Insulated attic bypass


Insulating the attic angled portions of our second floor ceiling is the first step in a very elaborate insulation plan we have for our old house. Our long term plan is a superinsulated house — a house that stays warm in the winter with only small fast fires and cool in the summer by trapping the low temperatures of evening. Going from empty stud bays, riddled full of telephone and cable holes, to such a final product is obviously a long road. I’ll talk more about the process and the benefits in a later post, but for now our task at hand is the attic bypasses.

They’re call bypasses because air entering the attic spaces over the first floor need a bypass to reach the attic spaces over the second floor and leave via the ridge vent. (See illustration on the right.) A small portion of our second floor ceiling is sloped. It starts at about 6′ at the top of the knee wall, and ranges up to the ceiling at a little over 8′. Our first step in insulating this area is to cut 8′ long strips of 2″ thick pink polystyrene insulation that fit directly into these bays. This will leave a 1.5″ air space against the roof surface for ventilation, and the spacing will be maintained by wood spacer blocks (which also provide an attachment surface). The 8′ strips extend 3′ vertically into the attic to prevent blown-in insulation (to be added much later) from blocking these ventilation pathways. The process went as follows:

Piles of wood spacers were prepped for nailing into the stud bays. These are made by ripping 2×4 stock into three strips, roughly 1.1″ x 1.5″ each. The 1.5″ defines the bypass spacing. The strips are then chopped into one foot pieces, and two nails are pre-driven into each for rafter attachment.

The wood spacers were then nailed at intervals into the rafters, 6 per stud bay.


A 4’x8′ sheet of foam insulation was then ripped to width for the stud bay, and measured for the cut-out for the rafter tie that forms an obstruction in each stud bay.


An angled marking tool was set to the rafter-to-tie angle, and used to define the cut angle for the rafter tie notch.


The width of the notch was sized using a scrap piece of 2×4.


The rafter tie notch was cut out with a drywall jab saw.


View of a completed notch.


The panel was fitted into place. Sometimes it had to be motivated to fit.


3″ cap nails were used to hold the strips of foam insulation in place. 1″ versions of these nails are used to hold down tar paper on roofs. The 3″ versions are for this sort of thing. These like to bend, and are obnoxious to drive upwards at the roof angle. The rafters were pre-marked so the cap nails would hit the wood furrow strips.


View of some of the completed stud bays. We are a little over one third done, and hope to finish the remaining bays in the next couple nights.

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3 Responses to Insulated attic bypass

  1. Leigh says:

    Excellent photo tutorial. Looks like you can look forward to a very cozy 2nd floor.

  2. Pingback: Beginning double framing for insulation » Farm Folly

  3. Pingback: Closing in the house » Farm Folly

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