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Building Science

Building Science Basics: Hot Air Rises

Warm air rising may seem obvious, but its effects can be subtle

Last week provided an object lesson in the stack effect, which is what building science nerds call it when warm air inside a building rises. Part of the ongoing barn project is turning the existing hayloft into a climate-controlled storage loft. The owner needs a place to gather the archives of a long career in order to work on a memoir. In addition to being heated, cooled, and humidity controlled, it’s also to be finished nicely with a lot of beadboard paneling on the walls and on the cathedral ceiling we’ve been framing.

This barn dates from somewhere around the dawn of time, so there are challenges. In the first place, the existing roof structure consists of 3×5 hemlock rafters on—wait for it—4 ft. centers. The roof is only 16 ft. wide and the cedar shingles on skip sheathing is as light as roofing gets, but still…

How have buildings like this survived 200 years of New England weather?

Not only is the roof framing wildly overspanned by modern standards, the existing timber frame is neither square nor level. Plus, two 6×6 tie beams running across the loft to keep the top plates from spreading did so with marginal success.

Into this space Brent and I have been imposing plumb, square, level, and structure. The first step was to add a ridge beam. Consisting of three 36 ft. long, 14-in. LVLs, how two middle aged guys muscled 750 lbs. of beam in place without a crane or an orthopedic surgeon is a story for another day.

I digress. I’m writing about stack effect here.

Cooked at the end of the day

The carpentry required to add new rafters under the existing roof was challenging. We determined angles using string and a T-bevel, measured from plate to ridge beam with pinch sticks, and…

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