Approaching Zero Energy in Concord, MA

An American classic found in almost every town in the country. The plaster was in great shape and the layout was excellent, so other than adding a few windows and updating the kitchen, we didn’t want to disrupt the interior too much. We almost doubled the living space of this house without adding an inch to the footprint. And the utility bills were cut by 60%.

SPECS:

Conditioned space: 2,000 before, 3,600 after
Bedrooms: 4 before, 4 after
Bathrooms 1 1/2 before, 3 1/2 after
Cost of renovation: $100 per sq. ft.
Annual utility cost:

• Gas $2,400 before, $858
• Electric $960 before, $471

STEPS TOWARDS ZERO ENERGY:

1. Upgrade the mechanicals:

• Furnace: sealed combustion 92% AFUE gas boiler (Munchkin boiler, www.htproducts.com)
• AC: 13 SEER split system in conditioned space
• Hot water: 85% efficient SuperStor tank attached to boiler.
• Outside air is supplied to the return plenum of the efficient air handler; 5 in. pleated media filter in the return path on each air handler.
• Energy efficient low sone exhaust fans in all bathrooms, fully ducted exhaust fan over kitchen stove

2. Bring the basement and crawlspace inside the house -- New 3 in. slab over 2 in. XPS over 1/2 in. drainage mat over old slab; 3 in. closed cell foam sprayed on perimeter walls (R-21), steel studs and drywall over foam insulation as per code.

3. Super insulate and air-seal the roof (R-60) -- R-39 (6 in.) High Density Spray Foam on underside of roof sheathing, R-20 two layers (staggered seams) of foil faced Polyisocyanurate roof deck insulation (3 1/2 in. total thickness).

4. Replace the windows with double glazed low-E replacement windows.

5. Insulate the walls (R-40) -- Cellulose blown into existing 2x4 walls (R-13), two layers with staggered seams of foil faced polyisocyanurate sheathing (R-26).

6. Buy Energy Star (or better) fixtures and appliances --refrigerator,dishwasher, stacked washer/dryer.

MATERIAL NOTES:

• We used closed cell foam in the roof because the rafters were only 7 in deep. Closed cell foam has higher R-value per inch. This allowed us to have the highest possible R-value on the roof without rebuilding the rafters.
• Foil faced Polyisocyanurate to the exterior of the walls gave us the highest R value per inch for the exterior walls. We didn’t want to disrupt the interior plaster to spray foam into the wall cavities.
• Windows - The tilt-in replacement windows were installed from the exterior rather than from the inside so that we could build out the jamb thickness to accommodate the deeper walls. New windows (Andersen, to match the replacaments) were added to the back yard to provide better views to the pond.

LESSONS LEARNED/WHAT WOULD WE DO DIFFERENTLY?
Better windows would be the next place to reduce energy loads in this house. A triple glazed unit, with heat mirror technology might further reduce the heating load allowing us to get closer to zero.