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Exterior Roof Insulation Retrofit (Vented Roof)

September 17, 2015

#ÜberRoof: A site-built ventilated SIP roof sandwich

Adding exterior foam on an old roof is a good idea whenever a reroof is planned—it is a once in 50 year opportunity to bring the house up to higher level of energy performance. Because old roofs were often framed with 2x6s, there isn't much room for insulation.

Nowadays, energy codes require a lot more.

  • First, strip the old roof and cover it with a peel and stick roofing membrane.
  • Next, cut off the rafter tails. This will make a simpler shape for the air barrier.
  • Close off the rafter bays with rigid foam or plywood.
  • Remove enough siding so that you can get to the house wrap, and seal it to the roofing membrane with flashing tape.
  • Apply Peel and stick membrane to connect and seal the house wrap to the roofing membrane
  • Install two layers of rigid foam with the seams offset. Tape the seams on the second layer. Extend the foam beyond the wall be about 4-5 inches. This will make it easier to weave into exterior wall foam, when siding replacement happens.
  • Apply 2x4s on the flat over each rafter and fasten with 6 inch screws. The 2x4s can extend past the foam to create a new roof overhang.
  • Add blocks of mortar mesh and insect screen between the 2x4s to keep bugs out.
  • Apply a new roof deck over the 2x4s and dry in with roofing felt or impermeable synthetic roofing underlayment.
  • Ladder-style blocking provides the structure of a torsion box which ties the fascia , soffit. and roof overhang together into a beam.
  • Make sure there is sufficient soffit venting.
  • On top- of it all, apply whatever kind of roofing you want.

Adding insulation to the outside of a roof is a once in a lifetime opportunity to bring an old house into the 21 century.


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