Replies

1. GBA Editor
   Martin Holladay | Jun 11, 2018 01:34pm | #1

   Howard,
   In general, it isn't a good idea to include an air space sandwiched between two layers of insulation. The main reason is that air leaks at the perimeter of the air space are likely -- and you'll end up with convection currents or air flow that reduces the thermal performance of the entire insulation assembly.

   If you ignore my advice and include an air space, the perimeter of the air space has to be meticulously air sealed.

   It makes much more sense to install 2 inches of rigid foam, followed by exterior furring strips to create a rainscreen gap, and then your siding.

2. 
   Sean_in_Montreal | Jun 11, 2018 01:34pm | #2

   Hey Howard, having a 3/4” air gap between your 2 layers of 1” eps would essentially render the first layer a drainage plain, and have no R-value, and there are cheaper ways to do that. What would be your reason for doing it your way?
   Also where are you located? 2” might not be enough depending on where you are.
   Sean

3. 
   peaceonearth | Jun 11, 2018 04:53pm | #3

   Thanks gents. I'm in zone 6, so should need R 7.5. The EPS is rated 4.2 per inch, so should be a little more than minimum. The main reason for considering this way of of doing the foam (which I likely won't now do) is the easier fastening. Fasteners can be shortened going thru strapping and 1" of foam, and then fastening vinyl siding thru 1" of foam into the strapping.

4. GBA Editor
   Martin Holladay | Jun 12, 2018 05:41am | #4

   Howard,
   If you really want the furring strips for the vinyl siding, and are restricted to 1 inch of rigid foam between the vinyl siding and the furring strips, the best way to proceed would be to fill in the air spaces between the furring strips with 3/4-inch-thick rigid foam, installed in an airtight manner. That's fussy work, but it would provide the best performance.

5. 
   peaceonearth | Jun 12, 2018 09:32am | #5

   Thanks Martin. I was aware of that as on option, but did want to avoid the expense and fuss that would entail. But I may rethink that.