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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.

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. This is especially problematic on north facing roofs that do not warm up in the sun. You can now get vapor barriers specially designed for this situation. One supplier is Siga 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 add insulation behind the barrier. Check out Martin Halladay’s posts on this topic on Green Building Advisor (GBA). One of his posts states: “GBA gets more inquiries about rotting cathedral ceilings than any other type of building failure. That’s why I’m conservative about recommendations for cathedral ceilings. It’s important to get these details right. If you screw things up, everything gets damp and begins to rot.” See below information on condensation problems in general.

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 paying a contractor to add more insulation is unlikely to save enough money on the bills to pay back the investment in a reasonable period of time. However, it might make sense if you did it yourself, which is easy if you are installing fiberglass. 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. If you did this very cheaply, by just buying rolls of fiberglass and installing it yourself it would shorten the payback period. Please see also my other answer (below) on cathedral ceilings.

Q: I watched a presentation of yours a few weeks ago, the one sponsored by Sherborn and Holliston. Impressive and informative…many thanks.
As a result of the presentation I’m interested in adding 2” insulation board, as well as Aerogel, when I re-side the house. My wife does not like foam off-gassing, so we will leave our fiberglass batting alone and add the insulation board outside of it.
There are LOTS of insulation panel types to choose from. Any recommendations on which are best?
In your presentation you mentioned the aesthetic problem with adding 2” or 4” insulation boards, i.e. that they strand the window a few inches inside the siding. I wonder if this problem could be overcome by cutting the insulation board surrounding the windows into picture frame dimensions by making – a 3-d mitre cuts of the insulation at the window corners. One would end up with thin insulation board at the edge of the windows, but gain a lot of insulation everywhere else.
Any thoughts much appreciated.

A: You can certainly do this kind of mitre cut to lessen the impact of installing the insulation. If you are adding only 2” it might still look good. I think it is best to discuss this with the window installer. 
The Aerogel product has the best R value per inch at R10, so 1” has the same insulation as a 2” ISO board, but it’s only an inch deep. It is more expensive psf than ISO board. I do not know what it’s flammability rating is though. 
A more fire-proof alternative than the foam boards is to pull out the fiberglass and replace it with rockwool batts (Roxul comfortbatt is the most widely available). It is about R4 per inch vs R3 for fiberglass. Rock wool cannot burn (it is literally strands of melted rock) so it is better fireproofing. You can add a 1.5” rockwool board (R6) instead of the ISO board outside the studs too. If you are going to go to all this trouble it is probably worth adding an airtight/waterproof membrane on the outside too. Siga makes these type of specialized membranes. They have to be properly installed (with the seams sealed) to create the air-tight barrier. After this your house will be far better insulated and far less drafty. 
I am glad your wife is concerned about off gassing, but I think there is an even bigger concern: fire resistance. Sometimes I feel like a lonely voice on the flammability of foam products. I intend to use rockwool when we add insulation on our rental property for precisely this reason. It is not just the flammability that bothers me, it is the thick black smoke created when foam insulation burns. I will also be adding the membranes for air tightness and humidity control too. 

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 insulation 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: I understand the carbon footprint for the plastic foam insulation is pretty high. 475 Building Supply and PHIUS are recommending cellulose and wood fiber bd which can be carbon sequestering. Short of that they are also recommending ROXUL, much lower carbon footprint.

A: Spray-foam insulates well and hence helps to reduce global warming. However, the gas used to make the bubbles in the foam is often a gas called a hydrofluorocarbon or HFC. If these gases sound familiar it may be because of their cousins, chlorofluorocarbons, or CFCs, which became infamous for causing the hole in the ozone layer. CFCs were banned in 1996. They were replaced by hydrofluorocarbons, which don’t deplete the ozone layer but were later found to cause global warming.  Hydrofluorocarbons can be about 1,000 times as strong as carbon dioxide in causing global warming. Hence, installing spray-foam has a high carbon footprint even if it insulates well. Because of this, some companies are now using hydrofluoro-olefins or HFOs to make the bubbles in spray-foam insulation. HFOs have very low global warming potential. If you are going to use spray-foam insulation I recommend these HFO-blown foams. 

Rockwool boards are also great insulation and are fireproof, which is distinct advantage over sprayfoam which burns with a thick black smoke. 

Dense-packed cellulose is also a good insulator. However, I am unconvinced by the argument that it is sequestering carbon out of the atmosphere because that wall will eventually be demolished and either decompose in a landfill or get incinerated. Hence, its carbon is returned to the atmosphere. I think it is more accurate to say that using dense-packed cellulose delays the carbon-dioxide emissions compared to the tree falling down and rotting in the forest, but I think it is inaccurate to think that it is permanently sequestering carbon dioxide in the sense that other carbon-sequestration technologies (such as turning carbon dioxide into rock and burying it underground) do. 

All types of foam, whether sprayed or boards, have similar end-of-life issues as dense-packed cellulose and are likely to get incinerated (releasing their carbon dioxide) or buried in a land fill (where the carbon will be permanently sequestered) when the building is demolished. 

Overall, I think that fiberglass, rockwool and dense-packed cellulose (treated with a fire resistant coating) are the best insulating materials, because they do not have the global warming potential of releasing HFCs into the atmosphere like sprayfoam does, they are fireproof, and they can easily be separated when the house is demolished and recycled or reused. On a renovation we are going to be doing shortly, we plan to use rockwool boards outside the air-barrier membrane (as the thermal-bridging prevention layer) and rockwool batts in the 2”x6” cavity walls for the insulation. On a different renovation we are planning to use dense-packed cellulose and rockwool boards. See my other answers to related questions on walls, SIPs, and controlling moisture in general in houses. 

It is easy to insulate a basement when you are finishing it. You just put roxul boards on the walls, cover them with drywall and paint. The only hard part about this is moving all the stuff out of the basement to do it, but since you have to do that anyway in order to finish it, it is actually quite easy. The floor is harder to insulate but even putting a layer of foam underlay on top of the concrete and then putting in thick carpet will greatly improve the insulation. Before anyone does this though I would recommend installing a sump pump and wiring it into the emergency panel and having a back up electrical supply (either a generator or a battery) because even a small amount of water in the basement would now mean that your carpet is ruined. This happened to us, 3 months after installing a beautiful new wool carpet that cost $3,000. We had to put the whole thing in a dumpster.

A: The easiest way is to drill a small hole (about a ¼” diameter) through the drywall in an inconspicuous place like in the back of a closet. Then take a piece of wire with the end folded over to make a hook and push it through the hole and pull it back out. If you see strands of glass it is fiberglass, if you see pieces of what looks like shredded newspaper it is blown in cellulose and if you hit something that feels slightly soft but nothing comes out if it is spray foam.