A: It is probably a matter of cost. 1/2” XPS is not going to do much, especially if you have R13 already. However, the contractor may be able to add 1/2” XPS without making any structural changes such as extending the walls. If you have to extend the walls it is almost certainly going to be too expensive to make any economic sense. When I had a similar problem I added a 10mm (approx 1/2”) of Aspen Aerogel’s Spaceloft which is R10 per inch or about R5 for the 10mm product. This is probably the best way to add insulation without making any structural changes.
What is the cheapest renewable source of electricity?
There are several options in MA to going fully renewable with your electricity. It is a matter of cost. From lowest cost to highest cost, these are:
- The first option is rooftop solar with a reasonably sunny roof (like my house), about 5-8c/kWh, by far the cheapest option for 100% renewable electricity. Even roofs in half shade (like my garage) can produce electricity at 12c/kWh.
- Even with a no-cash-down solar lease (or power purchase agreement) the cost is around 13c/kWh. A huge discount to the Eversource or National Grid basic rate at 29c/kWh.
- Move to one of the towns (Wellesley, Littleton, Boxborough etc.) that have a Municipal Light and Power (MLP) source of electricity. Often around 18c/kWh and many offer 100% renewable options, usually for a slight premium.
- NexAmp offers a 12% discount to the Eversource basic rate (the R1 rate, now 29c/kWh, so the NexAmp price is around 26c/kWh) for 100% solar, 100% generated in MA. There is currently a waiting list. There are other suppliers than NexAmp but the others offer a 10% discount. This is accomplished via the z-metering provision of the net-metering law that allows the generator of renewable energy to sell the credits to other people with electricity meters. You end up getting two bills each month, one from Eversource (net of the credits from NexAmp) and one from NexAmp for the credits. The two net to a 12% discount to the ES basic rate.
- If you switch to heating with primarily heat pumps (you can still have a backup fossil fuel furnace) you can ask ES for the R4 rate which is for electric heating. R4 is about 1.5c below R1. This is regardless of whether you have solar or not.
- Community choice aggregation offered through a town usually offers several options with a basic rate (ISO NE grid carbon emissions), a 50% renewable mix and a 100% renewable mix. The 100% renewable mix is usually more than basic rate but this is highly dependent of rates at the time the town bids out the contract.
- ES offers greener options through various generators for the supply portion of your current bill. I do not know current pricing but it used to be about 5-6c/kWh more expensive than basic rate for 100% solar, so say 35-36c/kWh