The road to a design workshop for our sustainable rehab took us to an energy model for the house (see also 09/09/2009 post). Our goal is to have a super insulated and airtight building envelop. But how much energy, CO2 emission and dollars will this save us and how much heating will we need?
- Insulation of building shell: R-21 in 2×4 stud wall, 16 inches on center
- Insulation of basement slab: R-5
- Roof insulation: R-52
- Windows: Double glazed, low E wood frame windows, U-value of 0.25 and Solar Heat Gain Coefficient (SHGC) of 0.5
- Exterior doors: Front door with U-value of 0.27 and all other exterior doors with U-value of 0.19
- Infiltration: 0.2 natural Air Changes per Hour (natural ACH)
- Heating: radiant heat (hot water radiators) with natural gas fired 90 AFUE boiler at 60 kBtu/h capacity
- Domestic hot water: natural gas fired water heater at 0.63 EF with 30 gallon storage tank
- Cooling: We decided against any air conditioning system, but would like to use ceiling fans and have a small window unit as a backup if needed. Over the past few years, we had a window unit in our un-insulated and drafty Elmhurst apartment. We may have used it three or five days in a year, usually when we had visitors. Knowing that we will now switch a very well insulated and airtight envelop, we don’t see the point investing in an air-conditioning system we won’t use.
- Ventilation: Energy Recovery Ventilator at 300 cubic feet per minute (cfm) capacity and 250 watts
- Solar hot water: 128 square feet of evacuated tubes with 150 gal storage tank
- Sun room for passive solar: Our enclosed back porch faces due south and offers the best and only opportunity for passive solar gain in the cooler seasons. We have the idea to create a ‘sun room’ with glazing on most of the south elevation, and awnings for shade in the summer months. The infiltrating sun in the cooler months would heat up the building’s rear brick wall. We hope to use this thermal mass and energy to precondition air with which we would ventilate the rest of each floor.
Corbett run three model scenarios for us, each with a slight modification to the above mentioned sun room:
- Design Load (kBtu/h): 53.2
- Annual Load (MMBtu/h): 63.1
- Annual consumption (MMBtu/h): 70.7
- Design Load (kBtu/h): 52.3
- Annual Load (MMBtu/h): 61.8
- Annual consumption (MMBtu/h): 69.2
- Design Load (kBtu/h): 48.6
- Annual Load (MMBtu/h): 57.3
- Annual consumption (MMBtu/h): 64.2
I have to admit that I was surprise. I did not expect that bringing the sun room into the conditioned envelope would reduce our energy needs. In other words, heating the sun room will help us to save energy. Sounds somewhat counter intuitive, doesn’t it? That the reduction in window size would result in better efficiency was more intuitive, as windows are the weakest link in the envelope.
Here is another interesting result that adds to the feel good factor: Corbett compared our performance targets (outlined above) to those of conventional building performance levels. Lo and behold, we could save as much as $1553.00 a year on energy cost (and that is with current energy prices…).
This energy model is by no means the final word, but provides us with a baseline that we can use for the workshop, design and decision making processes. I was sieving through the report details looking for opportunities and found them in the component load summary. This part of the report identifies how much energy (in Btu) is needed by or lost to various building components.
The single biggest energy loss is attributed to infiltration, in other words, having warm air leaking out of the building and cold air leaking in. This loss occurs at a rate from 28.8 to 30.5 MMBtu/yr., which is anywhere from 45 to 53% of the annual heat load! I like to think that this has opportunity written all over it if we take objective of a super insulated and air tight building envelope seriously!