Hi all, This is my first post, but I have been a lurker for a couple of weeks. My husband and I are replacing the windows in our ranch style house built in 1977.We bought it last year as a foreclosure. While we are at it, we would very much like to replace the T-111 as well. There is nothing under the T-111 except some black particle board that is referred to as "soundboard". We would like to take both layers off, exposing the studs. We have 2x4 construction, and there is a layer of vis-queen type plastic that is directly behind the interior drywall. We don't plan on messing with the insulation between the studs unless we need to. Being that the T-111 is not only the siding but also the sheathing, we would want to replace one sheet at a time. We are looking at using the Dow structural insulated sheathing and then using a rain screen (furring strips) to attach the fiber cement siding. We are replacing the T-111 rather than going over it because it hasn't been taken care of (not painted in over 20 years, dry rot in some places, etc). Also, we would like to have some stone veneer along the bottom 2ft or so; does anyone have any recommendations for installing this to allow for the rain screen for the fiber cement siding that will be above it? I am wondering if the plan I have outlined above would be okay for my climate. I live in Juneau, Alaska. It is VERY rainy here, and we get below freezing several times a winter. I don't want to do all this work and then have to rip it out because we have mold or condensation issues. Can anyone recommend a better "wall sandwich" if the one I want to do is substandard? Brenda

Sure, you can get away with 1" SIS in Juneau, but that would still only give you an R14 whole-wall R, in a temperate but ~9000HDD climate.  There's a good argument for taking it to R30 or better (especially if you're heating with oil or propane.)

Rather than replacing all the T-111, replace only the damaged peeling or rotted pieces with a suitable-thickness plywood or OSB, and leave the soundboard in place. Drill from the outside and blow cellulose in over the existing batts.  Then rather than Dow SIS (R5.5 @ 1") , put at least 2" (R10) of XPS or (R8) unfaced EPS, and you'll have an ~R20 wall.  If you use 3" of iso you'd have an ~R30 wall, but if foil faced goods are used you'd be running the hazard of creating a moisture trap, given the poly sheeting on the interior.  If you stripped it all the way to the studs a section at a time, stripping the poly sheeting would allow you to go much thicker on the exterior foam, and it would be toleratant foil facers, etc., since it would have a drying path toward the interior. 

With even R8 on the exterior of a cheap OSB sheathing there is effectively zero risk of wintertime moisture accumulation in the structural wood in that climate from vapor diffusion through latex paint on the interior- the poly was only necessary on the T-111 construction since there was no way to rainscreen it.  If you go with more than 2" of XPS (rainscreened siding or not) it would be a good idea to be rid of the poly since it would be too vapor tight.  But you could go as thick as 4-5" with unfaced EPS (R16-R20), and it could still dry toward the exterior.

Stone veneer at the bottom can be difficult with a foam-overcoat, but Jesse Thompson's slate skirt over 6" of exterior iso on a retrofit came out pretty nice:

http://www.greenbuildingadvisor.com...ders-maine 

(Scroll down to "An architect works on his own house" thumbnail links to pictures of the retrofits are at the bottom.)

Roofing insulation EPS & iso scavenged from commercial building re-roofing is dirt cheap (Thompson's place ran ~$2500 in material for 6" thick iso!), and there is likely to be a source in Juneau if you snoop around.  (Even craigslist searchs can find it sometimes.)  The long fasteners required for hanging it may run a grand or so, but compared to virgin-stock 1" SIS you may still be money ahead.

Okay. this sounds a little more involved than what I was prepared for. I am pretty good at remodeling, but I know when to throw in the towel. I didn't really want to deal with building my walls out to 8"+. I figured the SIS panels with the fiber cement would be a good start for my first siding project. It will just be window replacement for now! Thank you so much for your response. It really put it into perspective how involved something like this needs to be.

Brenda

Don't give up just yet!

You can still double the performance without going super-fat, without a huge expense- comparable in cost & labor to what you were proposing. Re-siding projects are an opportunity moment for upgrading the thermal performance of a building- once the the new siding is up the cost of taking the siding back down to add more insulation becomes prohibitive.

R11-R13 fiberglass batts in a T-111 construction with typical framing fractions results in no better than R10 wall, and that's only if the batts are installed laboratory-perfectly. In "typical" 1970s installations with compressions behind wiring + voids, etc it only hits ~ R8, R9 if they were more careful than average. If you did no more than replace the punky parts with OSB or ply, drilled from the exterior an blew in cellulose to compress the batts and fill the voids it would be substantially more air-tight and bring it up to a true R10. (This can be done with a box-store rental blower if need be, but it's often easier and as-cheap to let the pros do it, especially if there's any state/local subsidy available. Most box stores throw in a free day of rental if you buy ~$100 of cellulose.) Or you could just leave it, an focus your efforts & money on the exterior foam.

If you've blown the cellulose, adding 2" of EPS would bring the wall performance up to ~R18- about double what you started with. With virgin-stock that would run about 75cents per square foot, but with reclaimed EPS its more like 15-25 cents/foot. You might price it out for doing it with XPS instead- sometimes the cost is similar, and you 'd get another R2 out of it. (XPS sheathing can be had in 9' & 10' lengths too, and in t & g /shiplap versions, which reduces the amount of cut'n'cobble + seam-sealing labor.) At the local big-orange box store in Juneau pink 2" XPS (R10) runs ~$1.06/per square foot if you buy it in 2x8s, or $0.97 cents/foot in 4x8s. (I just looked online- they have 276 sheets of the 4x8s in stock.) This is do-able.

With only 2" of exterior foam the 1x4 rainscreen strapping for holding the siding only needs 6" screws to penetrate the studs sufficiently through the T-111 & fiberboard, a length which is cheaper & easier to handle than the much longer versions necessary for going 4-5" thick on the foam. On the fiber-cement sided portion you only need 24" on center, but on the stone veneer sections you'd have to reduce that to 8" o.c.. Your local orange box store can handle internet orders of 6" FastenMaster Headlok screws at a price that works out to ~68cents/screw. It's worth buying a couple driver bits for them too. Assuming 2' of the wall is stone veneer, that works out to ~2/3-3/4 screws per square foot (averaged over a 9-10' tall exterior wall), call it ~50 cents/square foot.

So your upper-bound materials cost on an R10 insulation overlay is in the neighborhood of $1.50/square-foot, but with reclaimed foam board it could be less than half that.

The difference between an R10 wall and an R18-R20 wall is something than you can FEEL in mid-winter- the surface temp of the exterior walls are measurably warmer, with a much higher radiation-temp.

See also:

http://www.greenbuildingadvisor.com/blogs/dept/musings/fastening-furring-strips-foam-sheathed-wall

and

http://www.greenbuildingadvisor.com/blogs/dept/musings/innie-windows-or-outie-windows

I would go thicker than 1" on the foam and use treated wood furring strips glued and screwed to any foam you use.

You can find a better quality wall sandwich in Google and compare the price and quality also.





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