Exterior Insulation on the Roof and Walls (vented roof)

November 30, 2015

 

How to detail the foam at corners and connections when tightening the energy efficiency of an old house. #ÜberRoof

 

Adding exterior insulation to the roof and walls of an old house is a great way to improve comfort and energy efficiency. It especially makes sense if the rafter cavities are already insulated, and it is most affordable when the roof and siding need replacement. 

 

Step-by-step:
  • The first order of business to to strip the walls and roof.
  • Next, cut off the rafter tails so that the connection between roof and walls can be sealed with peel and stick membrane. Next, cover the roof with a waterproof barrier or membrane.
  • Install two layers of foam to the roof, staggering the seams, and offsetting the joints, and tape the seams on the top layer.
  • 2x4s on the flat can provide a ventilation channel and support a new roof overhang.
  • New roof sheathing goes on top followed by some sort of roofing underlayment.
  • At the bottom of the wall, a piece of coil stock in bent into a J channel to keep the bugs out. 
  • The metal is tucked behind the house wrap. 
  • Two layers of insulation go on the walls with the outer layer’s seams taped for an air seal.
  • Attach a strip of bug screen to the bottom of the wall to keep the ventilation channels clear.
  • At the corners, weave the foam panels together to disrupt clear air paths between inside and out.
  • 1x3 strapping is screwed to the outside of the foam with screws that are long enough to penetrate deep into the framing. The strapping provides ventilation space behind the siding while providing a base for siding attachment. 
  • Box out a soffit at the top of the wall and tie it all together with fascia.
  • Soffit vents are needed, because this is a ventilated roof. Make sure to do the math on code requirements.
  • Install the siding, making sure to detail the bug screen tightly at the bottom of the wall.

Continuous insulation, multiple layers of air sealing, and ventilation channels turn a leaky old house into a 21st century home.

 

<code>

2012 IECC: SECTION R402 BUILDING THERMAL ENVELOPE

2012 IRC: SECTION R806 ROOF VENTILATION | SECTION R906 ROOF INSULATION

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Slideshow and download:

Top of wall details for a tight energy retrofit
Bottom of wall details for a tight energy retrofit
 IECC-R-value-table.png

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