Monday, September 7, 2009

But was it too much?

In the last episode, we attacked the second part of our attic airflow problem by installing a ridge vent to let the hottest air out of the top of the attic. So now I had a fairly complete intake system of open soffit vents and extra grille vents, as well as a real outflow system. But, as the careful reader will recall, the "net free area", or airflow capability, of the intake and outflow systems should match as closely as possible.
The ridge vent manufacturer helpfully supplied the net free area per linear foot of vent, and given that I knew the length of my ridge, I could calculate my total outflow. In addition to the ridge vent, I retained the two original "turtle vents" (holes with an inverted pan over them to keep the rain off), since they still seemed to operate properly - which we checked with a simple piece of paper held below the vent on a hot day - the paper was sucked up against the turtle vent, showing that the airflow was still out. This mattered - if I had changed the dynamics of the attic too much, the turtle vents might have become intakes, bringing very hot air off the shingle surface into the attic: definitely an Energy Inefficient situation.
So, by adding up the ridge vent area and estimating the turtle vent area, I got a rough number of the net free area of my outflow system. The hope was that my inflow system would roughly match, or slightly exceed (according to some sources) that number.
I didn't have specifications for my soffit vents, but some back-of-the-envelope estimates multiplied by the measured length of the perimeter of my house yielded a good estimate. I added to that the net free area of the grille vents I had added. The result: I still was short of intakes, by a pretty good amount.
In the meantime, I had noticed another possible Efficiency Issue with the house: the garage in the summertime tended to get very hot during the day, and tended to hold that hot air, right against the house, for the entire night. In most people's books, holding a large reservoir of hot air against a surface who's other side is being cooled is a no-no for efficiency. In fact, this problem yielded another chance to kill two birds with one stone.
How to cool the garage and add more intake area to the attic at the same time? Well, as good luck would have it, my attic access hatch was located in the garage. This was cause for another Eureka moment: I could leave the attic access hatch slightly open, leave the garage door open slightly to allow air in (but not so open as to allow the neighbors curious cats in), and voila! The outside air would flow into the garage all night long, up the attic access, through the attic, and out through the ridge vent. Instant no-moving-parts, no-electricity-required cooling for the garage and supplying of air for the attic. As readers of this blog might well imagine, words alone cannot express the excitement I felt when I realized this commonsense answer to my dilemmas.

Problem: Inflow area into the attic insufficient to match ridge vent
Problem: Garage holds heat all summer long
Solution: Slightly open garage door, slightly open attic access in garage, instant cooling!
Lesson: I don't always have to buy or install something to solve an Energy Efficiency issue

1 comment:

  1. That's cool. My garage gets pretty hot but there since a bedroom is right above it there is not much we can do, other than trying to not park the hot cars in it. Some people have installed fans, but I don't think they are considering the need for the intake.