Exterior Insulation: Wall Cavity Insulating Too – Good or Bad?
I am brining life back to a 100 year old home, deciding to add exterior insulating and replace the siding on the house after we had the windows replaced with new triple pane throughout. I am ripping off the old siding and tar paper, then installing the new exterior build-up:
-New weather barrier
-1.5″ thick IkoEner Air exterior continuous on the outside
-Rainscreen – 1″ vertical wood strapping
-LP SmartSide wood siding
Now that I have removed the siding and peeling off, I can see in some area gaps to the wall cavity that has no insulation or pockets of wood chips and feeling like now is the time to add insulation to those cavities.
Wall cavity build-up:
-Tongue and groove 3/4″ ship lap, 2 x4 fir studs, 3/4″ ship lap, 1/2″ furring stripes, horizontal lathe and plaster
-no vapour barrier
-Climate Zone 7 (Saskatchewan, CA): cold winters (-20C to -35C), hot summers (25-35+C)
QUESTION: Do I add wall cavity insulation from the exterior? Or is this a negligible gain that risks condensation? I’ve scoured articles and can’t find a clear answer.
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Replies
To answer the Specific question asked, blown in insulation is typically done from the exterior.
Caveats :
“Now is the time” to:
A. Use the Wood Wall Calculator at Applied Technologies Building Group to understand the implications of stud void insulation / continuous exterior insulation / moisture management etc.
B. Seize the opportunity to update utilities in the exterior walls (adding box volume, replacing wire and cable, adding new circuits, providing instrumentation cabling or raceways); noting clearances and locations of cables pipes and anything else one might penetrate or sever with furring fasteners;
C. Before you’ve removed the tar paper, be sure to document photographically “living in a tar paper shack“. It’s good for the hymnal (ie family story book).
D. Plan Your heating and cooling equipment upgrades, ventilation And circulation ductwork. ( I know you got new windows, I assume some are operable. Once you put a building wrap or the exterior insulation on, you will begin to suffocate and you will have to open windows just to breathe.)
E. Inspect evaluate and plan your attic upgrades (soffit venting, insulation levels, outdated, wiring replacement, used to use the attic for any of the ventilation and or circulation duct work and or equipment like an air handler, Slim duct fan coil, etc.)
With respect to polyisocyanurate exterior insulation: attempt to obtain the tested u values at the anticipated in-service temperatures, you would like this material to function at. The label R value Is based on an average temperature of 75°F, with a Delta tea of 50°F (a little early morning arithmetic tells me that this is for the condition of 50° on one side and 100° F on the other surface. In particular, look at its performance during your chillier months.
In zone five my exterior insulation experiences 0° F on one side and 40° F on the other side. The average would be 20° F. I used 2 lb./ft3 EPS (Type 9) On account of improved performance at low temperature, availability, and a workable level of moisture semi-permeability / semi-impermeability.
Take the time to document the location of your studs, rim joists, and all the discontinuities and irregularities. Marking up photographs can be very, very very useful marking up your foundation wall carefully no scribbles will be very useful.
Reset the nails used to hold your sheathing to the studs. Many will have worked their way out a bit.
You may want to add a few fasteners.
You may be able to avoid counter barring. Through careful arrangement of furring screws
Thank you for the detailed reply and recommendations. To add further context and comment:
A) This is very useful and informative - thanks for the lead
B) I started upgrading utilities last year by clearing the attic and gutting basement removing all knob and tube, running new plumbing throughout. No electrical in exterior walls except the 2nd floor where I ran new electrical down from the attic into bedrooms. Main floor no old electrical or utilities in walls. I will however, be using opportunity to run new electrical up from basement to a few spots for exterior plugs as well as a home entry exterior light (nothing ever existed).
