Posted By wes.frink on 14 Dec 2014 11:24 AM
Hi all, I'm looking at purchasing a 4600 sq ft house just outside of Indianapolis, IN that is a complete fixer upper. The house was built in 1991 with 2x4 walls, that borwn fiber board that is painted black on the outside, and then wood siding. The siding is in very poor condition from lack of maintenance. The house has a 12/12 pitch with 3 tab shingle. My thoughts to improve energy efficiency and closer to maintenance free are these:
Re-roof with standing seam metal to give it a cabin type feel to make the architecture.
Gut the exterior siding and fiber board under it and replace with Huber insulated R-Sheathing and then either pre-painted fiber cement or high grade insulated vinyl with a real wood look. Another questions along with which siding is while the exterior is opened up do I replace the wall insulation with closed cell? or leave it and cover with the R Sheathing. The windows are good double pane windows and the home is on geothermal for HVAC.
Any help or thoughts would be greatly appreciated. I run an electrical contracting company based on energy savings projects but siding and raw insulation are a bit out of my forte.
Huber R-sheathing is polyisocyanurate, which underperforms it's R-value considerably during the winter in an Indianapolis (US climate zone 5) location. When the temp through the polyiso layer is 25F (as it would be on winter nights) it is barely delivering R3 per inch of thickness. You'd be better off applying that money on EPS or rigid rock wool on the exterior of the sheathing, both of which increase in performance with falling temperatures. An inch of Type-II EPS is labeled R4.2, but that's at an average temp through the foam of 75F. When it's 0F outside you'll be looking at about R4.7-4.8 performance.
To
meet code in zone 5 with 2x4 framing with exterior foam you need at least R5 (labeled performance), so 1.5" EPS (R6-ish) or if you can find it, 1.25" EPS (R5ish) would be the minimum. The code prescription is for dew point control at the sheathing, and is independent of the total R-value code minimums- don't cheat the exterior-R if you want the sheathing to stay mold-free. More exterior R is even better.
If the asphalted fiberboard is in good shape, detail it as an air barrier and keep it. If it's warped and falling apart, replace it with 1/2" CDX or OSB (ZIP, with seams taped can be an effective air barrier, if caulked to the framing) but not before assessing the cavity insulation.
If the existing cavity insulation is R13 batts (likely, if built in 1991), and does not have a foil facer or interior side poly sheeting type vapor barrier, keep it- fill in any compressions or voids. Then when the sheathing goes back up, caulk the framing at the sheathing edges as you go to make it your primary air barrier, then tape the seams with the appropriate tapes. Replacing it with 3" of closed cell foam would be a waste of closed cell foam, since the thermal bridging of the 16" o.c. framing brings it's performance to it's knees. Even a full 3.5" of R6.5/inch foam is only an R1.5 improvement over a tight fitting R13 in terms of overall thermal performance.
Since you're not replacing the windows, you need to put the weather resistant barrier (WRB) layer between the foam & sheathing to be able to lap it correctly with the window flashing. In these stackups considerable resilience is gained by using the crinkly type housewraps rather than the plain-jane flat stuff or #15 flet, since the bulk water directed onto the WRB drains much more quickly, and the micro-gap is at least something of a capillary break.
Rather than long-nailing the siding through the foam, put up 1x4 furring through-screwed at least 1-1/4" into the stud 24" o.c. with pancake-head timber screws (to keep from splitting the furring), and hang the siding on the furring.
Both the bottom and top of the "rainscreen" venilation gap between the siding and foam should be vented to the exterior, using Cor-a-Vent &/or the roll mesh used for ridge venting to keep the critters from setting up condos in there.
