Replies

1. 
   Dana1 | Dec 10, 2016 03:54pm | #1

   The R value advantage of closed cell foam is miniscule compared to open cell after factoring in the thermal bridging of the framing- it's about the most expensive R1- R2 of whole-wall performance you could ever buy, and it comes with a hefty environmental hit (4x the amount of polymer as half pound open cell foam, HFC blowing agents instead of water, etc.)

   Most half-pound open cell foam these days comes in at about R20 @ 5.5", yielding an R14-R15 whole-wall R for 16" o.c. 2x6 studwalls. There are some 0.7 lb open cell foams that would deliver about R23 at center-cavity, but like closed cell foam, the difference in whole-wall performance at typical framing fractions isn't much- less than R1.

   Half pound open cell foam at 3"+ will air seal at least as well as closed cell foam.

   Can't help you on specific consultant recommendations in your area though.

2. 
   drsoderblom | Dec 10, 2016 04:04pm | #2

   Thanks very much for the information. It gets more complicated (of course) because the original 2.0x4.0 studs tend to be irregular, and so my intent is to add 2x3s (say) horizontally to attach the drywall to to get a flat surface. That should reduce some of the thermal bridging.

   For context, shortly after I moved in a decade ago I had a contractor put in a water-based insulating foam that filled the stud cavities through holes on the outside. I could see some of this later when I removed some plaster upstairs, and all the cells shrunk as they dried, leaving 1/4-inch or more gaps all around. What that means is that even a mediocre job that truly fills the stud bays will be a vast improvement in comfort and efficiency, but I would like to do better than that if I realistically can.

3. GBA Editor
   Martin Holladay | Dec 11, 2016 05:45am | #3

   David,
   What type of siding does your house have? Do you know whether there is a water-resistive barrier (something like asphalt felt) between the siding and the sheathing?

   Lots of different types of insulation might work here, including dense-packed cellulose. The most important factors are the skill and conscientiousness of the installer. You want someone who understands the need for an airtight approach, and who has a plan to reduce air leakage.

4. 
   drsoderblom | Dec 11, 2016 08:26am | #4

   The original siding is shiplap (probably cedar), but it's been covered with cedar shingles (upper storey) and aluminum siding (lower). That was probably done ~50 years ago, and I suspect one good reason was to deal with the flaking lead-based paint on the siding; i.e., cover it up. So there are two layers.

   The shiplap has 2-inch holes in each stud bay where the water-based foam was put in. It is a long-term goal of mine to remove the aluminum siding, take off the shiplap, get it stripped, repair itm and reinstall it. I did some of that on the inside of the porch and it looks terrific. But that's long term.

   Meanwhile, one obvious concern is moisture buildup on the inside surface of the sheathing, and my thought was to do this from the inside once the plaster and lath are gone: take pieces of the lath (about 5/16x2 rough) and tack them alongside the studs but against the sheathing, then cut 14-inch wide strips of roofing felt and tack them over the lath. That way there's a 1/4+ air gap behind the sheathing. That, combined with the general draftiness of the house should prevent problems, in my view.

   And I completely agree about finding the right people, starting with someone in the area to see this first hand.

   Thanks very much for the information.