Zero Carbon ® Blog

Q: Could you talk a little about financials of adding iso board to a pitched roof-cathedral ceiling with 8″ cellulose in rafter bays and shingles that otherwise wouldn’t be ready for replacement?

A: If you have 8” of insulation already, then adding more insulation is unlikely to save enough money on the bills to pay back the investment in a reasonable period of time. I have found with my client work that even with 6” of insulation already, the savings on going to 12” is only about $150 a year. If you have no insulation today it is worth adding 12” but going from 6” to 12” will not save very much money. Please see also my other recent post on cathedral ceilings.

Q: Insulation recommendations for cathedral ceilings?

A: Cathedral ceilings are tricky to insulate. Most building codes require them to be vented to remove the inevitable condensation caused by warm, moist, air rising to the top of the ceiling and penetrating the ceiling through light fixtures, skylights, cracks and just from diffusion through the drywall. The moisture in this air then condenses when it hits the cold roof surface. If this does not evaporate you will get mold and rot. You can now get vapor barriers especially designed for this situation. One supplier is Majrex in Switzerland. I have not used this myself but I intend to use it when converting unfinished space in our rental property to finished space. With a proper vapor barrier you can either add insulation behind the barrier or in front of it (lowering the ceiling). Check out Martin Halladay’s posts on this topic on Green Building Advisor.

Q: Do you know about the availability of high efficiency windows that will be acceptable for homes in historic districts with strict regulations to adhering to historic accuracy?

A: I think probably the best way to deal with this is to add the window inserts you can see in Chapter 3 of Zero Carbon Home. These are invisible from the outside so historical commissions usually are OK. If you use the glass ones and get the low-E version you can add about R2 to an existing window. If the existing window is R1 you now have an R3 window, which isn’t great compared to modern low-E triple-glazed windows at R4-5 but it is 50% better than a plain double-glazed window. They are also far cheaper than replacing windows and you can fit them yourself, you don’t need to hire a contractor. 

Q: What about installing the heat pump in a greenhouse? Maybe removable for the summer.

A: I have not done this in a greenhouse  but I think it is a good idea. Effectively I have done this by bringing plants indoors in the fall and keeping them in our sunroom. The sunroom receives a very small amount of heat from our house heat pumps (it is at the end of the ductwork) but, using LED grow lights (powered by my solar panels) I was able to get red ripe peppers at Christmas. They tasted great!

Q: So your walls are insulated with 4″ fiberglass batting?  Earlier I had the impression that your house has no wall insulation.  What about homes much older that ’74 with NO wall insulation? 

A: Yes our house has 2” by 4” stud cavity walls filled with fiberglass. The only answer, John, is to get wall insulation. You can do this without having to remove and replace all your siding by blowing in spray foam or dense packed cellulose from the outside. This requires drilling holes in the siding but these can be patched afterwards or a small section of the siding can be replaced. If you have no insulation today in your walls you are losing money through the  walls the way that water runs through a sieve. 

Q: What about sealing/insulating crawlspace walls and plastic on soil to insulate crawlspace? 

A: Insulating walls in crawlspaces is a very good idea. It is a lot easier to insulate the underside of the floor than to try to insulate the soil because you can just push fiberglass in between the joists. 

Response: Hi David, Thank you for replying. I had noticed condensation problems in the crawl space due to uninsulated HVAC ducts sweating in the summer. Venting the space in the summer allows humid air into the space where colder pipes sweat badly. Researched this and found a revised opinion on what to do with crawl spaces. The advice I read (sorry, no references) was to seal the wall vents, insulate the inner walls and cover the soil base with heavier plastic. Wetness due to water inflow may require drains and a sump pump. I am moving in that direction (DIY) having previously insulated the joists under the floor and ducting but still experiencing the moisture issue. Thank you again for sharing.

