Month: April 2021

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.

A: Yes, that is true. Air circulation evaporates water that inevitably condenses in walls in spring and fall. Please see my other answers to people’s questions on moisture on this page. However, it is almost impossible to seal an existing house so well that you get moisture problems. It happens far more often on new construction, particularly on houses built to the Passive House standard with requires almost no infiltration of air. The best way to deal with condensation in a house is to control the humidity which is easy to do with heat pumps. Please see my other answers on condensation and moisture problems in houses on this page. Condensation in houses is the most insidious problem in houses today.

A: This question mostly arises in conjunction with formaldehyde in foam insulations. Formaldehyde is not used in making cellulose insulation which is made from shredded newspaper, fiberglass which is made from fibers of melted glass or rockwool which is made from fibers of melted rock.

Strictly speaking spray foam is not toxic. But, if your house catches fire, it does burn with a thick black smoke that can be toxic and this is one reason I avoid it if I can and use fiberglass or rock wool (neither of which can burn) instead. 

Also, the gas that is used to make the bubbles in spray foam is usually a gas called an HFC (or hydrofluorocarbon) that, although, strictly not toxic itself, is 1,000 stronger than CO2 in causing global warming, so it is, in that sense, toxic.

A: Spray foam will last as long as the building. Dense-packed cellulose and fiberglass tend to settle down over time and so become less effective. However, taking off your siding or drywall is expensive and the reduction in heating bills will likely not pay for the cost of this work even if your insulation is in poor shape. It is better to add insulation opportunistically see Chapter 2 in Zero Carbon Home.