Real “minimum” insulation value for a Phase 1 retrofit
I have a 110 year old house with no attic insulation. It does however have plaster in lath, about 75% in good condition on the rafters, joists have foil faced fiberglass already, with 1″ deck boards over that. It have knee walls around 1/2 the permiter, a bonus room, semi finished over one area, and gables over 2 sections. knee walls are 4’x4′ triangular and go out to the copper lined box gutters (so you can see which seams leak) and soffit assembly. I can see light where many of the separate boards of the soffit, face board and gutter assembly meet. There’s a wall plate resting on top of the brick walls which I believe are 3 course, air gap behind outside course to the structural inner courses.
I want to eventually utilize the space, (11-1/2′ ceilings in the main open area with just 4 support columns below the hipped roof with widows walk) but for now I have a upstairs high velocity system, that long story short has some leaks and inadequate insulation, so it’s too small, and costing a lot of run. So I want to bring it into the conditioned space.
I have a steam humidifier that keeps up (3300sqft total, full stone basement) but it limted by single pane with poor quality storm windows. I have however seen frost under so exposed parts of the roof deck. Roof deck is original 3/4 or 1″ tongue and groove boards. Great stuff. Survived decades of water leaks on the original slate roof. All wood is full dimension lumber of course.
I need to 1) seal the attic and provide some insulation. 2) create vented space above rafters/plaster and lathe from all the hip and gabled roofs to the widows walk where i have a 800CFM exhaust fan. 3) install some soffit vents, but rely on existing leaks and covert one window to a gable vent. 4) insulate under all joists. Leave the plaster and lathe, it adds mass and structural strength . removing it is a lot of work, a big mess and I have to go back and tray and enforce the whole structure. Any radical structural change will probably cause walls to crack on the 2nd floor.
Here’s the problem. To meet code and keep the P&L, I’d need to get ridiculous and install 4″ XPS then 2″ polyiso, furring and then drywall. That would cost me around $9,000 in materials.
But for about $1000, I could install 1″ polyiso and furring strups (for strength) for now. (yes I know no thermal barrier…. but seriously, if fire reaches penetrated my plaster ceilings, the insulation and deck boards, I’m already done for. It would be temporary and I’d add more gypsum. Phase 2 is rockwool where I plan to have unfinished spaces, add 1.5″ XPS and a 2nd layer of furring then gypsum in finished spaces.
Here’s the thing, at R6 I’m already exceeding the rest of my structure which I estimates performs at R4ish, and my windows which limit humidity levels. In terms of ROI, above about R10 it’s going to fall like a rock. R38 seems pointless with a high mass home. ROI on $10k of foam for the attic would be around 20 years even without labor.
Any thoughts? What’s a minimum to prevent condensation on the foam with a vented cathedral roof assembly? I think R4-6 would be adequate.