C) Good call on documentation, I am making more pointed effort to document throughout
D) Heating and cooling - we've upgraded our boiler last year; no HVAC. Air conditioning is on temporary units but plan to add in ductless splits potentially next year if we can afford it. For now I roughed-in new ductwork to bathrooms and kitchen for a new HRV to provide fresh air exchange as the house sealing will now shift
E) Attic upgraded - no soffit vents due to construction type and not feasible as sill is only a 2" plate and exposed plank to outside. Instead I will be increasing the gable vent sizes while residing to provide required air exchange. Attic as of this spring is all new electrical, air sealed and R80
Remainder - thanks, I am able to clearly see all studs, joist etc now with siding and paper off so have been marking out to land my rain screen studs on strapping level.
The polyiso performance changes is something I overlooked and am afraid I'm committed to - half of the 1.5" insulation in hand and non-restock without exorbitant fees. I'll dig in to see what further testing data I can find.
The IRC that most of the US uses as a model code includes a table that shows how much exterior and interior insulation you need with a class 3 interior vapor retarder--e.g., standard latex paint on drywall or plaster--rather than an a vapor retarder with a lower perm rating. The ratio of interior to exterior can also be extrapolated to more highly insulated walls, but the table includes 2x4 walls and 2x6 walls. It's table R702.7(3) here: https://codes.iccsafe.org/content/IRC2021P1/chapter-7-wall-covering#IRC2021P1_Pt03_Ch07_SecR702.7.
In climate zone 7, with 2x4 walls, you need at least R-10 on the exterior, or about 43% of the wall's total R-value.
Aged, cold polyiso is likely to be around R-5 to R-5.5/in. Because this is a long-term potential moisture issue, and not an arbitrary energy requirement, I use conservative values for foam. At 1.5", your exterior R-value will be around R-7 to R-8, so the most you should put in your framed cavities is about R-9.
That's if you are committed to the R-7ish exterior insulation. I would fill the cavities with insulation, for about R-13, and install at least R-10 on the exterior, which would be 2" of polyiso. The more R-value on the exterior, the safer the assembly.
Unless you want to remove the sheathing, I would dense-pack the cavities from the exterior with cellulose or wood fiber insulation, which come with a borate treatment that help protect everything they touch. Or you could blow in fiberglass if that's the only option available to you.
Thanks Mike! That table and info you highlighted is helpful. I noted above I failed to factor in the polyiso performance and am committed to the 1.5" at least on half right now.
Removing the sheathing I have ruled out - the wood framing is excellent and the fir ship lap on the exterior is not shy of fasteners all in good shape and biting well. Coring holes for blowing in dense-pack insulation is definitely the route. I didn't think dense pack cellulose was a service or option in our area as I have rarely seen but found a few companies who do do it. My ignorance was on cellulose being a moisture trap but I suppose the new material is better performing as anything with borate treatments and fire retarder.
Are you doing the work yourself?
If yes then I would insulate the stud bays with Roxul.
You don’t mention how moisture inside the structure will be dealt with. Perhaps it’s already in your list of projects and not mentioned in the post?
To date moisture within may not have seemed to be an issue but I can assure you that with the insulating and air barrier improvements moisture absolutely will be an issue.
If your doing any work at all on the foundation exterior taking steps to direct water away from the structure will be well worth the time/expense.
Further if the basement has a base level of moisture above 60 then a standalone dehumidifier (Santa Fe is a good choice, there are others) would be money well spent.
I am doing all of the work myself, except potentially filling the stud cavities. Roxul would be my first choice if I was pulling of the sheathing but that is going to be too large of a task given the build quality and secureness of the existing.
I am installing an HRV for further moisture control and have a dehumidifier I ran in the basement to keep levels steady. My current compounding issue is I gutted 3/4 of the basement when we got the place as I found very poor finishing (no insulating drywall to strapping strips) on the exterior walls. I hope to get to insulating with VP on the basement this winter. Exterior wise I only had one grade issue that I rectified last summer with positive drainage throughout and no signs of ingress or high moisture content on the exposed concrete foundation walls.