Help me validate my thinking – exterior insulation
user-590753
| Posted in Energy Efficiency and Durability on
We are finalizing the construction details of our build in zone 5 – 2×6 wall construction. Originally we were looking at using 2″ of exterior XPS on our home, but found quickly with the Hardie siding that is causes quite an incremental cost due to labor for fastener and furring details. I really want to eliminate the thermal bridging so we are now considering 3/4″ GPS or XPS with LIB bracing and without osb. Then doing flash in batt with 2″ closed cell and R13 walls. Based on the ratio rules, I should be fine with use of external insulation. Is there anything I am missing in regards to durability for this construction?
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Replies
Hardie requires a rain screen now so you're pretty much stuck with installing furring on the outside of the house. Best bet is to avoid the XPS and go for the proper amount of foam on the outside utilizing PolyIso, EPS, GPS or Kooltherm.
I was already counting on the rainscreen. The issue is using 5 1/2" headlok screws to attach 2" CI. This adds a lot to labor and complexity vs just shooting nails through 1/2" or 3/4" rigid foam. This is not to mention the additional labor and materials to create window bucks. I want to build it tight and right, but budget also needs to be considered. It seems that the exterior insulation and flash and batt is a good compromise when considering the payback for energy savings.
>"I really want to eliminate the thermal bridging..."
There is no such thing as eliminating thermal bridging.
Continuous foam reduces or "breaks" heat flow through the thermal bridge, but it's still there. The only way to eliminate it is to put 100% of the insulation on the exterior of the structure, none between the studs (in which case the empty cavity would have a lower R than the studs, and thus a "thermal bridge" of a sort.)
>"Best bet is to avoid the XPS and go for the proper amount of foam on the outside utilizing PolyIso, EPS, GPS or Kooltherm."
That's right!
Sheathing with 2" of foil faced polyiso is half the density using half the (closely related) polymer of 2" spray polyurethane foam, and more friendly blowing agents to boot. The 2" of closed cell foam in the cavity would be a waste, barely moving the needle on the whole assembly performance relative to just installing R21 fiberglass or R23 rock wool due to the thermal bridging, whether there is exterior foam or not. So save the environmentally and financially expensive foam budget for the exterior, where it does the most good.
https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste
XPS is an environmental nightmare on both a blowing agent and polymer use basis, and has NO place in "green" building (unless reclaimed from building demolition/deconstruction). This just reflects the relative CO2 equivalent aspects, but the other environmental his from the manufacture of polystyrene is somewhat worse than polyisocycanurate or polyurethane too, but not the order of magnitude worse that you get from the HFC blowing agents used for manufacturing XPS:
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png
In zone 5 it only takes R7.5 to provide adquate dew point control at the sheathing with R20 cavity fill. With just 1.5" of polyiso on the exterior (~R9) you'd have decent margin on 1.8lbs blown fiberglass or R23 rock wool batts in the cavities.
If you go with 1.5" rigid insulation (polyiso), no sheathing and 1/2" strapping, you can hang the strapping with 3 1/2" nails with a framing nailer, no need for long screws. This would be higher performance than the spf+rigid you are proposing. Your proposed assembly works out to around R26 wall, this would be around R28.
You can also look at going with 2x8 walls 24 OC without exterior rigid insulation. I would still include a rain screen in this case. This would work out to around R24 wall using R30 batts and would be much cheaper to build.
On the outside of my house I used 4-1/2" Headlocks to attach the strapping over the 2" foam, pretty easy to run them in with a half inch drill, only real issue I had was the screws wandered if they weren't straight into the studs. It does take extra time but once we got a system down for installing the strapping it went pretty quickly. Same for the exterior window extensions.
I think this is always the challenge. I really don't find installing exterior rigid foam with long screws that difficult, but if somebody has not done it, they will quote higher because of the unknown.
The cost of headlocks for an exterior wall can also does add up, you quickly get close to the cost of insulation if you have heavier cladding.
The 4-1/2" Headlocks were $100 for a 250 count bucket on Amazon, I calculated I needed 3 buckets for the entire house, 24" oc. Foam board came in quite a bit more but I am doing the install myself so limited labor costs are being applied.
Lot of this is region specific. In the great white north those screws are close to $1 each at the box stores, you can quickly get up on the cost for 16OC studs with 18" screw spacing. 2" poyiso is around $25 here.
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