Reply: It sounds like you have two sources of moisture in the crawl space. The vents and the soil. I think you will need to deal with both to stop the water condensation on the HVAC ducts when the AC is running. Sealing the vents will help but without dealing with that wet soil it will probably not be enough. I think the plastic sheeting on the soil will help, but it is a band aid, not a cure. The real question is why is there so much moisture in the floor in the first place? Is the ground water high near you? If not, are your gutters in good shape? Overflowing gutters can put a lot of water in the soil right by the house and this will wet the soil under the crawl space. I have seen this and repairing the gutters (I put a perforated metal plate on the  top of the gutter to keep the leaves out) and replacing split downspouts (and adding extenders to the bottom of the downspouts to keep the water away from the foundation), worked. I also sealed cracks in the basement concrete with a can of spray foam – cheap and effective as a water barrier as well as an air barrier. Once you have solved this problem, I think replacing your hot water tank with a heat-pump hot water tank will help. It will not only cut your bills and carbon footprint but it will dehumidify the basement air too. Once you have done all this, I would insulate the HVAC ducts, but if you don’t deal with the moisture first, you will risk getting dampness and mold on the insulation.

Yours sustainably,
David Green
Make money by cutting your home’s carbon footprint to zero. We have done it. 

Q: What about solid stone walls/floors and insulation? Ensuring breathability.

A: Probably the best solution here is to use a product like rockwool boards for insulation. They are breathable so you reduce the risk of mold behind the boards. They are also fire proof, unlike most foam insulation. It is probably a good idea to also install a heat-pump hot water tank. Unless you have very high electricity prices these are cheaper to run than heating water with a boiler, even one powered by natural gas. If you use solar panels to generate your electricity it will be much cheaper than heating your hot water with natural gas. They also dehumidify your basement which will reduce the risk of mold. 

This concern was raised by a person who attended one of my Zero Carbon, Zero Bills webinars. Here is the question and my answer:

Queation: Hi David, I really enjoyed your talk last week on your zero carbon adventure. I did have a question, which didn’t get answered. I have a friend that works at National Grid and she told me if you sell your solar RECs, which I believe you did, you can not claim to be carbon free as those rights go with the REC? After your talk, I went back to her and she felt that this was definitely the case and that you would need to buy RECs to offset the ones’ you sold, to make the carbon free claim and you should also tell people you sold your RECs and brought other ones. 
Just wondering your thoughts on this 
Thanks and thanks again for a wonderful talk. I plan to share this with some folks 

Hi Rob, I have occasionally heard this perspective before and I think that reasonable people can disagree on this one.

I have two concerns with your friend’s perspective. 
The first is that I have not counted either RECs or SRECS in my calculation of my carbon footprint. I have actually cut my carbon footprint to zero by using the fab four, it is Eversource that is claiming it has cut its carbon footprint when it has not done so. So, I think it is more accurate that I say I have cut my carbon footprint and it is Eversource that has been allowed by the state regulators to claim it has cut its carbon footprint (by buying my SRECs) when it has not done so. But I have genuinely cut my carbon footprint and it is zero. The SRECS have not been counted in any part of my carbon footprint reduction. My carbon footprint is zero because I no longer burn heating oil and I generate all my own electricity, including that to heat my house, using heat pumps, powered with zero-carbon solar panels. 

My second point is that, where I do count the SRECs is in the financial subsidies. I genuinely receive this cash so I do not think it is right to remove it from the financial forecasts. There are many subsidies involved in going zero, from net metering and zero-interest loans (which are essentially funded through all of us in MA paying the highest price price for electricity in the US, other than Hawaii) to federal and state tax credits. SRECs are calculated per kWh I generate, the federal tax credit is calculated based on the price you pay for the solar system you install. Why should one subsidy not count just because it is based on kWh generated rather than on the price of the solar system? SRECs were replaced with SMART in 2018 and so it is only owners of legacy systems that receive them. I do not see why a subsidy should be dismissed for those who installed systems prior to 2018 but should be allowed for those who installed systems after 2018.

In my opinion, SRECs are just a form of subsidy, paid from a regulated utility to an owner of solar panels, which means they are ultimately paid by all electricity customers. This is essentially a regressive form of taxation because it takes money from everyone who uses electricity (including poor people) and funnels it to those who can afford to install solar panels which tend to be wealthy people or investors. I think this is rather unfair, but I do not make the laws Rob, I just abide by them.
Please share these thoughts with your friends, I am interested to hear what she says.