Month: April 2021

A: No because I have not added exterior storm windows. However, the purpose of exterior storm windows and interior-fitting window inserts is very different. Exterior storms protect the window frame and its paint from wind, rain and sun. But the air between the exterior storm and the real window is not sealed and it blows away with the breeze, hence it has very little insulating value. With little insulation benefit there is very little cost saving with exterior storm windows hence the reduction in carbon footprint and heating bill is likely to be very small. The purpose of interior-fitting window inserts is to stop drafts, add insulation and block noise. Because they are very effective at blocking drafts (see Chapter 4 in the book) and adding insulation window inserts will both cut your heating bill and carbon footprint. The financial payback is about 5 years. The increase in comfort of the room is immediate.

A: there are several good options for you. Blinds are cheap and effective at blocking sun. Growing deciduous trees that shade the windows in summer are also very effective if you are prepared to wait the 5-10 years for the tree to grow. Adding window overhangs also works very well but they cost more to add. The sunblock films you can buy at hardware stores do work to cut down the sun’s heat. They are cheap so you can’t lose anything to try it and see if you like it.

A: No. The low-E coating gets added to the glass at very high temperatures in the factory. You can buy films to apply to the glass from the inside, but these are not low-E coatings (i.e., they do not selectively reflect either UV or IR light) but they darken the whole window. They are good for blocking intense sun but they do not reflect the IR light that keeps you warm in winter.

A: We open plenty of windows and patio doors in summer, or more commonly in the shoulder season when it is around 70F outside, just not the ones with the window inserts.

A: You can but I do not. I leave mine in year round.

A: Please see the evaluation I did of the window inserts from four manufacturers. It is in the “Stuck at home COVID19” edition of the book which is available for free from my website, use the code COVID19 at checkout. If it does not work email me.

A: Yes, but only if the window material is glass. The only maker of window inserts that I know of that makes them in glass is Innerglass in CT, all the other companies I tested were made from clear plastic.

A: I have found that the physics of heat flow in basements is widely misunderstood even by people for whom it is part of their job. 
The most basic fact to understand is that heat flows from higher temperature to lower temperature. This is a fundamental as the laws of gravity. You don’t let go of a ball and see it rise. It falls. Always. The same with heat, it always flows from higher temperature to lower temperature, always and without exception. On average (I will deal with the area above the furnace in a moment) the house is at say 70F, the basement at say 60F and the outdoor ground below grade is at 50F. Heat flows from your house to your basement and from your basement to the ground. There is a lot of air circulation in a basement, sometimes deliberately because there are vents in the basement, and sometimes just because of convection currents caused by – you guessed it – that furnace. The heat rising from the furnace (lets say the air above the furnace is at 90F) stirs up the air in the basement causing the air to circulate around the basement and warm up the cold walls and floor of the basement (because they are colder than the warm air pushed up by the furnace). In the area right above the furnace the air, at 90F is indeed warmer than the floor of the house which is at around 70F and so heat does flow from this warm-air column to the floor of the room above the furnace. The further away you go from the area immediately above the furnace, the cooler the ceiling of the basement will get and at some distance, I am guessing about 15’, it will drop to 70F. Further away from the furnace the ceiling will be colder than the air in the basement and the flow of heat will be down from the house to the basement. So the ideal situation would be to insulate the entire ceiling of the basement except for a 15’ radius circle around the furnace. You could easily determine the size of this ring by just measuring the temperature of the ceiling of the basement using an infrared thermometer (such as the one I have, the Black and Decker TLD100 which you can get for about $30 at Amazon) during the time that the furnace is on.
I installed the fiberglass insulation on the ceiling of our basement in exactly this pattern. The ceiling of our basement is not insulated in a strip about 8’ wide and about 20’ long above the furnace. The long axis of this strip aligns with the long axis of the basement, which is where I think the convection currents carry most of the heat. Since, now we have heat pumps, we use the furnace only on about 20 days a year this has very little effect, but I think it is worth doing if you are still using your furnace year round.
However, what I did do that I think has a much bigger effect is to insulate the bottom and sides of the ductwork which run along the ceiling of the basement. I did this with 4″ thick fiberglass. This allows the warmth of the heated air inside the ducts to warm the floors above them while keeping most of their heat for the rooms to which the ductwork carries the heat like the bedrooms on the upper floor in the corners of the house. The heat in the ducts is no longer warming the basement. This is part of the reason the temperature in my basement dropped about 8F and the temperature of the air coming out of the vents in the bedrooms increased 5-10F. 
Overall, the trick is not to heat the entire house. It is to heat the parts of the house you live in.  

