Office Notes: Thought process behind my decarbonization project

August 3, 2021 | Tom Bassett-Dilley

I decided to take the leap and decarbonize my 1919 frame bungalow. This was born out of several things: first, we were at the end of service life on the gas water heater, within 5 years of service life on the furnace/AC (inefficient gas, poorly installed), and the soffits and fascias had begun to fall off the house—the squirrels were having a heyday. My walls still didn’t have retrofit insulation, and my enclosed back porch was poorly enclosed in the ‘40’s, with some band-aid level solutions I had installed to make it tolerable. Plus, the house featured an original basement, which is to say, a cracked slab that didn’t keep moisture, radon, or critters out, and no insulation down there. In other words, about 60% of my interior was nasty and uncomfortable, and the rest needed help, too. The driving motivation was a combination of a sense of stewardship and adventure—doing the right thing by my building (which would make for a more durable and much healthier and comfortable living environment), and experiencing first-hand technologies like the heat pump water heater, mini-split heat pumps, induction cooking, and good ventilation with energy recovery. Yes, it’s expensive, but I’m fortunate to be able to finance this given historically low interest rates. This project will teach me a lot, allow me to teach others, and beyond the enjoyment of it for many years, I think it will pay off in the long run. I began with the question, “could I make my house a Passive House?” The answer was yes, but it meant having to replace my roof (which was insulated in 2010, when I had no grand plans and little money), and having to pull out my existing windows (replaced in 2004, when I hadn’t heard of Passive House or triple glazing, and “airtight” was a term nautical designers, not architects used). So, while it was possible, I didn’t think it made sense to spend a lot of money throwing away serviceable items. My next question was, well, if I can’t make all the PH metrics, can I make the PH Source Energy limit? In other words, could I cut down my total overall energy usage to Passive House levels, even if space conditioning energy is a bit high? In doing so I would employ all the PH strategies at my disposal: minimizing thermal bridges, insulating, making the house airtight, using an ERV for ventilation, and using efficient lighting and appliances, plus some solar PV. The answer was yes, it’s totally doable! In fact, I could approach annual Net Zero by adding some more solar.

Check out these visualizations of our energy modeling:

Here are the strategies I employed to get there:

Exterior walls: these are leaky and poorly insulated, and were also covered with asbestos siding. I hired a remediation crew to remove the asbestos; next we will strip off the old siding and expose the original sheathing. We will drill holes in the sheathing so we can pump cellulose into the walls, and then we’ll cover the sheathing with a diffusion-open (meaning it lets moisture “breathe” to the outside) peel-and-stick air barrier membrane, sealed at all penetrations, sealed to the foundation, and sealed as best we can to the roof. We’ll then cover that with a diffusion-open wood fiberboard insulation layer, creating a thermal break at all studs. The fiberboard is essentially a negative-carbon product, having absorbed carbon while it grows, and being repurposed from waste product. Over the fiberboard go furring strips and new siding. This is more than a face lift—it’s like major reconstructive surgery!

Basement: an 850s.f. conundrum. Over the years, whenever I would come up with ideas to expand the house up or out, I always came back to “yeah, but I would have to deal with that basement.” Dirty, cold, wet, and leaking radon (just enough to be concerned), it’s a lesson in why capillary breaks and waterproof insulation are needed. The more I thought about it, the more I realized that insulating the basement plus good HVAC would mean I would have 850sf that I could actually enjoy for art studio, guests, music, laundry, etc. It would mean tearing out the old slab and putting in new windows, but it would really change the way I look at about 40% of the house. Also, replacing the three old wood structural columns with new ones (on stand-off bases) means I don’t have to worry about those degrading by wicking up moisture from the ground. For the windows, I decided to go with Marvin fiberglass double pane; while triple pane is my choice for comfort and energy, especially on larger windows, these are small units in less-used rooms, and I thought it made sense to save over a thousand bucks here.

HVAC and water heating: with a much-improved thermal envelope, I have the opportunity to have a much smaller, energy efficient heating and cooling system; and by replacing the old gas water heater with a heat pump electric type, I will have eliminated most the gas use (cooking and dryer only remaining)—almost there to all-electric! For space conditioning, this meant mini-split heat pumps, of course, like we use on most our Passive House and low-energy projects, since they’re very energy efficient. I wanted the system to function properly, so planned to replace the ductwork so it would be properly sized and well-installed (airtight). I chose a conditioning ERV (the CERV2 from Build Equinox) so that I would have demand-controlled ventilation, good airflow, great filtration, and a modest amount of conditioning when I’m ventilating. The water heater is Rheem Proterra 50-gallon heat pump unit I picked up at Home Depot. It will cool the air in the house in heat pump mode; I don’t see this being a winter comfort problem since it’s in the basement, and the conditioning system will offset the losses—but it will be good to live with it and get first-hand experience.

Appliances and PV: the last remaining items were the range (switching from gas to induction) and dryer (switching from gas to a heat pump condensing dryer). I decided to commit to these with the rest of the project so I could eliminate my gas line. I already have 12 solar panels on the house (kind of jumped the gun, but got them the last year that the Federal tax credit was 30%, before it got reduced) and plan to add some more to offset my increased electricity use (even though everything will be very efficient, I’m using electricity for heating, water heating, cooking, and dryer now, so overall electricity use will go up as gas goes to zero).

Other problem areas: common to a lot of older renovated houses, my front and back porches were enclosed long ago, and were not properly insulated. To fix that, I’m reframing the back porch walls and installing triple-glazed (Alpen) windows; on both, I’m insulating the floors with a few inches of closed cell spray foam for airtightness and condensation control, topped by loose fill insulation (less carbon intensive)—we’re still working out the details of this in light of material availability.

You can follow the progress on Instagram where we upload images from the project. Also, Tom presented this project on the Green Built Home Tour session on “How to Prioritize Sustainability Upgrades for an All-Electric Home” and you can watch that video here.

Tom Bassett-Dilley Architects | Contact