Replies

1. GBA Editor
   Martin Holladay | May 14, 2015 06:31am | #1

   Chris,
   Either asphalt felt (tar paper) or plastic housewrap (for example, Tyvek) will work fine as a water-resistive barrier (WRB). A WRB is essential. However, neither asphalt felt or plastic housewrap is very good as an air barrier (although Tyvek is better than asphalt felt at resisting air movement). Ideally, your contractor needs to pay close attention to airtightness when doing this work.

   The best thing you can do is to specify the installation of rigid foam with a minimum R-value of R-7.5 on the exterior side of your wall sheathing. (You are in Climate Zone 6, so R-7.5 is the minimum R-value for this foam layer if you want to avoid moisture problems.) This rigid foam layer should be installed with close attention to airtightness.

   For more information on these topics, see:

   How to Install Rigid Foam Sheathing

   Calculating the Minimum Thickness of Rigid Foam Sheathing

   All About Water-Resistive Barriers

   Roofing and Siding Jobs Are Energy-Retrofit Opportunities

2. 
   cfelker1 | May 21, 2015 10:28pm | #2

   Thanks for the response Martin. If I opted for the Tyvek and then upgraded my siding to the Alside Prodigy insulated siding, would that be a good option for adding a little R-value, longer pieces (less seams) and rigidity to the siding? Would this create any moisture issues in the wall since it is rated at 5 perms?

3. GBA Editor
   Martin Holladay | May 22, 2015 04:54am | #3

   Chris,
   I looked at a few web sites, and couldn't find an R-value rating for Alside Prodigy (not a good sign). The foam insulation varies in thickness, so the R-value is hard to calculate. The thickness appears to average less than 1 inch.

   That's not the approach I would take if I were you. I would install a continuous layer of rigid foam, then the siding. Insulated vinyl siding doesn't drain as well or promote drying as well as uninsulated vinyl siding, so a combination of real insulation and uninsulated siding is still best.

4. 
   cfelker1 | May 22, 2015 08:42am | #4

   The Prodigy siding is supposed to be R5 although it probably would not be a continuous foam layer. Just to give you more details on my situation...the siding reno really encompasses just the second floor of my house and one wall of the main leve and the second level is only 600 sq ft (smaller house). I have not too concerned about reducing energy costs as In the winter my natural gas bills are very reasonable (under $100) although in the summer the upstairs does get warm and about 3-4 degree F warmer than the main floor.I am concerned with moisture and inward vapour transmission int the walls. I am also getting my attic tooped up from R20 to R50 of lose fill with more venting added in the soffits.

   I do like the look of the Prodigy and the insulated back adding rigidity to the siding itself. What about Prodigy, 1x4 strapping (create air barrier and rain drainage), Tyvek, exterior gypsum (existing), fiberglass batts with kraft paper backing (one wall as poly vapour barrier). Understanding that a rigid layer of foam may work better, wouldthis approach be slound?

   Thanks again.

5. 
   cfelker1 | May 22, 2015 08:43am | #5

   I forgot to mention the siding is supposed t be a permeable rating of 5 perms.

6. GBA Editor
   Martin Holladay | May 22, 2015 08:53am | #6

   Chris,
   Problems with inward vapor drive (usually called inward solar vapor drive) are highly unlikely if your building has aluminum siding or vinyl siding. These problems require a so-called "reservoir cladding" like brick that holds a lot of moisture. So don't worry about that.

   If you aren't worried about saving energy, you can use any type of siding you want. You won't get much benefit from the foam layer in insulated vinyl siding if you install the siding over a rainscreen gap, because the gap will introduce outdoor air between your siding and your sheathing. Choose insulated siding if you like the stiffness it provides, but don't expect any thermal benefit if you install it over a rainscreen.