I'm planning on insulating an old wood frame house in Central FL. I currently have no insulation in the wood frame walls, just an empty cavity. A good deal of the siding is rotted and needs to be removed (there is no sheathing under the siding). Once I remove the siding, the wall cavity will be exposed. What's the best insulation choice at that point? Fiberglass batts in the cavity? If so, which side does the paper go in for my climate (central FL), towards the exterior or interior. When I put things back together I will be adding OSB to the studs, then siding. Would it be worth it to add 4x8 sheets of foam board insulation? If so what kind? Any suggestions would be appreciated. Thanks for the help.

Since you are pretty much starting fresh then this is a great opportunity to really beef up your envelope. Since you say "old house" I'm assuming it's 2x4 construction. Since you are in Zone 2, I would shoot for a wall value of at least R15. If you're trying to go budget, then fiberglass batt is the way to go for cavity fill. Put the facing towards the interior so any vapor dries toward the outside. But if you can afford blown in celluose then you can really reduce infiltration. Rigid XPS on the exterior is excellent at R5/inch. I'd go with that and blown-in. However if you do the exterior XPS and the fiberglass batt, get the unfaced batt and put your vapor barrier on the outside under the XPS. You don't want to trap any moisture in your wall.

Here's what I would do from outside to inside:

Siding
2" XPS
Membrane (Air Barrier and Vapor Retarder)
Sheathing (1/2" OSB or Plywood)
Stud Wall with Blown-in Celluose
Drywall

Thanks for the reply. I've heard the vapor barrier should be on the warm moist side. But here in FL that is the outside, unlike most of the country. So it's always a little confusing here in FL.

No problem at all. That's what I've heard too so that's what I went with. If you have any other questions feel free to message me.

Post #2 is a good suggestion. You might consider using 2" EPS to save initial cost over 2" XPS.

Posted By jonr on 08 Jul 2013 09:29 AM
Post #2 is a good suggestion. You might consider using 2" EPS to save initial cost over 2" XPS.

And to save 99% on the lifecycle global warming potential of blowing agents used to make the foam. At 2" the HFC blowing agents' GWP would likely exceed the 50 year carbon footprint of the energy savings in a FL location!  With EPS you're looking at sub-5 year "payback" on greenhouse gases.

But 1.5" foil-faced polyiso would also get you there for about the same money as 2" of EPS with a similarly benign GWP, and much easier to air-seal well using FSK tape.  (Polyiso also has a higher kindling temp than EPS, and won't drip or melt even while burning- it's generally safer/better from a fire spread point of view.)

At R10 the insulating sheathing has a very long financial payback in a FL location.  With R13 cavity fill and adding 1/2" plywood or OSB sheathing the whole-assembly R runs about R9-R10, and would meet code min in FL.  An inch of polyiso (R6) still has a long-term financial rationale from an energy use point of view, and is still a very decent thermal break over 2x4 framing for the climate, delivering ~R15-R16 "whole wall", which is about right. See Table 2, p10 of this document. 

Air sealing the gypsum to the framing and to all electrical & plumbing penetrations is worth it while you have it open, or you could take an open-cell spray foam approach, which is a cost adder, but would make the place VERY tight without a lot of detailing labor.  If it's a DIY and you're discounting the labor the foam approach may not be worth it, but invest in a 12 board foot FrothPak or two for sealing over the electric boxes, etc. If insulating with batts, go with un-faced goods, and take the time to tuck the corners & edges in then tug it back out slightly proud of the studs before closing it up, to ensure a complete cavity fill, with no voids or thermal-bypass channels around the batts. Similarly trim for snug fit around electrical & plumbing.

The more airtight you make the house, the more you are able to control humidity with the A/C without excess cooling. That's on top of it being a cost effective energy saver.

I would never build or remodel another house without a layer of Thermax on the sun exposed exterior. Properly installed it affords vapor and radiant barriers with more R-value per inch than any readily available product on the market. It makes sense to stop water vapor at the exterior since you can't control the weather but generally do control the interior space.

Use a nail base SIP. Will leave you with an exterior wood sheeting to apply your siding to. Give me acall, 832-689-4547 www.buysipsonline.com