I am debating if it is more economical, easier, to put up polyiso inside instead of outside the house.

Outside I need 1.25" in Z5 requiring a lot of hoop jumping to deal with siding installation, brick, interior vapor barriers, furring strips, all adding up to lots of cost.

I am wondering if I should do this instead:

gypsum
1/2" polyiso on interior
2x6 stud wall
R21 HD batts or demilec agribalance ocSPF
1/2" sheathing
housewrap
siding

I lose 1/2" of room space but the wall breathes to the outside, like most homes. I don't have to worry about fasteners sagging over time, exterior water being trapped inside exterior-applied rigid. I also still get a thermal break, just on the inside. Though it is not as effective, it is still a R3.7 boost to the wall assembly.

Has anyone done this? What issues do I consider?

Makes more sense than putting the vapor barrier on the wrong side. Johns Manville talks specifically about interior side installation.

Consider cellulose for the cavities, 3/4" polyiso (interior, taped) and Zip sheathing (exterior, taped). Use gaskets (not caulk) where needed. Consider EPS if you want exterior foam (perhaps in addition to interior polyiso).

the exterior foam will help to keep your studs warmer, and help your insulation to work better. If you use the right screws on the strapping, you will not have a problem with the siding "sagging". Any water that gets behind the foam should be able to drain out, but proper flashing techniques are always key to keeping water out of wall assemblies; foam or not.

Bob, if I do exterior, I need to:

1) have the framers furr out the windows by framing them with an extra 2x4 then 1x4 on top of that.

2) I have to get them to stop after framing for a week to allow me time to put up the exterior insulation, then furr it up, tape it, etc. Then come back to install the windows.

3) I'm worried about the fact that the polyiso expands/contracts and the tape will fail eventually. This will let air and water in

4) The tape will eventually fail completely

5) If I need to reside (plan to be here 20 yrs), I worry about 1x4s having nails in them, then pulled out later and renailed.

6) I am concerned about the inside dew point with nails and screws through the insulation - this will cause condensation

7) I need a special order vapor barrier

8) The biggest issue I have is the cost. two layers of polyiso even with me doing the labor for 5300 SF is like 390 sheets. It's expensive! I'm not sure what/if there is a federal rebate even, but if there was one it will likely be minimal so this is all cost prohibitive.

Bob, if I do exterior, I need to:

1) have the framers furr out the windows by framing them with an extra 2x4 then 1x4 on top of that.
Nope; Foam to the window openings; "strap" around the openings with a 6" wide strip of 3/4" advantech

2) I have to get them to stop after framing for a week to allow me time to put up the exterior insulation, then furr it up, tape it, etc. Then come back to install the windows.

3) I'm worried about the fact that the polyiso expands/contracts and the tape will fail eventually. This will let air and water in
It does not expand/contract anywhere near that much
Use a good tape; not junk. Siga for instance.

4) The tape will eventually fail completely
see above.

5) If I need to reside (plan to be here 20 yrs), I worry about 1x4s having nails in them, then pulled out later and renailed.
Siding should last for generations unless you're using vinyl.

6) I am concerned about the inside dew point with nails and screws through the insulation - this will cause condensation
you're overthinking this, unless you are using less foam than you should.

7) I need a special order vapor barrier
Why?

8) The biggest issue I have is the cost. two layers of polyiso even with me doing the labor for 5300 SF is like 390 sheets. It's expensive! I'm not sure what/if there is a federal rebate even, but if there was one it will likely be minimal so this is all cost prohibitive.
You can pay now or later. Or you can use recycled foam for 1/2 to 1/3 the cost.

"Nope; Foam to the window openings; "strap" around the openings with a 6" wide strip of 3/4" advantech "


Thanks. Can you explain a little more? Are you saying to put the foam up to the RO edge, then put this on top of the foam, screwed through the foam as if it was acting like a furring strip? Or are you saying to box frame the inside of the window opening out - making the RO 1.5" wider/taller and making a "sleeve" to slide the window into? I would be concerned about nailing the window into the 3/4" thick edge of this material.


"Why the need for the special vapor barrier." If I put up only one layer of 3/4" I will need to allow the wall to breathe inward. If I put up two layers, I still have 800SF of wall that will have brick with no foam behind it (traditional insulation). So I'll still need the special vb.


"You can pay now or pay later" - this is actually short sighted. Paying for it all up front is harder to justify than paying a little extra per month. What is this going to save on my heating cooling bills per month? $40? At 5k extra, it would take over 30 years to repay (based on inflation and investing that money at 5% return per year). The reality is I will never regain that money if I did the math - because I'd move, be moved into a home, or be dead by then. At $10k extra. Sheesh. Forget it. This is more about comfort and doing what's right.

Are you saying to put the foam up to the RO edge, then put this on top of the foam, screwed through the foam as if it was acting like a furring strip?
yes. You can also "box frame the inside of the window opening out - making the RO 1.5" wider/taller and making a "sleeve" to slide the window into" and nail the Advantech to this as well. the box aids in taping the window.

"short sighted"? I'd argue it's exactly the opposite: you're building a home that will last, presumably, a minimum of 50 years, possibly 200 years, so build it for that period. If you are only concerned with the next five or ten years, why use brick? build it out of Homosote & paint the bricks on. Fuel gets more expensive every year, whereas what ever you build into the house only increases in value every year. The real estate community is slowly adding the HERS rating to listing sheets - at some point, when buyers can compare, smartly built, well insulated houses will be worth more than the others.

It would, as a matter of energy policy, be nice to always see ratings that indicates that house A is actually cheaper to own than house B (that may cost less upfront). Not to mention the environmental impact.

Not having (or not using) a rating discourages energy efficiency beyond code minimum.

the larger point is the importance of finding cost effective ways to build. With exterior foam, used polyiso cuts the material cost by up to 2/3, but does not affect the labor cost. Exterior cellulose, or batts, using some type of larsen truss - which could be homemade, or I joists, or a manufactured truss similar to a floor truss, saves on both material cost and labor cost, and allows the wall to breath out. Interior foam helps, but unless it's EPS, prevents the wall from drying inward. It's also hard to get interior foam tight with all the electrical penatrations, and wall, ceiling and floor framing. These all identify the benefits of double stud walls; inexpensive to build and insulate and vapor porous both ways. No question, though that "high performance houses" are a work in progress.

The crossover point between what makes sense for an exterior-foam approach vs a trussed or double wall approach is about R40 whole-wall, give or take. Exterior foam at >R40 gets extremely awkward with very long timber screws for the furring that still need to hit the studs to meet the loading requirements. Even R40 takes 4-4.5" of rigid iso and 6.5-7" screws.

Long-nailing siding through the foam the very high thermal conductivity of 10,000 nails severely undercuts the performance of the foam. Even with thin foam, fur it out, and nail to the furring. If you really think you will need re-side in under 25 years, you must have the crappiest siding on earth, but OK, go ahead with 2x furring and longer timber screws, which still works fine with up to ~2" foam, not so much with 4.5" foam. (With 2x furring you get a somewhat flatter wall too.)

Double-studwalls or deep TJIs with OSB sheathing aren't super vapor-porus, and the OSB will run pretty cold, making rainscreening the siding critical. In many passive house designs a primary air barrier of OSB also acts as the vapor retarder, since it's about 1perm, give or take. Placing the OSB air/vapor barrier somewhere in the middle of the total R to keep it above the dew point, and using fibeglass faced gypsum for the exterior sheathing at the rainscreen boundary offers maximal drying rates in both directions.

Well I want to first say I appreciate all the advice so far.