[Greenbuilding] rigid foam inside Larsen truss

Laren sylvanabode at gmail.com
Sun Jan 7 17:35:35 CST 2018


Reuben wrote:

> I have a line on 3-1/2" rigid foam panels

> My house has 2x4 walls into which I've blown dense pack cellulose.

> was going to add a Larsen truss (another 2x4 wall with a big cavity
> between inner and outer walls) and blow cellulose into the rest for
> an eventual 11" thick wall.

Hi Reuben;

  Seems excessive for your relatively mild climate. Do some payback
calculations, but not just comparing the absolutes of doing it or not doing
it. Also look at the paybacks for each inch of insulation.   I anticipate
that
the last few inches will save less energy than was required to produce
and install the cellulose. Overkill is UN-GREEN, and simply wasteful.
Diminishing returns hit pretty severely when it comes to insulation.
So, do a few calculations to determine the optimal strategy.   It might
be greener to spend a little more on 100% renewable electricity, than
on redundant insulation.    With extreme walls like that, you would
likely be looking at nearly all your heat loss going out windows,
doors, and ventilation, and the last few inches of cellulose simply
not having any payback, at all, for the time and money they cost
you.

  BTW, with adding that much foam on the outside of your
existing walls, you are very safe in terms of condensation,
with or without more cellulose outside of it.

> I'm wondering if there are reasons not to place these as the
>middle layer of a cellulose sandwich?

 My recommendation is to add the foam as the outer insulation
layer, without the expensive additional layer of cellulose and
and framing,that would not save hardly any energy. For instance,
you live in (IIRC) a climate with maybe 4500 annual HDD (Heating
Degree Days) not considering solar gain and interior generated
heat (which together may reduce the effective HDDs by a lot).
So if we look at a worst case, assuming just R13 for your total
existing walls, only R12 for the foam (you didn't say what kind)
and another R2 outside the foam, we get a total of around R27,
So, an  annual heat loss of 4000BTU per sqft at the most.  Now,
the thicker (another 7 1/2" of cellulose) wall would come in at
around another R26, so lets just say R27 to make the math simple..
It means you might save, at the very most, half (2000BTU/sqft/yr)
for having to build those Larson thingies, and paying for them, the
cellulose, and probably an extra layer of sheathing.   2000 BTUs is
0.586kWh. So, even if you used electric resistance heating to make
up for it, in your region with your cheap hydro electric it might save
2 cents per year, at most   And, the cost would be at least a couple
of dollars in materials.    However, I suspect you may have a wood
burner, and that you already keep your interior temperatures lower,
which in your climate radically reduces your annual heating load,
So, instead of the minimum 100 year payback (calculated above)
you could likely be looking at more like 200+.years, to break even.
And, if you have a decent mini-split heat pump, multiple time that.
In other words, putting your money, time and effort into such
extreme insulation layers, over the top of what is already quite
good insulation for your climate would be simply wasteful,
therefore not economical, and not green. Put your efforts into
a part of your energy usage where there is a lot of cheap and
easy improvement to be made. For what you are talking about
putting into wrapping your house with second walls, just to
save a miniscule amount of heating, you could likely buy a
used Nissan Leaf, electric car, and move most of your driving
off your fossil fuel list, and save yourself enough money to
fund other energy saving projects.

> Seems like a quick and in this instance cheap way to get a lot of 'r'

    A good deal on foam definitely can be, if everything else is right for
it.
The question is, after the foam, isn't adding another layer of framing going
to be a very bad idea, because your walls would already be well insulated,
for your fairly mild climate? Think about wrapping your house with the foam,
instead of, and without, the second exterior walls.    It could save you a
lot
of money and work, and still save nearly as much energy. Put the money
and time that you save, into other, more effective, energy saving projects.

> But I also don't want to do anything stupid when it comes to moisture.

 You ask a lot of good questions. That is the best way to avoid mistakes.
Remember, your house envelope is not the only place you use energy,
and now there are good options to improve other areas of our energy
lives.

BTW......about a week ago, we bought an 8ft pallet (45 sheets) of 2.18"
XPS, to wrap our 1925 house in a climate much colder than yours. We
got it (new) for 25cents/sqft, and won't need extensive wood framing,
outside of it.    Against a mini-split heat pump, using 100% renewable
electricity, our payback (since we will be re-siding anyway) will be less
than five years.  Our pre-leased electric car, which did not cost more
than a comparable used gasoline car, saved us more than the foam
cost, in just the first six months. Put your money where it will do the
most good, for you, for humanity, and for the planet.

-Laren Corie-
Designer of Energy Efficient Homes, Since 1975
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