Thursday, May 31, 2012

Getting Closer to smaller living: Stage 1.2 - Insulation and Heat

Insulation...  A lot of Tiny Home people gloss over this, but to make a really efficient tiny home you have to think about it.  If you use typical construction using 2X4 and 2X6 boards.  that gives you the ability to have 3.5" of insulation for outer walls and 5.5" for the roof and floor.    Rigid foam board insulation is the easiest to deal with and the lightest.  And interestingly it also delivers the highest R value per inch.  The higher the R value the better.  The cheap white beaded foam board is about R2.5 to R3 per inch.  so you can stack  two boards in the wall cavity, 1 2" thick and 1 1.5 inch thick to get the wall full,  this will give you and R of 8.75.  Typical homes have a wall R value of 10.  You want higher than that.  So we look at the Blue or Pink board.  This is extruded Polystyrene and it has an R value of 4.5  to 5 giving a 2X4 wall with 3.5 inches of it an R value of 15.75.  Now that is a wonderful insulation value for the wall.   If we want to go further, we can buy the high end foil faced Extruded.  That has an R value of 7 to 8 which delivers a Whopping  R24.5 for that 2X4 wall.     The drawback is that the last two use CFC's in their production which means they will outgass.  so you need to be sure to have a sealed vapor barrier inside the home to stop the CFC outgassing from going into your home.   Now round down to figure losses for you cutting into the insulation to run wires and  using the pink/blue board you can have R15 outer walls, and a R24 ceiling and floor.  This is better insulation than most Mc-Mansions.  What I recommend is using the cheaper Blue or Pink board for the walls and the foil faced for the ceiling and floor.  Yes you can use it everywhere and you will save more on Heating and cooling. but the foil faced stuff is harder to find and is 2 times the price of the pink or blue board.

Figuring on lower numbers,  assuming I cant get 1.5" thick board so I have to only use 3" in the walls and 5" in the ceiling and floor.  R13 wall and R22 ceiling, 1 door and 4 windows.  a typical Micro home will lose about 5000 Btu per hour.   A person at rest puts off 222 Btu per hour, yes you are a heat source, so we can remove that from the 5000 Btu/h number for heating.  So I need to make up 4800Btu/h to maintain the temperature in the home, this is assuming 70 degree inside and 10 degree outside temperature.   So how do we heat the house?  Well a typical 800 watt electric heater will deliver about 2,750Btu/h and a 1500 watt electric will produce about 5,100Btu/h.   You can easily heat a 180 sq foot micro home with a 1500 watt oil filled portable heater.  If you had electrical service then assuming $0.10 per Kwh cost.  running your heater 12 hours a day drawing 1.5Kw means it will cost you $1.80 a day to heat your home when it is 10 degrees out and you are not getting any heat from the sun coming in windows or hitting the house. and remember, the sleeping loft will be warmer than the floor, so night temps can be set lower to save energy.

So my idea of a 12,000Btu/h RV furnace is really overkill, but probably not a bad idea.   Excess capacity means it will run less, and when solar power is considered, I would rather heat from a Propane source than an electric source.

Air conditioning is the opposite,  5000Btu/h heat gain rate from outside and add in the 222btu/h per person in the home and you need to get rid of 5222btu/h.  luckily  tiny AC units are huge.  The smallest one I can find is a 5000btu/h AC unit.   but if you have friends over you can quickly overwhelm it by the additional heat, plus heat from cooking also adds to the load.

So how much propane?   well,  Propane has about 21,500 Btu per pound.  so a 5 pound grilling cylinder has about 100,000 btu in it. (rounding down for losses)  that means that I can run that 12,000 BTU furnace for  8 hours 20 minutes before the 5 pound tank (if it has 5 pounds in it) is empty.  That is total run time.   If I   need to make up 4800btu per hour, that means I have heat for 20 hours 30 minutes.   A 20 pound grilling cylinder gives me 89 hours of heat (again assuming a full 20 pounds)  that means in the dead of winter you need to get a 20 pound grill tank refilled every 3 and a half days.  This also means that the tiny coleman 5 pound cylinders can be used as an emergency gas source  for 8 hours of heat if you needed to.   This is assuming only Furnace use.  this does not account for hot water heater or cooking.   RV water heaters use as much as the furnaces, they just dont run as often or as long.

This is where insulation really kicks in.   If you were to increase insulation and reduce heat loss by only 500btu/h That 20 pound tank will last an additional day plus 4 hours.   All of this is assuming worst case.  dead of winter in the arctic circle with no sunlight at all.  in reality it heavily depends on the weather and your location.   sunny days reduce heat loss significantly, even when it's 10degrees F out the sun can give you 1300btu/h through a typical 32" by 32" window.  also heat gain through the wall as the house heats up from solar radiation, etc..   plus if it's only 40 degrees out, your heat loss is a lot lower than  the calculated, etc...

By my estimates, a 20 pound grill tank can easily heat your home to a normal 65-68 degrees for at least 7 days in the winter. Spring and fall are looking at a monthly cylinder fill.  Worst possible case,  $80.00 a month heating bill in the arctic circle using only R13 walls and R24 ceiling.  if you were to add storm windows, triple pane instead of double pane and higher insulation, you can do a lot better.

Again, this is assuming that the home is off the grid.  no electric except for solar, and you have to cart in your own propane in your car/jeep on your own.  If you will be in a location that has full hook ups or you can have electrical service ran, these issues are not a bother to you.   But it's good to know what the effects are,  remember your monthly expenses in living are tied to the decisions you make before you build.  spending more when you build means you spend less while you live there.

In my off the grid case,  the loft will be a lot warmer on it's own and if I was to run 12V outlets up there to use a  12 volt heated blanket even more gas can be saved.  However, they use about 80 watts of power.  So now that is a higher load on the solar storage system.   80 watts over 8 to 10 hours is about  800 watt hours used out of the storage system.   Everything in off grid living is a trade off.  And if you want to be portable, you can not use alternative systems like wood heat.  I have seen tiny wood stoves, but honestly I cant see them being used safely in a micro home.  the only small ones are the old cast iron pot belly type and they utterly scare me.   If someone made a tiny modern type I may change my mind.

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