16 Comments

1. 
   jameshowison | Jun 20, 2022 11:50am | #1

   Any way to think about the comfort improvement from more consistent temperatures of the walls? ie the focus above is on the fuel costs to achieve the thermometer set point (as well as embodied carbon); but is there any way to model the impact on surface temperatures (or mean radiant temperature?) and to discuss the comfort implications of that?

1. 
   Malcolm_Taylor | Jun 20, 2022 12:40pm | #2

   James,

   That's an interesting question, although my (largely intuitive) take is that only very rarely would walls thermal performance affect occupant comfort in the way glazing, or surfaces you touch (like floors) can. Even in fairly poorly insulated wall assemblies, the interior surface is pretty close to the air temperature.

2. 
   charlie_sullivan | Jun 20, 2022 03:08pm | #3

   That's a real benefit, although I think you'd see a bigger difference if the walls were worse to start with. But it's a good reason to take care to provide good air sealing at the same time, as that can also have a big impact on comfort in specific spots where it was particularly bad.

3. Expert Member
   KOHTA UENO | Jun 20, 2022 09:49pm | #8

   TLDR: for a correctly-insulated opaque wall, you probably won't get much of an interior surface temperature or comfort bump from continuous exterior insulation.

   The way I have always been taught to estimate surface temperature is to use the "air film"--the the heat transfer coefficient from a surface to the air--table below is from ASHRAE Fundamentals Chapter 26. For the interior wall, that's a vertical surface and horizontal heat flow, so about R-0.7. You then need to think of this air film layer as "imaginary insulation" at the inboard side of the assembly. The temperature at the interface between the innermost layer (e.g., gypsum board) and the air film is the surface temperature.

   So the effect of an R-0.7 air film inboard of an R-13 wall will be pretty miniscule. But doing the same on an R-2 or R-3 window is a pretty significant fraction of the whole R-value--thus the cold surface temperature of windows... and why window surface temperatures drive wintertime comfort.

   
1. 
   jameshowison | Jun 21, 2022 12:45pm | #9

   Thanks all, very instructive :)

   I write this sitting next to an uninsulated concrete block wall (stucco, block, plaster finish, paint), so it's very much the other way around. Yet many contractors have advised that "the savings wouldn't be worth it" to add exterior insulation (Austin TX, so it's a balanced heating/cooling climate), and the Manual J doesn't shift much when I model it.

   Yet I know, given my handy iPhone based thermal camera, that the surface temps can be quite different (saw a 40°F wall during winter freeze, right now the wall shows 83.3°F with a thermostat set-point of 78°F). My impression is that when the system is fighting that radiant, the comfort implications are substantial (especially when the thermostat is at the center of the house). The surface temps are also slow to change, given the thermal mass of the blocks and the lack of useful diurnal swings in summer.

   So my situation is more extreme, because the walls are un-insulated to start with, but there is still the Manual J and energy savings vs perceived comfort question. Maybe it's more like thinking about surface temps of glazing than it is the usual discussions of insulation?

1. 
   charlie_sullivan | Jun 21, 2022 04:01pm | #10

   Yes, an uninsulated concrete block wall is very similar to a double-pane window without low-e or argon. So that is a much bigger comfort issue than a 2x6 wood frame wall with fiberglass would be.

2. 
   charlie_sullivan | Jun 20, 2022 03:13pm | #4

   This statement, from a commenter on the original thread, is incorrect:

   "“Air leakage can reduce insulation efficiency quite substantially if there are a lot of leaks. Large thermal ‘holes’ can make the addition of insulation futile, as all the energy will still flow out of them.”

   In fact, as long as you maintain the temperature differences between inside and out using a thermostat, the heat loss through a wall at R12 or R17 will be a constant, regardless of how much heat is also leaking out through other paths. Certainly, it's more economical to close up a big hole that is leaking 1000 BTU/hr than to add enough insulation to save 1000 BTU/hr, but the insulation will save the same 1000 BTU/hr whether you plug the hole or not.

1. 
   Malcolm_Taylor | Jun 20, 2022 03:31pm | #5

   Charlie,

   You are of course right, but perhaps the original comment was more trying to point out something like that a well insulated house with all the windows open won't be very efficient.

   The same is true of differing levels of insulation in the building envelope. While the well insulated walls of a house will lose the same amount of heat whether there is insulation in the roof or not, the disproportionate amount of heat lost to the roof can't typically be offset by improving the walls.

1. 
   charlie_sullivan | Jun 20, 2022 03:43pm | #6

   And I did not look back at the original comment--I might be taking it out of context.

1. 
   Malcolm_Taylor | Jun 20, 2022 06:52pm | #7

   Kohta sure gives you your money's worth when he chimes in!

2. 
   kbentley57 | Jun 22, 2022 05:35pm | #11

   Charlie,

   That was my comment. I may have typed it out weird, but my intent was basically what Malcom presented. I should have said something to the effect of "You can have all the insulation in the world, but it wont matter if there's a breeze blowing through the house."

3. 
   AndyKosick | Jun 23, 2022 07:34am | #12

   For the record, the local Habitat has been nailing vinyl siding through 2” XPS for over a decade. It’s all fine. It works, not a single failure of any kind. There’s houses in my neighborhood that I walk by regularly and look at. All fine. Product warranty aside I wouldn’t hesitate to do this on the my own home.

4. 
   jwyman | Jun 23, 2022 04:39pm | #13

   In the past two years we have done a deep energy retrofit to our 1941 New England Zone 5 cape. We removed the 3" of Rockwool between the 2x 6 rafters and blew in dense packed cellulose, stripped the roof and added a Solitex Mento permeable weather barrier, 4" of board insulation, 2 x 4 strapping vent space, plywood and a new roof. We went from six cord of wood in our Tarm boiler to three. Next we stripped off the asbestos siding, added a high quality weather barrier, taped all seams and were ready to add new siding. The original plan was to add exterior board insulation, but due to the drastic difference in heating the house along with the added cost of padding out all windows and doors, could not justify the cost. instead we added a rainscreen and prefinished fiber cement siding.

5. 
   vivian_girard | Jun 23, 2022 05:51pm | #14

   My main concern in this particular case would be with the ability of the contractor to perform the job correctly. We don't know how many sqft are involved with this job, but $3000 seems like a very low price for covering a house with polyiso -even a thin sheet. One of the biggest challenges (and expense) would be working around the existing openings and roof line once the wall has been thickened. The issue of electrical boxes and hose bibs seems trivial by comparison. Something doesn't seem right with this proposal.

6. 
   bimbomb | Jul 02, 2022 02:12pm | #15

   I'm in Phoenix (Zone 2). We have a 1960's masonry home with a windowless west facing wall with direct afternoon sun exposure. The interior of that wall is like a radiator in the summer, and we struggle to cool the rooms on that side of the house. I was considering 1-inch polyiso over brick and Hardie Panel on top for just that wall. I know the proper answer someone will quote is to wrap the entire house, but all of my other walls have adequate shade or border underutilized interior space. Any thoughts on exterior insulation of a single problematic wall in Zone 2?

1. 
   Malcolm_Taylor | Jul 02, 2022 02:37pm | #16

   Bimbomb,

   The exterior insulation on that wall would make a big difference. It might also be one of the rare instances where a radiant barrier facing a rain-screen cavity would help a lot.

   Since most of the heat gain is from exposure to direct sunlight, another option might be to fur the wall out with 2"x2"s and use an open cladding.