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boerssd 11-21-2012 12:39 PM

Insulation Retrofit - Double Wall Construction
Need some advice on the best way to re-insulate my home during a large scale renovation I'm doing next year. The critters have come through the old siding (knot holes, cracks, warping) and perforated the house wrap, and tunneled in the insulation. My home is a nightmare to heat and cool.

First thing you need to know before you offer advice is that my home is double wall construction on most of the walls. So - from the inside of the house to the outside of the house - these are the layers...
1) 1/2" drywall
2) 2x4 studs insulated with 3.5" Fiberglass pink
3) 6mil plastic vapour barrier
4) 1/2" OSB
5) Insulated dead space - 2" I believe
6) 2x6 studs insulated with 5.5" Fiberglass pink
7) Tyvek or equivalent house wrap
8) Wood bevel siding

My initial thoughts are to:
1) pull the outside of the house off - until I get down to the OSB that is sandwiched between the two walls.
2) Put down an inch or two of spray foam into the dead space between the two walls (on top of the OSB) - primarily to air seal the envelope.
3) Re-insulate the 2x6 stud wall with Roxul.
4) Put another 1/2" OSB layer of sheathing on the outside of the house - primarily as a rodent / bird barrier (little buggers).
5) Re-wrap the house in a Tyvek or equivalent house wrap
6) Re-side the house with a horizontal siding that incorporates at least some form of mechanical joint between the rows (tongue and groove, channel rustic, etc.)

So - my questions:
1) Am I nuts or does this approach make sense in general?
2) Yes to spray foam that way, no to it, or change where it is in the layers?
3) Does any type of radiant barrier make any sense instead of, or in conjunction with - the house wrap?

I get one shot at doing this while the whole outside of the house is peeled off. I want to do this once and do it right.

Another side note - all of the outside windows will be replaced at the same time is this tiny little project.


nealtw 11-21-2012 03:12 PM

I see one problem. The foam on the outside of the osb with poly on the other side. Sooner or later that osb will get wet, roof leak or plumbing or something, it will never have a chance of drying out. If you are in a cold zone the poly should be next to the drywall on the inside and if you are in a warm zone you different rules, but I don't think you ever want to trap wood between two.

boerssd 11-22-2012 09:10 AM

Technically - the spray foam does not qualify as a vapour barrier until it reaches a thickness of greater than 2 inches. Also - in double wall construction, the position of the plastic vapour barrier is no longer at the drywall - it is between the two independent walls as listed above.

You do point out my obvious concern though. I'm hoping that 1" of sprayfoam will essentially seal the air envelope, without fully trapping the 1/2" OSB between two vapour barriers.

The spray foam installers I've spoken to don't think this is an issue - this is their recommended approach. I have to have my grain of salt handy though and vet this with folks who don't have a stake in the project. These guys are selling spray foam at the end of the day.

Forgot to mention - Ottawa, Ontario area - so yeah - I'm in a cold zone...


nealtw 11-22-2012 09:57 AM

Open cell foam is not a vapour barrier, but just because you can't trust it to stop moisture. Can you trust it to let moisture thru?
A properly installed house wrap is an air tight envelope. I did find this interesting.

boerssd 11-22-2012 10:14 AM

Thanks. That link would seem to indicate that my suggested application would be ok. I will continue the research and see where I get to.

I guess my other question is - given the thickness of the walls - literally 12" of insulated space - am I just going one step too far by considering the spray foam in between?

There is always a point of diminishing return... Would replacing the compromised (rodent tunnels) insulation and re-wrapping my house give me the value I'm looking for? The spray foam addition adds about $4k to the renovation...


nealtw 11-22-2012 10:40 AM

Ya, you hate to spend the money for little return, but then you will never have a better chance to get it done.

GBR 11-29-2012 07:30 PM

I'm with Neal on this. Adding SPF to the outside of OSB is problematic from a moisture standpoint. End of article:
Granted that was an early summery of ocSPF, but you don't want it attached directly to OSB, need a gap for drying:

With the poly inside you limit the drying and with only 2" (R-10), the sheathing temperature will be at 41*F for your location, condensation forming there at 35%RH and above, at 70*F room temp.--- from any (1/8") gap in the air barrier (poly)- especially around the exterior wall outlets/switches;

You are stopping the thermal loss at the foamboard, inside the wall, the extra 5-12" doesn't help much. Better to use more f.b. on the exterior, leave the cavity insulation alone, then the higher sheathing temps will stop condensation from forming:

Moisture in cavity from air gap in a heating season- pp.10:

Drain-able housewrap like Dupont "stucco wrap" or rainscreen walls, fig.24:


boerssd 11-30-2012 12:49 PM

Gary - thanks for the response. Lots of reading material! Appreciate the time it took you to put that together. I'll give it all a read and reply back here. Sounds like the spray foam on the inside cavity is not a great idea.

I do have a few questions though before I get through the links...
1) You say "leave the cavity insulation alone". I know before I even take the siding off that I'm going to find a lot of mouse tunnels etc. Would you leave it alone even with that?
2) Strapping on 2"-4" of foam insulation on the exterior of the house could be troublesome. My walls are already 12" to 14" thick. Adding to that can be complex. My gable ends have absolutely no roof overhang - the roof would need to be built out to accommodate another 4".
3) Is the foam board a necessary part of the rainscreen application? I was considering this approach, but minus the foam board. Essentially adding 1/2" OSB to the outside of the house, the dupont commercial wrap D (same as stuccowrap), and then the strapping and venting for the siding on top of that.


boerssd 11-30-2012 07:17 PM

There is a paragraph in that second article you linked that leads me to believe that I should NOT put foam board on the outside. It makes me question whether or not I should even put up another layer of OSB. Here is the paragraph:

With foam sheathing (of the appropriate thickness – R-value based on condensation control as noted above) the type of cavity insulation becomes irrelevant. Fiberglass batts, cellulose, low density foam, high density foam, ground up “blue jeans,” whatever, it no longer matters, they all work. Just don’t install an interior side plastic or foil or vinyl vapor barrier. A wall assembly has to dry in at least one direction and with that foam on the exterior the only practical drying direction is typically inwards. Remember that the assumption here is that the foam on the exterior is impermeable, which is not always the case – but once again we are being conservative.


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