Somerville is the location of our latest DER (deep energy retrofit). This project is a little different from the others: for one, NStar is the supplier of the incentive package. Nstar’s program requires a HERS rating to be performed prior to doing any work. This is the guide to what measures need to be incorporated to increase the performance of the dwelling.
With this said, new techniques and details are used and thought up as each one of these projects unfolds. On this DER we chose to execute a “chainsaw” technique. This is where you remove all the details or projections of the dwelling. You then wrap all the surfaces (roof and sidewall) with the 2 layers of Dow Tuff-R rigid foam. The Chainsaw technique eliminates the troublesome soffit and rake intersections. Instead you have a solid connection of foam to foam with each layer taped to combat air infiltration.
The sidewall layers consist of James Hardie house wrap with all seams taped. The first 2” layer of Tuff-R rigid is then screwed to the dwelling. Again, all seams are tight and taped. The second layer is staggered from the first layer so no seams are within 1’ from either side. Both these layers are fastened temporarily with washers and screws at a couple of points on each sheet. The permanent technique is strapping laid out to mirror the interior structural framing. This strapping is then attached every 16-18” with 7 ¼” screws that connect to the structural framing and not the wall sheathing.
The roof layers:
Once the roof surface is down to the original sheathing a layer of Grace Ice& water barrier is installed and laps over the side wall rigid where they meet. Next comes the double layer of Tuff-R installed by a similar technique as the sidewall execution. The strapping is substituted for 5/8” sheathing. The sheathing gets screwed to the existing roof rafter to tie the structure together and integrate them. A second layer of Ice & Water membrane is then applied the new roof sheathing. The sheathing is left long on both the soffit and rake of the dwelling until the trim build-outs are completed.
There are numerous details that I have breezed over but this will give you a sense of why this technique was used and how the photos relate to the work being executed.