Zero Carbon(R) T-shirt

Zero carbon footprint all organic cotton made in USA T shirt

Strut your zero carbon credentials with a T-shirt with a lifetime carbon footprint of zero! This T-shirt is made from wonderfully soft organic cotton. The cotton was grown on a family farm in Texas and the T-shirt was sewn in South Carolina. Domestic organic cotton has the lowest carbon footprint of all major fibers used in making T-shirts. The small remaining carbon footprint (including that of washing and drying it over its lifetime) is completely offset by us buying carbon offsets. These carbon offsets are independently verified to be both real and incremental (i.e., the carbon reduction would not have happened without the offset). The Zero Carbon(R) T-shirt – soft on your skin, soft on your planet.

This shirt is popular with companies for use as a corporate T-shirt printed with their logo. Substantial discounts are available for quantities of more than 1,000 shirts. If you are interested in this please email me at dgreen@greenzerocarbonhome.com

You can see it by clicking here: Zero Carbon™ T-shirt

Direct Air Capture Carbon Sequestration Technologies

Hi Mark another thought that might make its way into the building code initiatives:

This is in response to an email about Mitsubishi’s new modular carbon capture technology. 

Hi Jan, I am skeptical. Not because of the chemistry – there are dozens of chemistries for sequestering CO2, for instance NASA used lithium hydroxide on the Apollo moon rockets – but because of the cost. Current estimates are about $100-$200 per ton captured. You can buy a carbon offset in the regulated market for about $50 a ton and about $15 a ton in the unregulated markets. They have the same effect.

Unlike solar panels and wind farms where there is no consumable cost, all these carbon capture technologies use a consumable (Lithium hydroxide in NASA’s case, some source of amines in this Mitsubishi example) which means there is a cost you cannot get below for chemical-based carbon capture. Hence economies of scale, that have been so successful in solar and wind, cannot reduce the marginal cost to zero. The cost of the consumable chemicals sets a floor on the cost per ton of the carbon capture. 

The cheapest way to sequester carbon is to cut down a mature tree, plant a new one to replace it, saw the tree into planks and use them to build a house. This sequesters the carbon for about 100 years. There is no incremental cost as we do this already and people are perfectly happy paying $4 for a 2×4. What it needs is a building mandate to use wood rather than steel and concrete. Wood can be made into glued/laminated beams that can build up to 4 stories high. 

Q: My insulation contractor recommends adding 1/2″ XPS (expanded polystyrene) insulation to my walls. I already have R13 fiberglass.

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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. 
  5. 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.
  6. 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.
  7. 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

Case Study: 1,300sf, 1934, colonial-style home with limited solar potential

Despite being a vintage home that did not allow for adding wall or basement insulation, and without changing any windows, this house’s bills and carbon emissions were both cut in half.

The house is in Arlington, MA with 5,600 heating degree days.

The program consisted of:

  1. Insulation was added in the attic, some air sealing was done in the attic (but neither insulation nor air sealing was done in the basement),
  2. Air sourced heat pumps were added
  3. A fairly small solar array was added to the roof (which has considerable shade from nearby trees)
  4. The natural gas furnace was retained
  5. A wood-burning stove was added for both ambience and heating

Here are the details:

Congratulations are in order! You have cut your carbon emissions by 56% and your bills by 32%. The cut in the bills is despite the price of natural gas being 32% higher than it was when I measured your house back in 2019 and electricity having gone up 13% since 2019. Natural gas will soon have doubled in price from 2019 and electricity will soon be 50% more expensive so you have insulated yourself from those increases as well. You are also not funding Putin’s war in Ukraine.


In the year before I worked on your house your total bills were:
$2,632 for gas, $830 for electricity
$3,462 total
In the last 12 months they were:
$1,033 for gas, $722 for electricity net of solar, $600 for wood
$2,355 total

Your total carbon emissions before the “fab four” were 11.4 tons per year and after they were 5.8 tons, for a cut of 56%.


So I saved you $1,107 per year so that is what you owe me. This is 32% cut in your bills. And this is with the prices for both natural gas and electricity having gone up considerably since I started work on your house. If I recalculate your “before” costs at today’s prices for gas and electricity your “before” bills were $4,207, so you can think of your savings as having been more like $1,851 or a 44% cut compared to the recalculated costs before the Fab Four. So roughly roughly you can say you cut your carbon emissions in half and cut your bills in half too. I assumed you were using electricity from the grid at the average ISO NE emissions per kWh rather than the zero emissions from the “green” electricity you buy because you pay extra for that.


With the limited amount of sun available on your roof I think this is a very good outcome. You are absolutely right that it would be better for everyone if the utilities supplied more green electricity. In addition to the money savings and carbon emissions savings you have almost certainly increased the price of your adorable little house with both the heat pumps and solar panels.


