Best Insulating for 2×4 and 2×6 walls in old construction (no exterior insulation)
Friends:
Greatly appreciate all the good ideas shared here in GBA but still struggling to come to happy decision for renovating my house. Been at it for long time and finally getting to finishing the interior and looking for ideas and/or reassurance on how to “best” insulate/finish the walls. I am well aware of air sealing etc and am working on that throughout whole house. Ceiling/attic is fully taken care of (R38-60) – now just needing to deal with walls.
House in zone 6a in New Hampshire. Eighteen hundred sq. ft (1800 sq.ft) house is half 2×4 and half is 2×6. Exterior was rebuilt 15-18 years ago (sadly before the value of external insulation was really much understood) with Hardi board, Typar and 1/2″ ply. Typar seams were “somewhat” taped (it is what it is). The exterior shell is (has been) fully water tight for 15-18 years (I know because there is no interior finish just open walls with fiberglass batts and I have recently repaired/replaced many rotted window trim made with the disastrously bad Windsor 1 finger jointed wood – that’s another story!!:-) But now I am going to finish interior.
In my situation which I think represents many thousands of houses in the USA (maybe millions) it seems incredibly difficult to find a reasonable, economical balance between all the aspects of wall insulating, i.e., environment, cost, R value, potential for vapor/condensation problems, etc.
What does anyone think of the following plan that seems to be my best idea:
(1) for 2×4 walls: 3″ xps board cut and installed in wall bays with can spray foam seal all around; covered with Intello Plus on studs (completely intact with only a couple penetrations – no elect.); covered with 1×3 horizontal strapping (potentially with thermal break using pieces of sill seal etc.); covered with 2×3 vertical strapping (creating electrical space which can be reasonably filled with fiberglass bat); covered with 1/2″ drywall and paint.
(2) for 2×6 walls: 3″ xps (like above); covered with 2 1/2″ fiberglass bat (this zone is where electrical will be installed); covered with Intello Plus on studs with appropriate gaskets etc for penetrations (I am still considering the furring option, but since not needed for electrical it seems an unnecessary added cost/effort); covered with 1/2″ drywall and paint.
Now, I know that xps foam is not considered the friendliest environmental product, but balancing all the related issues, it seems to come out on top: meaning for me, that even with the Intello no other product (other than spray polyurethane) gives as much R while minimizing vapor getting to the sheathing. I should add that I have concluded that the design has to be capable of surviving/performing if and when interior Relative Humidity gets into high 40’s or around 50% in wintertime – and yes the house will have adequate venting to try to control RH but reality of occupants following proper RH control can not be counted upon. Summer time cooling is not a design factor though ac is used on rare occasions.
So, I would appreciate anyone’s thoughts and ideas on how I could save some money and still get a decent wall insulating job done that I can confidently feel that it won’t rot away for at least 50 years:-)
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Replies
A 3-1/2" R13 fiberglass batt will give you more insulation than 3" of XPS (R12), and is cheaper and easier to install. If you use faced batts that's your vapor barrier. For the 2x6 walls a R21 high density fiberglass or rockwool batt, faced, is not a bad wall.
If you're going to the trouble of strapping out the 2x4 walls you should fill the entire space with insulation. Rather than criss-crossing, you could just do a double-stud wall. Another thing to consider would be filling the stud bays with fiberglass, then a continuous layer of foam, then 1x3 strapping to hold the foam in and give something for the drywall to attach to.
Service cavities are over-rated. Just run the electric and insulate around it. Don't run plumbing in exterior walls.
Having used plenty of XPS insulation in our 70's split entry(4" exterior, 1 1/2" on basement floor and 1 1/2" on the bottom of the roof trusses before sheetrock) I would opt for furring the 2x4 walls with a 2x2(or split a 2x4 on the table saw) and filling the bays with rockwool. 3" of XPS gets you R15(claimed) where 3 1/2" of rockwool gets you R15(claimed) and I'd use rockwool any day over fiberglass. If you furred the 2x4 wall out to 5-5 1/2" then you're looking at R23. Living in Anchorage Alaska and staying toasty.
Nice! . this is a good approach. I am in Anchorage too. Cheers!
Similarly to others, I'd suggest rockwool in walls & use the EPS for thermal break & air barrier/vapor control ...
The only difference is I would use a product like Amvic Envirostrap that has a slot for strapping to sit within the wall plane ...
https://www.amvicsystem.com/products/rigid-board-insulation/envirostrap/
~formerly worked for spray/injected foam (both closed/open cell) contractor (NH/VT)
Another approach would be 1inch thick foam board ripped to 1-1/2 width tacked to the studs. 3/4" osb or plywood secured over the foam (framing nailer or screws). You get the thermal break and added depth of cavity. Plane any bowed out studs before the foam. If you get a system down, you can do a whole room relatively quickly this way. You knock down a big thermal break at the headers, but what I am always reminded of is how much double top plate area there is. I like to hold the foam and OSB on the bottom plate up off the floor so i can can foam that gap.
We like using rockwool, even if you have to buy it from Lowes. It's just so easy to work with.
Did you consider to furr in?
Like in CCHRC Cold Climate Building Techniques & Concepts: furring in an outside wall
https://www.youtube.com/watch?v=BFp3y0KIaqk
https://www.youtube.com/watch?v=afTGDYCvCV0