Sanity checking an energy improvement proposal (II)
A few weeks back, I mentioned that I had an energy audit performed on my family's home and discussed some of the issues found by that process. Then yesterday I presented a simple energy balance model for the house. Today I want to continue the story by discussing a model for what the house might look like after the $19k (give or take) in energy upgrades that we are discussing with our energy efficiency contractor (Snug Planet).
Recall that the model yesterday looked like this:
This shows both the inflows and outflows of energy during the heating season (here taken to be October-March) in millions of BTU. My estimates for after the work look like this:
I have kept the same scale so that you can see visually that this represents roughly halving the energy consumption of the house during the winter. It's first worth saying a few words about what's not changing:
- The losses through the walls. These were already modest and it would be very expensive to improve them further by removing the siding, adding rigid foam, moving windows etc outwards, and then residing the house. Thus no upgrade work on the walls has been proposed by Snug Planet. This will make more sense to do when the siding is nearing the end of its life.
- The conductance losses through the windows. The house has forty windows and thus replacing them all with good quality modern windows would be very expensive and likely not pay for itself any time soon. To be honest, I also feel some resistance to pulling out 150 year old windows that still work and look like they could last another 150 years.
Next my assumptions for how the work would reduce the outflows:
- Assuming that a good job is done of sealing off the tops of the interior upstairs walls with rigid foam as proposed, then the interior walls should no longer be a heat leak and that term disappears altogether.
- Additional insulation will be blown into the attic to bring it to R-45 and it is straightforward to calculate the effects of that using the same procedure as yesterday.
- Once the basement is air sealed the air down there should stratify and allow relatively little downward heat transport. I modeled this by assuming the basement floor acquires an effective R15.
- Finally, I assume that infiltration is cut from a blower door measurement of 5155cfm@50Pa to 1800cfm@50Pa. At this point, I do not have a firm target for this from Snug Planet, and so the 1800 is a conjecture based on the results of another energy retrofit of an older house in my area where the windows weren't replaced.
The next question is how the inputs will change in response. My first assumption is that the electricity usage of the house during the winter will fall to 80% of the summer level - this is somewhat of a guesstimate based on replacing the heaviest energy using appliances and figuring that usage of the baseboard electrical heat could be eliminated. The other terms (solar gain etc) are unchanged. So then the firewood usage would fall from 3 1/4 cords to 2 cords.
Under these assumptions, and with a 15 year 3% on-bill recovery loan, in the first year, we would pay $1570 in loan payments but save $2165 in energy costs. Obviously, the exact numbers are sensitive to final project cost and what does or doesn't go into the project. But the larger point is that this project does have the potential to work as advertised: we would save $500 in the first year and the amount of saving would (on average) increase in each following year as inflation increased energy prices but not our loan payment. Finally after fifteen years the loan would be fully paid off and we'd just have energy bills half as large as they would otherwise have been.
One interesting aside I wanted to make: in our case we rely on wood and renewable electricity so this operation will not really save fossil fuel emissions. However, if instead we were using our standard utility electricity offering and natural gas or coal, then this would reduce the operational carbon emissions of our house by a factor of two, and cost negative money. Since I'm sure our house is not atypical in this regard, I think this raises questions about the pricing of carbon offsets.
- Infiltration is the largest single energy loss at present, and so the project economics are very sensitive to the final blower door number. It's critical that we not do all the expensive stuff in the attic and basement but then neglect to do the small fixes required to get the infiltration number down. I'm going to be looking to get a commitment ahead of time as to what the final blower door number will be and who is doing what to achieve that.
- We currently pay $7.50/Mbtu for wood heat, but $35/Mbtu for electric baseboard heat. So the project economics are also very sensitive to how much we reduce electric usage versus wood usage. This in turn requires giving some more thought to how to distribute the wood heat around the house. Right now we keep the downstairs at about 75F and air movement keeps the upstairs at about 65 during the day, which with the baseboard thermostats set to 60 keeps the electric heat off except for at night. With less air movement in the house, we probably need some additional measures to allow convection to move heat upstairs. There are two places in the ground floor ceiling where square holes in the paneling have been patched. I suspect these were holes with grates over them that were the Victorian solution to this problem. I may open them up again.
- My assumptions about heat loss to the basement depend on the idea that there won't be much air movement down there. If the dehumidifier or heat-pump water heater stir up the air a lot, this may not be true.
What do you think? Leave a comment below.
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