These last few questions raise issues related to dampness or moisture control. While moisture control is not explicitly part of a zero-carbon retrofit, I think it is very important to pay attention to it or you will get mold and eventually rotten timbers. This is not a trivial problem. A house in our town was condemned by the board of health because mold issues sent the occupant to the hospital with asthma. The risk of condensation/mold/rot increases if you do a lot of air sealing because air sealing reduces the flow of air in the house. It is this air flow that evaporates the condensation and prevents mold. Early-generation PassiveHouses, which rely on exceptionally tight building envelopes, often developed mold issues. These problems can be overcome with good humidity control.

In the winter, drafts rob you of heat (and dollars), but they are also a source of dry air that evaporates any moisture that has condensed in your walls, basement or attic. Condensation happens when the temperature of the air falls below the dew point. This is what causes dew on the grass in the morning. Overnight the temperature dropped below the dew point of the air. The dew point of the air varies with its humidity, but it is often around 40-45°F. This is why you see dew on the grass in October and April because we have fairly warm days with lots of humidity in the air, followed by cold nights. If there is dew on the grass, there is dew in your walls.

At these times of year (spring and fall), condensation in your walls and roof is inevitable because if the house is at 70°F and the outside is at, say, 40°F. Somewhere in the wall, the temperature is at the dew point and condensation happens. Air flow up your cavity walls and through your roof evaporates this condensation. This air flow is not driven by wind. It is driven by the stack effect which creates vertical air flow in the house caused by warm air rising. Condensation does not cause mold as long as it evaporates within about 24 hours. Condensation is inevitable, but mold is not. 

The best way to prevent mold in a zero-carbon retrofit (or any house) is to control the humidity. If the humidity in the house is lower than that in the walls (which it will be as long as the house is warmer than the dew point of the air, which it almost always will be unless you keep your house at 50°F) then condensation can dry to the inside of the house by diffusion. Even drywall allows moisture to dry through it by diffusion. This provides moisture a way out of the wall cavity even if there is very little air flow up the wall cavity. Air-sourced heat pumps come with humidity control. On mine, I can set the humidity level as easily as I can set the temperature. I set it to 40% year-round which feels comfortable because it does not leave your skin feeling dry, but the air is still dry enough to dry out the bath towels or the laundry.

In addition to the humidity control on the heat pumps, I also have a heat-pump hot-water tank in the basement. This dehumidifies the air in the basement. Humid air is less dense than dry air and so it rises. Warm air is also less dense than cold air, so it rises too. If the air is both warm and humid, it really rises. Basements are often damp and are often warm (at least in places) because of the furnace or boiler. The warm humid air in the basement rises through the house, reaching the roof where the moisture often condenses on the cold roof surface. The source of mold and rot in cathedral ceilings can often be traced to dampness in the basement. Hence, if draft sealing is part of your zero-carbon retrofit, I think it is essential to control the humidity in the house and the basement. 

Heat pumps with humidity control, heat-pump hot-water tanks and draft sealing play very nicely together. 

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 at our rental property.  Repairing the gutters (I put a perforated metal plate on the top of the gutter to stop leaves blocking the gutters and causing them to overflow) 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.