I have a question about some improvements I have planned in regard to insulation in my house. I live in Essex MA, and in 2013 installed a two ton GSHP with ductwork in my 2000sqft ranch with unfinished basement. I've slowly been air sealing and insulating to a point where my attic is R-60 (although maybe less as it seems the cellulose may have settled a bit), my foundation walls are R-13, and my walls are R-13 (2x4 construction, plywood exterior, 6mil plastic under the plaster). I've planned to replace my siding this year, and with it my windows (they are old andersen casement double panes). With the siding off, I plan to add 1 or 2" of rigid foam (or more) to take my walls closer to R-20. I plan to install tyvek over the plywood, add rigid foam, then vertical strapping to attach fiber cement plank siding. I've read several articles detailing the process along with the re-framing of the windows so they aren't innies so I'm comfortable with the process in general. One question I have, however, is how much foam, and with the installation plan, will I have any issues with condensation on the new foam configuration? Input is greatly appreciated!

With the 6 mil poly inside I think you need to be careful that the wall can still dry to the outside.

You don't want vapor barrier inside and out.

Hopefully Dana1 will chime in, He really know about the vapor permeability of the rigid foams.

Chris

With poly vapor barriers on the interior you really need to keep insulating sheathing at 1 perm or better, preferably with a rainscreen gap between insulation & siding (but vinyl siding would be OK.) If you're taking it to 2", unfaced Type-II EPS would be the best choice, which would come in at a bit under 1.5 perms, which is still more vapor-open than the plywood sheathing, and more vapor-open than 1" XPS. (2" XPS would be 0.6-0.7 perms, which would be fine if you had ANY drying capacity toward the inteirior, but you don't.)

Type-I EPS would run about 2-2.5 perms but is fairly crumbly stuff in thinner sheets when it doesn't have facers. The facers would reduce the vapor retardency to unacceptably low levels.

Better than foam would be 2" high density rigid rock wool, which has a vapor permeance of about 30 perms. (Roxul ComfortBoard is available through a few distributors in MA, but you have to really hunt for it, or special-order it.)

http://www.roxul.com/files/RX-NA_EN/pdf/Technical%20Data%20Sheets-%20updated/Building%20Envelope/ComfortBoard%20IS.pdf

http://www.roxul.com/files/RX-NA_EN/pdf/Technical%20Bulletins_Guides/13A19%20ROXUL%20Fasteners%20Guide%20-%2004-15-2014.pdf

http://www.roxul.com/files/RX-NA_EN/pdf/Technical%20Bulletins_Guides/13A08%20ROXUL%20COMFORTBOARD%20IS%20Application%20Guide%20-%2004-11-2014.pdf

http://www.greenbuildingadvisor.com/blogs/dept/musings/installing-mineral-wool-insulation-over-exterior-wall-sheathing

If using foam, use the crinkle-type Tyvek between the rigid insulation & plywood, which gives it a hint of capillary break and some drainage space. With rigid rock wool either type of housewrap would be fine.

Ok, I'll check into the Roxul. I have some connections at National Lumber so we'll see what that turn up. Can someone help contrast 2" vs 4" (adding R-8 vs R-16 to R-13 walls)? I know there is a plateau when it come to the effect of R-value on energy consumption. The material cost will be double but labor will be about the same, and the goal is to get the house as comfortable as possible with the 2-ton geo.

THink you should avoid cement siding. Go with something liike LP smartside instead. with a rainscreen over furring, anything that hits the cement siding between the furring will crack the siding fairly easily. Most of the wall will have the gap. LP Smartside, as an example, looks as good and is similarly priced (actually a bit lower and looks better imo). Have fun.

2" vs 4" will introduce quite a bit more cost and issue in terms of siding weight and screwing the furring into the studs through 4" of insulation.

sbeausol: Insulation upgrades like the one you are doing are common around your area, and examples can be seen at< http://www.greenbuildingadvisor.com.

Your ideas will work fine for the most part, provided you have sufficient foam outboard of the stud wall to prevent condensation, and provided the wall is well airsealed. Strongly suggest you either caulk the sheathing joints with a urethane caulk, or tape all the seams with 3M-8067, or SIGA Wigluv, both of which will stick to the plywood. Many of the builders doing Deep Energy Retrofits have found that 4" of poly-iso foam will bring the wall close to R-40 and will help in drastically reducing the heat load. However, this can be problematic with an interior vapor barrier, so the Roxul may be your best bet.

I've installed both cement board and cedar shingles over 4" of exterior insulation with no issues. You can order long zinc coated screws from Fastenal, GRK, or Fasten Master (you can get the last two thru National). The screw should go 1-1/2" into the studs. Do not skimp by using galvanized drywalls screws.

"I've installed both cement board and cedar shingles over 4" of exterior insulation with no issues."

Installing only, and living with the product, are two different things. Anyone with kids knows they don't seem to care/understand about the cost/repair/maintenance of things. all it takes is one poorly placed throw of a ball or something with reasonable force to hit and crack the siding. I put LP on my place, 3/4 furring. I wouldn't put a cementitious product but that's me, so I figured I'd make the comment.

It's true that throwing baseballs at cement siding will probably crack it; same with vinyl siding, and they will dent aluminum siding. Balls will also destroy asphalt siding as my son proved on the old barn, and do a number on cedar shingles, even when installed over solid sheathing. So if you are using your house as a target, avoid these sidings and most others - baseballs can be damaging. Solid concrete should be fine.

Personally, I've always been and continue to be a proponent of one siding that as proven to hold up for centuries - wood. Although maybe not against baseballs.

The LP Smartside is pretty tough. Very happy with it and looks great. Plan to use it again on my next build.

gokite: Having used it, I'm not a proponent of cement board either; have not used LP SS but plan to try it.

LPSS is easier to handle/install and less detrimental to one's health when cutting as well.