[Greenbuilding] [BULK] Re: super-insulating

[Lawrence Lile]

>One unfortunate feature of this approach is that 1) it typically
doesn't 
deal with band joists, interior to exterior wall joints, and other 
places the foam won't go because there's no interior wall there, and 2) 
it doesn't necessarily give you that much of a handle on air leakage, 
because of 1).

I'd agree, Curt, I've done the job from both sides, for good reasons in
each case, and adding R-5 to the outside of your building and then
adding new siding is a better way to go, because it covers all the
short-circuits, rim joists, T-walls, and other sins of conventional
framing. But I would not recommend it at all! 

I'll pull a "Nick Pine" and explain this with a little painless math

That being said, if you are already at R-21, your walls are not the
biggest problem.  

Your Mileage Will Vary but let's just take a hypothetical house because
I don't have your house measurements:

A 2000 sq ft, square house would be about 45 feet on a side, with about
1620 sft of exterior walls including band joists.  If it was all R-21,
and your design temperature is 10F ( I have no Idea where you live in
Oregon) then on a peak heating day you'd use about 

1620 * (65 - 10)/21 = 4242 BTU/Hr. 

If you increase by R5 (1.5" of foam) that number drops to 3426, or a
savings of 815 BTU/hr.  At maybe $2 a square foot the job will cost you
a minimum of $3000 or more and a major project.  If you get a real cold
snap to zero outside, then this difference is already erased.  In other
words, the savings is almost in the "noise" 

Let's say you have ten double glazed windows, each 15 square feet. All
of these windows together might total 4125 BTU/hr, or the same as your
walls.  

150 *(65-10)/2 = 4125 BTU/HR

Ten sheets of 1/2" foam will cost you under $100.  A 1/2" sheet of foam
has an R-value of 2.5, and the airspace with foil against it is good for
another R2.77, so the manufacturer sez. The total, with a double glazed
window, is about R7.3.  If you put up foam inserts at night on all South
windows and as much as you can tolerate on all the other windows, you'd
save 

150*(65-10)/7.3 = 1130 BTU   4125 - 1130 = 2994 BTU/hr

So you are saving almost 3000 BTU/HR for 100 smackers.  That's why
adding wall insulation beyond a certain point doesn't make sense, even
if TSHTF or in my case, the Compost hits the Air Handler.  If you are
building new, a layer of foam makes sense to me.  But in an existing
house, I would only do that job if it needs siding anyway, or if it was
seriously underinsulated.  


If you already have R-21 walls, then concentrate on windows and doors.
Add a screen door with a glass insert.  Add insulating curtains or foam
inserts to windows and seldom used doors.  Caulk the edges and trim.
Add storm windows if you must (I hate them).  Look at how air moves in
and out of your house.  Does your furnace have a source of outside
combustion air that doesn't have to pass through the house?  How about
the water heater, ditto?  Is there an energy recovery ventilator?  How
efficient is your heating system?  Could it stand an upgrade, or an
auxiliary system like a wood stove?  All of these things added up will
save more energy, and probably have a better economic payback, than
adding insulation to an already well-insulated house.  






Lawrence Lile, PE, LEED AP
Project Solutions Engineering