I've never done a slab-retrofit, but I watched a friend fight through that misery over the past 18 months...
(He ended up wishing he'd jackhammered the old slab and started fresh, but then again he started out everything wrong wrong wrongly. :-( )
Except where the slab is bearing structural building load you need not worry about XPS foam board compressing over time. Do NOT use polyisocyanurate (often comes foil-faced) in that application. But you DO need to insulate it to make it reasonably efficient. XPS is the right stuff (pink, blue, or green- extruded expanded polystyrene is the stuff.)
In much of AK you need to insulate it to keep from destabilising the permafrost too, but I'll go out on a limb here and guess that's not your issue, given the "moderte climate" descriptor. :-) ) Don't bother with reflective foils & radiant-barriers, since it requires an air-space to have a measurable effect. When the material is in direct-contact conducted heat is 99.9999% of the heat transfer, not radiated heat. Depending on how "moderate" your climate its, you can often get away with insulation only on the outer 2' of slab perimeter, and not insulating under the middle of the slab helps lower the air conditioning load. Around here (MA) most people use the 2" R10 stuff, but I can imagine 1" R5 is "good enough" in warmer places.
If you can tolerate losing 3-4" of headroom it's probably easiest to insulate and pour over the existing slab. If you can't, demolishing a perimeter zone to insulate you can probably cut it down to under 2" of overall lift with a pour-over. (I'd defer to the pros on the best way to do this.)
Even in cold climates people use both tank & tankless HW heaters as the heat source- usually natural-gas or propane-fired have far lower operating costs than electricity (but in some markets with lower than average electricity costs the propane price spike this past year pushed it way over the line.) Using the domestic hot water as the hydronic fluid and using the hot water heater directly for double-duty is allowed in some places, but be sure it's designed to never stagnate if you go that route. Setting it up so that the cold water enters the heating loop first gives you a little bit of slab cooling in non-heating months (wth no negative impact on heating season efficiency, since you're going to be heating that water anyway.) In MA there's a specific requirement to guarantee so many seconds of pump circulation for every so many hours to avoid stagnation & bacterial growth issues.
The where's when's and payback on solar are so project specific that it's impossible to address, but the low-temperature requirements DO get the best efficiency out of solar collectors. But unless you have pretty good insulation, great solar aspect & climate, and high heating fuel costs, it will usually be hard to rationalize a major solar-thermal investment purely on financial grounds. |