Erik on insulation: like everything in life, the more you know, the more you realize you don’t know. Until…

(Erik on insulation: like everything in life, the more you know, the more you realize you don’t know. Until…)

Until… you have clarity and you think finally know it all. But then later you realize you don’t.

Ok, so here is my overview of the tradeoffs associated with insulation… specifically the pros and cons of using spray foam and rigid foam in insulating your new or used/retrofit house project. I know at least a bit of what I speak because we just built a house which uses a number of different insulating products:

1. XPS (blue Dow Styrofoam) — under and side slab and 2″ in attic ceiling/roof
2. closed-cell foam and open cell foam — walls and roof
3. foil faced polyiso — also in walls
4. dense-packed cellulose — also in walls and roof

The issues with insulation choices. The generally fall in to one or more overarching principles.
a) Reduce energy use (name your favorite reason: global warming, energy independence, $ savings, etc)
b) KISS (“Keep it simple, stupid”)
c) Local economy

OK, so finally… here they are:

1. embodied energy
2. global warming potential
3. vapor barrier — permeance
4. air barrier
5. dew points
6. drying potential — “keep the water out, but let it get out if it DOES get in”
7. cost
8. r-value per inch
9. longevity
10. full-life-cycle — recycle-ability
11. ease for installer to do a good job
12. “insulate local”
13. “heat with wood” LINK

OK, so just listing those issues just tired me out without even writing
about them.! Perhaps I will discuss them one at a time in separate posts.

But let me for now, jump right to the conclusion after factoring in all the above issues as I see it:

1) Do not:
Do not use XPS or Closed-Cell Foam (unless it is the type using water as a blowing agent)

2) If building new:
insulation: 12-inch double stud walls (not staggered — no need) with dense packed
cellulose behind mesh from a local installer with the heavy duty blowing
equipment that I will figure out the proper name for.

air barrier: external taped “zip system”

roof: cellulose in 2×8 or 2×10 with OSB-gussets holding 2x4s to provide thermal break.
probably do a vented roof so maybe use 2×4 on edge with another layer of OSB on top.

3) old houses — deep energy retrofit
insulation: add cellulose to the walls. Maybe hang cellulose-filled larsen truss or EPS SIPs on the outside.
roof: if there is an attic, either do deep cellulose on the floor (seal holes first!) or if a used
attic… water-blown (not HFC-245fa) closed-cell foam in the rafter bays. Or maybe open-cell

4) aside: solar for everyone (if you have sun)
4.1 electricity: PVs actually pay off pretty quickly in MA even without factoring in tax credits

4.2 hot water: if DIY: follow and oversize collector and storage if possible. Otherwise
consider heating water with geyser hot water heat pump and more PVs

4.3 space heating:
4.31 Make thermosiphon air-collectors (see, solar siding (ala Nick Pine) or a thermally isolated
low-mass sunspace (ala Nick Pine and Laren Corie) with water storage for cloudy days
4.32 Join the solarheat Yahoo group.
4.33 This almost has to be a DIY job, unless you use the small Solarsheat collectors like those at

4.4 the basic idea with solar is that it is at some point going to be cheaper to add solar electricity and/or
solar heating than more insulation to your home, especially after a certain point if it is a retrofit job. It is
pretty simple math to figure out what that point is by using Ohm’s Law for Heatflow (ala Nick Pine) — basically
calculating the thermal resistance of your home by factoring in all the different walls/windows/etc.

5) aside: employment
if you are unemployed…. this seems like a great field to go into with endless work to do!
This is probably what I would do if I were out of work! You could either get in on consulting people what to do …
like Passivhaus consultants. Or be a construction contractor specializing in deep energy retrofits or solar projects.


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