Replies

1. GBA Editor
   Martin Holladay | May 27, 2010 11:42am | #1

   Benedict,
   There is no single answer to your question. As I have often pointed out, you shouldn't replace your windows if you expect the investment to save you money. It would probably take you 50 or 80 years to save enough energy to pay for new windows.

   However, if you want new windows, get the best ones you can afford. I'm partial to triple-glazed Canadian windows with fiberglass frames.

   If your roof is leaking, you should obviously replace the roof first. If your roof isn't leaking, start with any job you want to see done. If you are planning new windows and new siding, those two elements of the job should happen at the same time.

   When your siding is being stripped, look at the sheathing carefully for signs of moisture intrusion, staining, or rot. If you sheathing is dry and sound, I wouldn't hesitate to install thick exterior foam, regardless of the 6-mil poly on the interior. Just be sure your contractor does a careful job of flashing windows, doors, and penetrations -- and include a rainscreen between the siding and the foam.

2. 
   BH2os | May 27, 2010 12:00pm | #2

   Thanks Martin -

   A preliminary check behind the current siding (cedar) showed moist 30# felt. The OSB was OK at least in this spot. This area is on the South side that exhibited the most paint problems. My plan was to strip down to the sheathing, add Tyvek Drainwrap, then 1" (is that enough?) EPS, furring, then Hardie. What do you think?

   I'm assuming I can't use the foil-backed rigid correct?

3. GBA Editor
   Martin Holladay | May 27, 2010 12:06pm | #3

   Benedict,
   In Vermont (climate zone 6), your foam sheathing needs to have a minimum R-value of R-7.5 (if you have 2x4 walls) or R-11.25 (if you have 2x6 walls). This is the minimum R-value to maintain the sheathing above the dew point, which is important if you want to prevent condensation in your wall.

   I think you are wise to choose EPS rather than foil-faced polyiso, in light of the fact that your walls have interior poly. If you assume that the EPS has an R-value of 3.9 per inch, you need at least 2 inches of EPS over a 2x4 wall, or 3 inches of EPS over a 2x6 wall.

4. 
   BH2os | May 27, 2010 12:29pm | #4

   Martin -
   Rainscreen = furring strips? Should those be treated or painted do you think?
   Are there schematics that I could give my contractor as the the best way to deal with the windows, doors etc..when adding the insulation to the outside?

   What seemed at the start to be simple looks trickier be the minute.

   Thanks for your help,

   Ben

5. GBA Editor
   Martin Holladay | May 27, 2010 12:56pm | #5

   Benedict,
   Yes, vertical furring strips can be used to create a rainscreen. You can also use a three-dimensional plastic mesh like Cedar Breather.

   If you (or your contractor) wants to learn more about flashing details for rainscreen siding applications, I suggest the following resources:

   The Stategies and Details page at GBA

   The EEBA Water Management Guide

   The Builder's Guide to Cold Climates from the Building Science Corporation.

6. 
   BH2os | May 27, 2010 01:23pm | #6

   Martin -
   I DO need the WRB on the sheathing correct?

   So a good wall system could be:

   Drywall
   Poly
   2x 4 w/ Batts
   OSB
   DrainWrap
   2" ESP
   Home Slicker/Furring Strips
   Hardie

   Thanks again

7. GBA Editor
   Martin Holladay | May 27, 2010 01:34pm | #7

   Benedict,
   Your WRB can go between the sheathing and the foam or it can be installed on top of the foam. Some builders even put a WRB in both locations. But your decision to put the WRB over the sheathing is fine.

   I know that you are planning a retrofit of an existing house. But you asked about a "good" wall system, so I feel duty-bound to note that some of the existing features of your wall are not optimal.

   1. In a good wall system, you would omit the polyethylene.

   2. In a good wall system, you would fill the studs with some type of insulation that is more resistant to air flow than fiberglass batts.

   3. In a good wall system, plywood would be preferable to OSB.

   That said, your retrofit plan is a good one, and there isn't any need to change the poly, batts, or OSB.

8. 
   BH2os | May 27, 2010 01:40pm | #8

   Thanks for your help Martin. It won't be perfect. But it will be better.

9. 
   BH2os | Jun 11, 2010 01:19pm | #9

   I'm about to finalize my siding and exterior insulation plan and had a couple of questions.

   As I mentioned above I was going to use Tyvek Drainwrap as a WRB over the OSB and under 2" of EPS foam. Do you think the "channeled" nature of the Drainwrap would cause any significant airspace that would degrade the warmth of the wall cavity? Should I use plain Housewrap instead?

   And should I use one layer of 2" EPS or 2 layers of 1" or doesn't it matter?

   Thanks for any insight
   Ben

10. GBA Editor
    Martin Holladay | Jun 11, 2010 01:36pm | #10

    Ben,
    Q: "Do you think the 'channeled' nature of the Drainwrap would cause any significant airspace that would degrade the warmth of the wall cavity?"
    A: No. In this location, Drainwrap is a better choice than plain housewrap.

    Q: "Should I use one layer of 2" EPS or 2 layers of 1" EPS?"
    A. Two layers of 1-inch EPS (with seams staggered both horizontally and vertically) will perform better than one layer of 2-inch EPS, because there will be less air leakage and heat flow at the seams.

11. 
    BH2os | Jun 11, 2010 02:09pm | #11

    Thanks for the clarification Martin.

    Should the seams be taped in this installation of the foam?

    And lastly, as to furring strips, does anyone have ideas as to the most appropriate material to use?

    Plane wood
    Primed wood
    Pressure treated
    Plywood
    Trex
    Metal

    Thanks again

12. GBA Editor
    Martin Holladay | Jun 11, 2010 02:20pm | #12

    Benedict,
    I would tape the top layer of foam. Ordinary unpainted softwood strapping should be fine.