Renovation: rigid insul. under clapboard
I’ve read a number of articles here on FHB and at Building Science about the use of rigid insulation under siding, but of course, none of those are exactly applicable to my case. I’m hoping that if I describe my situation, those of you with experience in this will share your thoughts. Thanks in advance for your advice and for taking the time to read the lengthy explanation.
Exisiting conditions
+/- 2000 sq ft, storey and a half (approx 26×42′) +/-80 years old. 2×4 construction. Most exterior walls have drywall with fiberglass batt insulation. A couple of rooms have original plaster walls and blown in cellulose. Most first-floor walls have very thin poly (?–clear plastic) between the drywall and the insulation.
The house is in Nova Scotia (conditions similar to northern coastal Maine). The house is on a hill and is protected from both summer and winter prevailing winds. The long side of the house is oriented South-East; the front faces South-West and gets lots of direct sunlight. This link says more about the weather than I ever could (I swear it’s not as bad as this!!!): http://www.on.ec.gc.ca/weather/winners/novascotia-e.html
The Goal
I want to get rid of the vinyl siding, and replace all of the windows in the house. I’m taking the opportunity of a big job to go even bigger, and reframe a number of openings to brightent the place up (and hopefully bring it back towards it’s original look). While I’m at it, I plan to add rigid insulation to the exterior to help with air-sealing and, obviously, insulation. All of the new windows are EnergyStar rated vinyl with 3 inch brick mold and nailing fin.
The Plan
1. Remove all vinyl-siding, set aside for recycling.
2. Remove shingles, bringing the walls down to tongue and groove.
3. Reframe window openings as necessary; extend jambs out past existing T&G surface to accomodate insulation and strapping.
4. Apply tyvek or other housewrap (This is the part I’m least certain about!)
5. Apply waterproofing/flashing to window openings
6. Install insulation, tape seams, and install strapping
7. Install sheathing (match thickness of strapping) around window openings (on top of insulation) to create solid surface all the way out to the nailing fins.
8. Install clapboard siding.
Questions
Most of my questions (I think) are centered around the use of tyvek or similar.
1. From Building Science and FHB, I think I undersatnd that it might be better to allow moisture in the walls to move into the house, and be handled by Heat-Loss Recovery ventilator. Because I’ll be opeing up most interior walls (at least a bit), I’m wondering if I can just try to rip out as much of the thin poly as possible. If the poly is discontinuous, will it still act as a localized vapour barrier (which could pose problems) or can I assume that vapour will find its way through whatever openings exist? The second floor of the house has no poly.
2. Is the right place for the tyvek between the rigid insulation and the t&g, or on top of the insulation? If I add 1 inch at R5, and have R12 in the walls, this would equate to about 30% of the insulation being outside of the vapour barrier. Am I right that this would put the condensation surface on the outside of the tyvek?
3. I’ve always wondered about nails/staples penetrating tyvek–doesn’t that reduce it’s effectiveness? What if I try to staple only above the framing, then apply strapping directly over that.
4. This design incorporates something of a rainscreen element–what thickness of strapping would be required to make this effective? I’ve read about people ripping pressure treated lumber–but from what I’ve seen of cut treated lumber, the treatment doesn’t fully penetrate anyway, so is this necessary? Is untreated strapping a recpie for disaster?
5. Are there special considerations I need to make in using clapboard siding in a set up like this?
Thanks again for all your expert advice!