can be as easy as adding new soffit vents to improve air intake, adding a ridge vent (or having it properly cut), or adding an attic fan to increase air movement. All of these, by the way, have beneficial effects on mold and fungus growth.

A longer term approach is to plant trees carefully, determining where they will provide the maximum amount of shade without endangering the home.

Remember that southern face on my house? I have a 100 year old walnut tree on that side and, on the east, a quaking something-or-other (I'm a home inspector, not an arborist. Sorry.) The old folks had the right idea, a century ago.

They did forget to plant on the west corner. I've put in an Asian pear tree. It will be a few years until it gets tall enough but I can wait. When it does get big, it's a two-fer: shade and fruit.