[Greenbuilding] rigid foam inside Larsen truss

[Lynelle Hamilton]

So well said, Lauren!



On 2018-01-07 6:35 PM, Laren wrote:
> 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 themost. 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 regionwith 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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