Hello, long time lurker but my first post. I'm in central Florida and my main concern is passive cooling more than heating. I'm looking at building a house within the next year and see lots of people insulating their foundation. The house design I am working on will have approx R40+ walls and ceilings, and be as airtight as possible. I've been looking at passive house documents, and trying to generally follow them where it makes sense for my application. I am torn between a non-insulated foundation so that it can soak up the cool ground temperature and help keep the house cool, or insulating the foundation and using radiant heating/cooling. I plan on just staining the bare foundation through the entire house... I know there are lots of other factors, just wanted to get some opinions...

Quarantine,

I would just build ICF from footer to gable ends. Building with ICFs will provide you with all that you want...high insulation rating, airtight, energy efficient, passive solar, and it would be essentially hurricane proof. It would take a lot of worry out of what you are looking to build. If I were going to bulid a house in Florida, I would give it serious consideration. Yes, I 'm into ICFs (not in your area) so perhaps I'm biased, but I would at least certainly price it.

renangle

Posted By Quarantine on 03 Jun 2010 12:51 AM
Hello, long time lurker but my first post. I'm in central Florida and my main concern is passive cooling more than heating. I'm looking at building a house within the next year and see lots of people insulating their foundation. The house design I am working on will have approx R40+ walls and ceilings, and be as airtight as possible. I've been looking at passive house documents, and trying to generally follow them where it makes sense for my application. I am torn between a non-insulated foundation so that it can soak up the cool ground temperature and help keep the house cool, or insulating the foundation and using radiant heating/cooling. I plan on just staining the bare foundation through the entire house... I know there are lots of other factors, just wanted to get some opinions...

In FL the deep subsoil temps are in the human-comfort conditioned-space range.  With sufficient control of solar gain, an R40 house that's earth-coupled wouldn't need anything like a heating system(!), but would still need active dehumidification/ventilation. Heating with an electric element in the ventilation stream would be sufficient (as in the Urbana IL Passivhouse, which has a much bigger heating load with R60 walls than you'd ever see in FL with R40.)  Cooling with a chilled slab would handle the sensible load at still fairly warm temps, but without active dehumidification the latent loads would be crazy- you simply can't get by with a chilled-slab + ERV alone in a tight house with dew-points hanging in the 70s much of the summer.

DO insulate the stem walls/slab-edges but don't insulate the slab- use the thermal mass of the earth to your advantage.  Use only CA Title 24-complaint "cool roof" covering, and be very careful about glazing size, type, & location, overhangs, etc.  It's probably worth springing for the Passive House software to model the house, particularly for the solar-gain control.  At R40 on the envelope you should be able to to design it to meet the PassiveHouse standard as an earth-coupled design if you pay attention to details.  Dealing with the latent loads will be the trickest design aspect, but a single-pass chilled coil in the ventilation stream may do it for both sensible & latent cooling.

Thanks for the reply Dana... Well for cooling/heating I've been toying with the idea of using ground water. My aunt lives about 1/2 mile from the location and her well water normally hovers right around 68F to 70F. I planned on building an uninsulated storage tank in the middle of the living area containing about 1400 gallons of water for thermal mass. My goal is to control the temperature of this water by cycling groundwater into it or heating it with solar if needed. Then circulate this water through an air/water exchanger in the ERV or through a separate circulation system. The main thought is with the highly insulated shell, if I can keep the storage tank in the mid 70's I can keep the house there too... Water for the house will be fed from the storage tank. The tank will NOT be pressurized, I will run a 12v 5gpm pump to feed the house system. The storage tank is going to have a float system to keep it filled when needed from the well. I also plan on using a solar/battery system to run 12v ceiling fans, some LED lighting, 12v well pump, and circulating pump for the storage tank. I can get 1KW of solar cells for $600 to $800, then going to build the panels myself. The 7200+ solder joints are going to suck but I have plenty of time on my hands. With building the panels I think I can get it down to $1.50 to $1.75 a watt... The overall goal is to keep the house cool and "green" and only need grid power for the large appliances(fridge, washer, drier) and my abundance of computers... EDIT: Why are my posts showing up without formating?

Thin film PV panels are already commercially available for under $1.75/watt, and it's anticipated that better-efficiency CIGS technology PV should hit below $1.50 within the decade. I'm not sure it's worth anybody's time to build your own for similar money.

You'll still need something to control humidity- 68F (or even 60F) water in a ventilation path coil will not remove sufficient moisture from the ventilation stream to keep mold under control. The dew point of 75F/60% RH air is ~60F- you have to be significantly colder than 60F to dehumidify.

The tank may be overkill, but if it's a multi-story house it may be useful for sensible-cooling of the upper floors with hydronic panels/ceilings/floors. That would be an expensive solution though- circulating the air between floors might be a better bet.

Pre-conditioning the ventilation air earth-tube style might be better investment than a big tank. Plastic ducts only on the earth-tube, to minimize mold hazard conditions inside the tube itself.