Building Off Grid home need R50 in roof.

[off-grid man]

I am currently building a 3000 sq ft home for my friend off grid (no electric company) solar,wind and generator power only. They have been living in a three bedroom mobile off grid since 1998 and love it so much they are now building this larger home. I am trying to acheive R40 in the Roof rafters, R30 in the ceiling joist, and R38 or better in the outside walls. I have blown in cellulose isulation, fiberglass mating, or spray expanding foam to choose from. Also a company call (Inuslation 4 Less) has radiant barrier that is only 3/8" thick and claims test showing R14 capability, so I was also going to use this as well. Can anyone give advise on best way to acheive the R values and what kind of thickness am I looking at. I am also going to use Geothermal HVAC (S.E.E.R. of 27) and I am building a heat exchanger to use the wasted heat from the 22KW generator engine coolant and engine oil to heat the domestic hot water. Thanks, Off-Grid Man

[bmwpower]

Subscribed. This sounds interesting.

What powers the generator?

[suemarkp]

Geographically, where is this house located? I don't know if you've ever done a Manual-J heat gain-loss calculation. If you do, you may be surprised at how quickly you reach diminishing returns with insulation. Once you're at R30, going all the way to R60 will only cut the heat loss by 50% while doubling the insulation costs. Even at R30 the loss through that wall or ceiling is getting rather small and things like air leakage or window/door R value become the major factors that affect things. Windows and doors seem to be the largest insulation holes you can have, so minimize those and buy real good ones (R-5 seems to be about all you can do with a double pane, low-e, argon filled window). You must have a certain amount of air infiltration to have a healthy house, so a heat recovery ventilator is about all you can do to help that. Combustion appliances in the house really need to get their air from the outside which may not be possible with a stove. You'll need more air infiltration if combustion appliances use room air for combustion.

But if you can afford the extra insulation, or want it anyway, it will reduce heat gain and loss. I'm suspiscous of the radiant barriers that claim an R value. To me, these are different things and I'm wondering if the R value is an "effective" one instead of a true one. If you're in a hot sunny area, then a radiant barrier should help your cooling. If you're in a very northern region where winter sun is wanted and summer sun isn't bad, then I'd skip it.

I would expect expanding spray foam to be the most effective insulation because it will reach all the voids and won't sink over time (but I don't know what its history is - perhaps it breaks down over time). But if you ever need to run wires or pipes through that insulation after the fact, you may have quite a challenge on your hands. I think the next best method is criss-crossed layers of R-19 or R-25. Wooden joists, studs, and rafters have an R value of about 1 per inch, so you should try to get insulation on one side of that wood. Doing criss cross framing in the ceiling and two separate 2x4 walls would achieve this. The walls could be two vertical layers of R-13 with a horizontal run of R-13 or rigid foam in between (assuming a 10" wall). You could also apply rigid foam insulation to the inside or outside framing, as this will also slow heat loss through the framing.

Why are you insulating both ceiling joists and rafter spaces? Is the roof vented (preferably with a ridge vent)? If so, shouldn't the rafter area be left uninsulated except perhaps for a radiant barrier?

Foam insulation is typically R-5 per inch (but there are many types and the R value varies).
Fiberglass insulation is typically R-3.5 per inch.
I don't know what celluose or rock wool are per inch.

[off-grid man]

bmwpower, the generator is propane. I had planned on using the wasted hot exhaust gases and storing the heat in a Phase Change tank which would allow me to extract the heat energy to use for heating domestic hot water hours after the generator has been turned off. I can tell you about that another time. I use plumbers solder (Tin/Lead) as the Phase Change storage material and synthetic hydrualic oil to transfer the heat energy to the domestic water heater. The system is call Co-Generation, it has been used on large boat and shipps for decades. Check out a web site called (Polar Power) they make these units for private and military systems. Co-generation... Also check out a site for ( GFX waste water heat exchanger) which used the wasted heat energy from the hot water going down the drain from your shower, dishwasher, and washing machine to pre-heat the water saving energy.

suemarkp, Great information, but since the home is completely off gird and the only power source is solar, wind and generator I wanted to try to acheive the highest R value I can. The inverter battery system powers everything for a 3000 sq. ft. home including DC water pressure pumps, and three window mount 5000 BTU air conditioners. I will use the generator to power the Geothermal HVAC system only on the hottest and coldest days. The three window AC's can run all day and all night to provide cooling for most days and I will be installing a radiator in the HVAC duct and running hydrualic oil throught it that has been heated by the generator for added heating in the home on most cold days. The R14 with radiant barrier I also would have to see to belive but I have seen this radiant barrier do amazing heat reductions in smaller buildings. It works best if it has a 3/4 to 1 inch air space behind it. I am using 8 inch wall studs and roof rafters. I could add a 2X2 on to that to get a 2x10 for 10" clearance. We are using double pain, agron filled,Low-E windows. We are installing a fresh air system to bring in clean air. All combustion appliances will be vented to outside air. The home is located about 60 miles north of Huston Texas. The spray foam is the best but the off gassing is a real issue due to medical condition of one of the persons living in the home. Celluose is a fire issue after less then five years the fire proofing stops working, and rock wool is not as good as Fiberglass. I plan on using rigide foam sheets cut to fit with fiberglass and radiant barrier to try and acheive the R values I am looking for. Thanks for your great information it was useful.

[suemarkp]

You've probably thought of this too, but,

Consider planting deciduous trees to provide daytime shade but allow winter sun. East and west are easy, but they need to be real tall on the south side and you may not want a 50+ foot tree within 25 feet of the house.

Put awnings or large overhangs over southern windows.

Avoid western windows. Go for southern ones.

Consider having a black insulated surface that is exposed to the night sky. You could pump a fluid through this and make this fluid rather cold (perhaps make your hydraulic HVAC assist system both a heating and cooling one).

I believe Houston is more of a cooling climate than a heating one. Think about whether you want to insulate under the floor or the lower basement walls from the earth. It will help cooling but hinder heating if you leave it uninsulated or lightly insulated. I read that dirt is about R-8. This won't apply if you build on a crawlspace.

[skipatroller]

If this project is still in the planning stage, you may want to consider ICF construction (Insulated Concrete Form). If you do this the walls are Hurrican proof -- 2" of Foam, 6 or 8 inches of Concrete, and 2 more inches of foam. The R-value of this type of construction is somewhat tricky as you have a thermal Mass insulated on both sides. (Some ICF companies claim that this is a R-60 equivalent) ICF have some real advantages. Check out some like http://www.buildblock.com/technical.htm

If this is beyond the planning stage, Mark's suggestion of adding a insulator in series with the wood studs is the best thing you can do. Think of it as a simple electrical resistance problem. For a 2x6 frame construction wall you can have R-21 insulation, but that is in parallel with the ~R-6 of the studs that make up about 20% of your wall surface.

If you cover the outside of the wall sheathing with a R-5.6 "foam board" (we used "Blackore"), now you have R-26.6 in parrallel with R-11.6 for the 20% of Wall that is the studs, plates, headers, etc.

Yes, I ignored the Drywall, wall sheating and siding in the above example.