I have a 70's era home with "batt and board" wood siding, 2x4 structural walls. We would like to re-side the house this summer and put in new windows, old ones are wood, single pane. We would like to take advantage and improve our efficiency while doing this project, seeing as there is only R-11 kraft-faced batts in the walls. I am thinking about just pulling off the batts, leaving the painted plywood, adding insulative foam board and then siding over. There is tar paper between the plywood and sheating. What steps are necessary and materials needed to carry this out? Is it even a smart way to do it? I am a electrician and a competent carpenter, but I am not experienced with this sort of retrofit. I would appreciate any tips or help so I don't screw up by assuming I know what I'm doing. Thanks.

What is your design targets for R-value and wall thickness, and what is your zip code (for weather/climate info.)?

There are lots of details to attend to- particularly around flashing etc. eg:

http://www.finehomebuilding.com/item/11050/flashing-windows-in-walls-with-exterior-xps-insulating-sheathing

In general you can get better air-tightness with 2 layers of foam taping (with housewrap tape, or FSK tape if using foil-faced goods) staggering the seams. If you're stripping the siding felt, primer the seams of the plywood with a decent acryilic-latex primer and tape them with housewrap tape. Glue the rigid foam to the sheathing with blobs of purpose-specific foam-board construction adhesive, and use as few screws as necessary to hold it in place as the glue sets. Then use vertical furring through-screwed to the studs for someting on which to hang the siding. The gap the creates is a protective "rainscreen", and should be protected from bug/varmint intrusion top & bottom, but should still be allowed to communicate air freely behind the siding, which increased the drying capacity of the wall & siding enormously (when un-faced foam board is used), drying the siding more quickly as well.

Think carefully about how you're going to mechanically support the siding, and trim the exterior corners , especially if you're going more than 2" thick on the exterior foam. (Tetails will vary.)

No matter how much foam and air-sealing you do from the exterior, caulk the studwall top/bottom plates, and the sheathing/stud interface in every studbay. And no matter how much foam you put on the exterior, the extra cheap-R you'd get out of filling the stud-bays with sprayed cellulose will usually be "worth it" from a performance & comfort point of view. (Nobody has ever complained about having too much R-value in their walls, and cellulose is a fraction of the cost of foam, and usually outperforms batts in real-world performance.) For more money you could use new-school super-fine fiberglass like Optima or Spider to get 10-15% more R in the studwall cavities than with cellulose, but whether it's worth the extra money kinda depends on the total stackup and the design goals.

Whether you would need an interior vapor retarder of any kind depends on the ratio of the R-value of the foam and any cavity-fill you install, and it's local-climate specific. Anywhere in the lower-48 of the US with 50% or more of the R value as exterior foam you can pretty much skip it- stick with standard latex paints (no vinyl or foil wallpapers, please) which allows the structural wood to dry toward the interior. In a wide swath that could be reduced to 25% foam/75% fiber and be perfectly fine too, but it really depends. If the exterior foam is thicker than 2" or has a foil or fiber facer you have to take even more care to give the stackup capacty to dry toward the interior, but without allowing moisture to build up in the sheathing in winter from interior-air or water vapor permeation condensing on the colder wood.

Dear Dana1,

Thanks for the good suggestions here and elsewhere. Your post above is really helpful. One additional question for you and others:

Glue the rigid foam to the sheathing with blobs of purpose-specific foam-board construction adhesive

What types of adhesives are we talking about? What would work well with foil-faced polyiso? I'm looking to install 2 layers of 2" each polyiso with staggered and taped seams.

caulk the studwall top/bottom plates, and the sheathing/stud interface in every studbay

Again... any specifics that work well for caulking here? I had thought to have closed-cell foam sprayed at the top/bottom plates.

I think I am more than "set" on the general plan, but really needing good specifics on products to use for caulking, taping, foaming, etc.

