[Greenbuilding] Siding over foam insulation?

[Bob]

Justin Close wrote:
> A, hopefully, quick question this time around.   :)
>
> One of the bidders for our siding job on our house, in response to my 
> question about installing an inch or two of foam board over my 
>   
Justin,
I agree with the good advice you've been getting, but wonder about the 
cost-effectiveness of stripping down to the studs, and certainly of 
foam-filling the cavities.  All of the new construction I've tested that 
used spray foam has used some variant that allowed them to benefit from 
the air sealing and fully conforming properties of foam but falling back 
on a cheaper-per-cubic form of insulation for the bulk insulation.  One 
technique (working from the inside, so not applicable) is 
"flash-and-batt"; an inch or so of foam on the outer layer with batt 
fiberglass bulk fill.

I would consider leaving the existing subsiding and blowing cellulose 
("dense-packing") or  fiberglass  ("spider-packing")  into the closed 
cavities.  "Tubing" is the technique I trust, but it should also be 
possible with a direct blow (through access holes), provided the holes 
are larger and spaced closer than the old two-hole per cavity approach.  
I'm more familiar with using cellulose, where you need to get a density 
of 3.8 to 4 pounds per cubic foot of cavity, but am assured by an 
installer that they have success with fiberglass using what he called 
"spider packing".  (I haven't had time to get full data on the 
technique, but watching them on a couple of jobs I'm willing to give 
credence to his claim.)  Be sure they seal the access holes well.  
Unless the foam layer has low permeability, you might also want house 
wrap to serve as a drainage plane.

The inside vapor barrier issue is a very real hazard.  If you do open 
the walls and find it, just cut along the studs and eliminate it.  If 
there is an inside poly barrier, I retract my suggestion about blowing 
the walls, at least with cellulose. I would make a small (easily 
repairable) inspection hole by removing a bit of the interior surface in 
an inconspicuous location (closet or inside a cupboard) to check for the 
poly.  You are only likely to find the poly if the interior is drywall.  
If the home is old enough for plaster walls you can forget about vapor 
barriers.


A side issue, that can be as - or more - important is to check the seal 
around the windows.  Since you didn't mention replacing / reseating them 
as part of your plan, you probably can't properly flash the rough 
openings.  (See Joe Lstiburek's technique)  However, if you can save a 
bit on the residing / reinsulation, you would do well to add it to your 
plans.  Water leakage at the window openings can undermine all your 
careful efforts.  If the house has some age, I'll be surprised if you 
don't find some water damage at the windows has already occurred.  Caulk 
is only a temporary protection; it needs to be renewed as regular 
maintenance.  Proper flashing is the back up (real) water protection.

Sorry.  There are simple questions but few "simple" answers when houses 
are concerned.  I hope this helps.

Bob Klahn
YS Building Science