Did you burn the entire cord of wood in the last 12 months? If not let me know how much you burned as I suspect I have overestimated your carbon emissions.

Any thoughts on honeycomb shades for windows?

Q: I have seen these claiming R4.7. Any thoughts?

A: I think they are a very good idea. They are cheap, look good and are very effective. Also, thick, pleated, lined curtains that go to the floor (or window sill) can add R3-R5 to any window. Window inserts can add R1-R2 to any window, but they also block drafts which neither shades nor curtains can do. Many older windows, especially sash windows, leak like sieves, so for these the window inserts are probably the best idea, but if your windows are not drafty then I think either curtains or shades work great and are pretty inexpensive.

Heat Pumps for Hydronic (forced-hot water) Radiator Heating Systems

Q:  For homes with existing hydronic (hot water) radiators, what do you recommend?  Is it possible to use air-source heat pumps to heat the water on those systems?  What about increasing the radiator sizes to operate with Lower water temperatures?

A: Yes you can do this. Daikin, Jaga and SpacePak all make air-sourced heat pumps designed especially for FHW systems. I think it will be a lot cheaper to use the existing radiators and leave the old furnace in place in case the heat pumps can’t keep the place warm in the middle of winter.

Questions to ask a solar-panel installer:

  • For how many years does the manufacturer guarantee the panel-power output?
    • Can I make a claim using only the data on the monitoring app, i.e., without having to get the panel independently tested?
  • For how many years does the installer guarantee the array-energy production per year (in kWh / year)
  • What is the cost per kWh of the electricity guaranteed to be produced over the warranty period of the panels?
    • Cost should be after the federal and state subsidies
    • 5c/kWh is very good but even 10c/kWh is still a 55% cut and it is fixed
  • What inflation rate did you use in the financial forecasts? If it is more than 3% ask them to redo the calculations. I used 0%.
  • Did you allow for taxes on the SMART subsidy? SMART is taxable income.
  • For how many years is my roof guaranteed against leaks?

Questions to ask a heat-pump installer:

  • Does the Manual J show my house will be at 70℉ when it is 5℉ outside?
  • If you have no ductwork ask, “Can you quote this 1) with ductless units and 2) with ductwork in the basement for the ground floor plus ductwork in the attic for the upper floor?” Also ask for a quote for radiant-floor heating, this was surprisingly cheap in our rental house (but this will not do AC for you). Make sure the attic work is well insulated or you will get ice dams.
  • I have not tried AC using cold water in radiators. Some manufacturers say it can be done, but I doubt it without getting pools of condensation on the carpets.
  • For forced-hot water radiators, make sure the Manual J calculation is done with a water temperature of 110℉ not 140℉
  • How much money will I save on heating if I am paying 23c/kWh for electricity? How much if I am paying solar rates (5-8c/kWh)
  • How much for a heat-pump hot water tank? How much will it save me per year?
  • How long is the warranty, does it cover parts and labor?

David Green’s House Has 4.6 ACH50 – Proving That You Can Get To Zero Carbon Without Passive House Level Air Tightness

I had a blower-door test done at my house and the result was 4.6 ACH50. ACH50 is a common standard for air infiltration and stands for Air Changes per Hour at 50 Pascals. Pascals are, like pounds per square inch, a measure of air pressure. 50 Pascals is about the pressure caused by a 20 mph wind. 4.5 ACH50 is equivalent to 1,035 CFM50 (cubic feet per minute at 50 Pascals). This means that the natural air exchange on my house (i.e., at 0 Pascals) is about 0.23 ACH (sometimes called ACH0). This means that the entire air volume of my house is replaced every four hours due to drafts around doors, windows, walls and chimneys. The natural air infiltration rate in my house is 238 CFM0.

This proves what I have long suspected, which is that it is absolutely not necessary to seal your house to the level of air tightness required by the Passive House (PassivHaus) Institute in order to cut your carbon emissions to zero.

The Passive House standard is often held up as the ideal standard for low-energy consumption houses. But I have never seen any financial analysis accompanying this conclusion. This data proves that you can cut both your carbon emissions and bills to zero (and I am making a 15% return on investment too) without the expense of creating a very tight building envelope.

Very few builders can build to a the Passive House standard of 0.6ACH50 and doing so often requires many hours of skilled labor plus the addition of an ERV (energy recovery ventilator) which, alone, can add $5,000 to the cost of the house. I know one contractor who recently did the air sealing on a Passive House project. He gets paid about 3x what a typical laborer on a construction site gets paid. Labor hours add up real fast at those rates! Hence, the Passive House standard for air infiltration can only be achieved at considerable expense – an investment that will never earn a return.

Much like geothermal, solar hot-water panels and thickening your walls with insulation, a super-tight building envelope makes energy sense but does not make financial sense.