Thanks,

Rob

The tried & true long-lived & flexible caulks tend those marketed as be acoustic sealants, but literally ANY decent caulk is better than none. In addition to caulking the sheathing-stud and top & bottom plate-stud interfaces on the interior of the stud bays, seal the plate-floor and plate-ceiling interfaces etc before the drywall goes up. Also seal the wallboard to the window & door framing, as outlined here:

http://www.buildingscience.com/documents/information-sheets/4-air-barriers/air-barriers-airtight-drywall-approach/images/bscinfo_401_figure_01.jpg

See the various air-sealing detailing info sheets linked to here:

http://www.buildingscience.com/documents/information-sheets/4-air-barriers

Foamboard construction adhesive is available from a number of vendors and mysteriously called "foam board construction adhesive" or "foamboard adhesive", comes in caulking-gun tubes:

http://www.liquidnails.com/products/product.jsp?productId=34

http://www.loctiteproducts.com/p/pg_ca_foam/overview/Loctite-Power-Grab-Foamboard-Construction-Adhesive.htm

Even though the foil facers of the iso protect the foam from solvents in standard construction adhesives, I'd still use the foam board typeadhesive, since you'd otherwise be trapping a foam-solvent between the layers.

On the iso, seam seal the facers on each layer with FSK tape (2-3" wide aluminum tape- be sure it's adhesive service-rated for at least 25F below and 50F above your anticipated weather temps), and stagger then layers by a foot or more. http://tapeinfo.com/TapeInfo/SingleCoatedFoilTape.aspx



If you're using foil faced goods on the exterior, the stackup MUST be set up to dry toward the interior. In cold climates you also have to do the math (at least the simple-arithmetic) on the R-value ratio between foam & fiber to minimize the winter-hours where the sheathing is below the dew point of the interior air. The dew point of 68-70F, 30-35% air has a dew point of 35-40F. If air in the cavity and finds 35F or colder sheathing you'll have a condensing event. If that's only for a few hours at a time and the majority of hours in any given week the sheathing is well above that your risk of accumulating moisture and getting mold or rot conditions if the interior wall is air-tight.

If wallboard is leaking interior air in a convection loop the risk goes up. There are models (eg WUFI) for determining the risk at a particular location/climate and particular indoor temp & RH, but for most residential apps assuming a 37F dew point and 70F interior temp, and doing the math on R-ratio for the average temp of the coldest month (usually January), and padding it 10 percent is usually "safe". eg, the average January binned hourly temp in my area is around 24F. The 37F point in stackup at 24F exterior with 70F interior would be a ratio of (37F-24F)/(70F-37F), or 12/23. That means at a minimum the R value of the foam needs to be 52% of the R value of the fiber. In an R13 cellulose/fiberglass 2x4 studwall Ia'd neet a minimum d 0.52 x R13= R6.8 in exterior foam, but bumping the ratio by 10% up to 62% of the fiber-R would make than 0.62x R13=R8, for a total stackup of R21, 38% of which is exterior foam.

At that point the vapor retardency of the interior finish wall need not be any lower than what standard latex provides, which gives it ample interior-drying capacity, and the vapor-permeability of the exterior foam can be extremely low, as it is with poly or foil-faced goods. If going with foil-faced exterior foam, you MUST take care to keep the interior air-tight, but at least vapor semi-permeable (>>1perm), since it can't dry toward the exterior. If the exterior foam is XPS and 2" or thinner (or unfaced Type-I EPS and 4" or thinner) it'll still retain some exterior drying capacity. If the exterior R is below the calculated & padded ratio, it's advisable to use unfaced goods only, and use a semi-impermeable vapor-retardent latex primer on the interior, and air-sealing the wallboard becomes even more important. Rainscreen gap behind the siding also enhances exterior drying, which is relevant for when the exterior foam is thinner and semi-permeable side, but not with foil-faced goods (which simply can't dry toward the exterior.)

Any number of websites can provide you with monthly-average temperature data for verifying your stackup, eg:

http://www.average-temperature.com/Stations_US_States.aspx

Forgot to mention: For cold climates with a January monthly average of 10F or lower (such a Minot, ND), derate the R of thinner exterior iso by ~10% from it's labeled R. (You can similarly up-rate thin exterior XPS/EPS by 10% at those temps.) eg: A layer of R7.5 iso sheathing will perform at only ~ R7 at temps that low, but R7.5 XPS sheathing will perform at R8 or better.

For temp averages 25F-ish or higher, the derating/ uprating is less, so assuming the labled-R would be sufficient.