Smaller structures take more gas per square foot.
“I’m going to turn my garage into a home office. How much insulation should I use?”
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A common mistake, when estimating energy costs, is to consider the square footage of a structure.
“If my house is 2000 square feet and costs $200 to heat in the middle of the winter, and my garage is 200 square feet, it should cost $20, right?”
This is roughly true in terms of heating them UP, but not for KEEPING THEM HEATED. Heat LOSS is a function of wall area.
Consider a 2000 square foot house and a 200 square foot garage. If they are both simple squares, the house is approximately 45 x 45 and the garage is 14 x 14. The total wall length for the house is 180 feet and the garage is 56. So the garage will cost approximately one-third to heat
House 2000 square feet
Garage 200 square feet
House walls are 45 feet long (square root of 2000)
Garage walls are 14 feet long (square root of 200)
Total wall length of House is 180 feet (4 times 45)
Total wall length of Garage is 56 feet (4 times 14)
Garage has 31% of the lineal wall footage of the house.
All things being equal, the energy cost to heat the garage should be 31% of the cost to heat the house, not 10% (based on square footage).
The bottom line is that smaller structures need more insulation.
HOW MUCH INSULATION?
So the next logical question is:
“If my house is insulated with R-15, what do I need to insulate the garage with to get the energy costs down to 10% of the house?”
I’ve read in many places that if you double your insulation, you cut your energy in half (inversely proportional). I’m not completely convinced of this, but it may be correct, so, in our example above, you’d have to insulate the garage with insulation of about R-45.
Other factors come quickly into view at this point.
AIR INFILTRATION — Allowing air to leak in or out can quickly undermine any attempt to reduce energy costs. Air leaks must be fixed first.
FRAMING — Wood framing does not insulate as well as actual insulating materials and dilutes the R-value of the wall. Top and bottom plates, window and door perimeters, jack studs, and cripples, even wiring and piping increase the conductivity of the wall.
ROOF and SOUTH-FACING WALL — This is a tough one. A black roof or south-facing wall can add a fair amount of energy to a structure during times of full sun. Ideally, the insulation could be ’switched off” if the roof or wall were actually heating the home. Since that’s not possible, I’ve dreamed up a few systems to capture this heat, but haven’t built any yet.
WINDOWS and DOORS — usually have an R-value in the single digits. South-facing windows can let quite a bit of heat in during full sun.
GEOTHERMAL — If your house is build on dry sand, I don’t suspect you would lose much energy to the ground because sand is a poor conductor. On the other extreme, if you are on a marsh, you would continuously lose energy to the ground. The two factors are how conductive is the soil, and how much underground water movement is there.
Conclusion — Okay, so how much insulation?
I guess the bottom line is how much do you want to spend upfront, and how much do you want to spend in ongoing energy costs? If you want low energy costs, go with the high R-value in the wall (19 or 25), quality windows and doors, and a south-facing window with an awning for the summer. If you want to shave the upfront costs down as far as possible, put in minimal components and just heat the space during work hours with a programmable thermostat.
Tags: energy, garage, heating bill, home office, insulation, r-value