Posted By petemac on 17 Apr 2012 09:10 PM
Hi folks,
This seems to be a really active forum with some excellent participation.
My wife and I are in the process of buying a ranch in NJ and are planning to reconfigure the 1st floor, expand either the rear or front of the house and add a second floor. In essence we're hoping to create our dream home. We'll almost have a blank slate to work with but we'll need to save some walls so that it's technically considered a renovation and not a new construction (for tax reasons).
To me, this is the only real time we can take advantage of newer, more efficient technologies like radiant heating and top of the line insulation. This is where I'd like your input.
1. The sub-floor of the 1st floor looks like slats of wood running diagonal to the floor joists (this is from looking up while in the basement. I wonder whether this is substantial enough to leverage the Warmboard-R, which is their non-structural product line. Thoughts?
2. Let's just say that I decide to rip it up. Typically the subfloor runs underneath the bottom plate of the walls (right?). Can I just run an edge saw along the bottom plate to separate it from the rest of the subfloor, then lay Warmboards down?
3. If I choose closed-cell insulation, would I ever see the return on investment for Warmboard/radiant heating, or would it be overdoing it?
Thanks in advance for your insight.
--Pete
There's no cash return on radiant floor, but where and how much you put the closed cell foam can make or break the cost-effectiveness of the foam. Putting R6/inch insulation inside of stud bays is essentially money wasted if you have other means of air-sealing the wall. The R1/inch thermal bridging of the ~20-25% framing fraction dominates the heat loss of studwalls built that way.
In a 2x4 studwall with a 25% framing fraction you could have R-infinity (made from Unobtainium? :-) ) for the center cavity R-value, but the average value for the whole wall with the thermal bridging of the framing factored in would be ~R15.
In a 2x6 studwall with 25% framing fraction with R-infinity cavity-fill delivers only ~R23.
A 25% framing fraction 2x6 studwall with 5" of closed cell foam in the bays delivers ~R15.
But if you did a 2x4 wall with cellulose fill and put 2" of closed cell (or rigid polyiso) on the exterior of the sheathing, thermally breaking the studs with R12 foam you'd have an R30 wall at the same thickness as your R15 closed cell 2x6 wall, for about half the insulation cost. This is a reasonable stackup for carrying some ROI for a NJ climate if you're heating with propane, electricity, or oil, but is about the limit of what makes economic sense for heating with condensing gas.
If you use reclaimed roofing iso (from commercial re-roofing and demolition) over the sheathing rather than virgin-stock there's an argument for going with 4" even with condensing gas.
If the foundation walls and band joist are not currently insulated in the basement there's a long term economic argument for 2" of closed cell foam there too, but 3" of EPS with the edges foam-sealed would usually be cheaper, or 1" XPS trapped to the foundation wall with an R11/R13 batt insulated studwall. On any new foundations under additions, a minimalist R16 insulated concrete form approach is usually reasonable, if not, insulating on the interior with foam is. There's an argument for about two inches of EPS under any new basement slabs too- add an inch of XPS over that if you intend to make it a radiant slab. XPS retains the staples used for holding the PEX better than EPS does, but is a bit more expensive per unit R. With 2" EPS and 1" XPS you'd be at about R13 under the slab, which makes sense for heated slabs, whereas R7.5-8 is fine for slabs not being used as radiation for the heating system.