November 4, 2011

Save Money by Insulating Your House

Winter is upon us and some nights are dipping below freezing. One task on my mind is to improve the insulation of our house for better energy efficiency and comfort. Currently, I am focused on the garage. Although the garage is usually considered an unheated space and not insulated, this is not completely true in our case. We have a raised ranch house with the bottom floor being the 2 car garage, laundry room, and family room. The 3 bedrooms we have are above the garage, and the rest of the upper floor is the living room, bathroom, and kitchen. The floor of our master bedroom is often cold, and I suspect that the insulation between the floor and the garage below is not very good. Heat also leaks from the family room and laundry room into the garage through a thin wall. To make matter worse, the north side of the garage exterior wall is currently not insulated. It is just 2x4 wood studs with exterior sheathing and wood siding, on top of 9" concrete block foundation. The insulation R-value of this wall construction is about 2, which is pretty low.

We can calculate how much heat is being lost to the outside by measuring the temperatures on the two sides of this wall. I used a hand-held infrared thermometer to measure the temperature at 9PM on November 3rd, 2011. The outside temperature was 39.4°F; the inside of the exterior wall was 53.3°F; the temperature inside the garage was 57.6°F. With these 3 temperature readings, I can calculate the R-value of the concrete foundation to be 2.9, and the wood stud wall above it has a temperature reading of 51°F and a corresponding R-value of 1.9. Based on a rough area estimate, the heat loss is about 1000 BTU/hr in this mildly chilly night. We use a 95% efficient natural gas furnace for heat, so this amounts to about $10 a month in heat that’s lost. If I add the heat lost through the 2 garage overhead doors and the concrete floor, it’s about $30 a month in heat loss!

What to do? At a minimum, I should put fiberglass batts in the 2x4 stud cavities and finish with drywall. This would bump the upper wall to R-14. But wait, the concrete foundation is still R-2 and the overall wall would just go from R-2.2 to R-3.7. At R-3.7, heat loss is still about 660 BTU/hr. I can put 2 inch extruded polystyrene (EPS) rigid foam insulation panels (DOW blue Styrofoam) on the inside of the concrete, use masonry anchor screws to secure 2x2 furring strips with 16"-on-center, another 1.5 inch EPS foam, and finish with drywall. I would leave the bottom 1 inch concrete exposed, to avoid water damaging the drywall, and this would give the foundation section R-16. This combined with the upper section R-14 would give me an overall R-14.9, which reduces heat loss to 160 BTU/hr ($1.73/month). Another option for insulating the bottom half of the wall is to anchor 2x2 furring strip horizontally on the top and bottom, sit 2x6 on top of the 2x2, put 2 inch EPS between the top and bottom rails, and put 2x4 studs with 16"-on-center. Fill the 2x4 cavities with fiberglass batts and finish with drywall. This second option gives an overall R-14.7 instead of R-14.9, and the cost of materials is about the same. The main difference is that the second option is 6 inches thick vs. 4 inches thick in the first option, and the second option allows you to put plumbing and electrical outlets if desired. I have built a wall in the laundry room as described in the second option, but I left the bottom 6 inches uninsulated to allow for the dryer vent pipe to run under the 6" wall. Leaving the bottom 6 inches uninsulated would lower the R-value from 14.7 to 10.8.

I can't wait to measure the temperatures after I insulate and see how much improvement I really get compared to the calculated values.

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