[Greenbuilding] [BULK] tub/shower and window questions

[Lawrence Lile]

>The builder who will be doing this phase has proposed that I'd be
better advised to upgrade the windows than go beyond the standard 6"
wall--that essentially, the windows will be the weakest link in a pretty
good envelope, at that point.  The new walls will constitute about half
of
the exterior surface of the house, with the remainder being the wall
assembly I described above.  Assuming an R-40 roof, would anyone care to
offer advice on this?  If the windows are the weak link, what would be a
reasonable step up?


Here's my take:  When I built a house, after some analysis, I put in 6"
walls, filled them with cellulose insulation, and wrapped the whole
thing in 1.5" of foam insulation plus tyvek. 

Inside, I carefully caulked all the seams in the plywood skin, and then
carefully sealed all the penetrations in the interior sheetrock.  This
included caulking around the edges of all the electrical boxes, and
(before sheetrocking) squirting foam around the backs of the electrical
boxes and in the holes where romex penetrated the top plates, to seal up
the swiss cheese that the electricials leave after they get done.  We
caulked seams in the exterior plywood skin, caulked the top and bottom
plates to the skin and to the floor, and carefully sealed every
penetration.  

My take is, spending labor money on caulking and sealing WAY beyond
normal practice is money better spent than on extra superinsulation.
You swiftly get into diminishing returns with insulation: If you have an
uninsulated wall, an inch of foam does a whole lot of good.  If your
wall is already two feet thick, an inch of foam accomplishes almost
nothing.  Somewhere in the middle is a balance point where economics
makes less sense.  


Do you want to go to R-38 walls, doubling the normal insulation level?
Cut your floor area in half, and you've done about as much good.  You'll
spend twice as much money framing a superinsulated house, because you
have to build two frames.  It gets expensive fast.  

Your friend is right, that the windows become the Achilles heel very
quickly.  Earlier on this list I showed that $100 spent on moveable
interior insulation for windows beats the energy savings from wrapping
the whole house in 1.5" of foam.  Anything that improves window
performance makes a great difference in overall energy performance very
quickly.  

If you do the math, you'll find that 50 to 75 percent of your energy
flow in a wall is in the window.  Doubling the efficiency of an already
efficient wall won't affect the whole by very much. 

Now don't get me wrong - the people using superinsulation and Larsen
trusses are doing a noble thing, and we should be pushing the technology
in that direction. However, that's not how I chose to spend my limited
money.  


Lawrence Lile, PE, LEED AP
Project Solutions Engineering