Insulating Knee Walls
I enjoyed the article "Building an Energy-Efficient Spec House"
(8/07). I am happy to see that the insulation was kept in the
rafters, rather than following the knee walls to the floor.
We've tried to insulate knee walls many different ways, with
I also long ago stopped attaching knee-wall studs directly to
the sides of the rafters as shown in one of the story's photos
[reprinted below] because it was so hard to insulate and seal
around the studs. Instead I run a top plate, which eliminates
the twisting in the studs and helps to distribute the flexing
that rafters are prone to. Before building the knee wall, I
first insulate the rafters with fiberglass, then install a poly
vapor retarder before attaching the knee-wall top plate.
I also think that it's generally a good idea in accessible
knee-wall spaces to cover the insulation with drywall or
plywood. The 1x3s shown in the article's photos might not be as
effective in 15 years' time, and even reinforced poly can tear
or sag over time.
Brooklyn Park, Minn.
Author Paul Huijing responds: While I agree that using a top
plate on the angled knee wall will result in a stronger wall,
the air-sealing problems you mention are more likely with
fiberglass batts than with blown-in cellulose.
As for omitting the drywall on insulated rafter bays behind
the knee walls, my insulator has been doing this for about 10
years and has never had a problem. He uses a very strong
reinforced plastic held to the rafters with 1x3 furring strips
nailed on 16-inch centers.
Step Flashings Not the
I would like to take exception to the claim in "Rain-Screen
Retrofit" (9/07) that step flashings contributed to the house's
problems. Looking at the pictures, what I see is that the water
damage was concentrated across the first floor — the most
lived-in part of the house. But I don't see any telltale signs
of water drips below the first step flashing off the bay
As a third-generation roofer, I can tell you that we are taught
never to violate the housewrap in any way. The key is to make
sure that the step flashings are tall enough to accommodate the
siding chosen for the house. If builders or siding contractors
are worried about water behind the siding, then all they need
to do is use the corresponding tape provided by the housewrap
manufacturer and seal the tops of the flashings to the wrap
above. If the tape is good enough to use at laps and cuts, it
should be good enough to use there.
One problem I've seen over the years is the first step flashing
hasn't been put in front of the siding coming up the wall from
below. This is especially an issue with EIFS, which are
sometimes as much as 3 inches thick. If the EIFS are installed
after the roofing is completed, water that runs down the step
flashings will run behind the siding into the wall. Many
contractors resort to caulking the bottom flashing to try to
dam the water, but we all know how long caulking lasts under
Homeowner Safety Should Come
While I respect the comments made by Mr. Holt in "Expanded AFCI
Requirements Spark Controversy" (In the News, 9/07), the key
issue at hand is safety, which is what AFCIs were designed to
Homeowner safety needs to be a priority in the home-building
process, but it seems builders are too hung up on cost.
Electrical fires kill and destroy property; the Consumer
Product Safety Commission (CPSC) believes that AFCIs could stop
up to 50 percent of these devastating fires, yet many builders
are opposed to them.
Whether you use the CPSC cost estimate of $15 to $20 per AFCI
or the street cost of $30 to $35 (not including installation),
it's still relatively insignificant compared with the deaths,
injuries, and hundreds of millions of dollars of property loss
caused by electrical fires annually.
According to the story, Mr. Holt's belief is that "few
electrical contractors ... have experience with the new
combination AFCIs." To the trained eye of the contractor,
installation of the combination AFCI is essentially the same as
the standard version, which the NEC has required for
The difference, and what the NEC has recognized in its
expansion of the AFCI requirement, is that the combination AFCI
detects both parallel and series arcing — a technological
leap forward in homeowner protection. It's also important to
note that the expanded NEC requirements have the support of the
National Electrical Contractors Association (NECA) and other
AFCIs are a major benefit to the homeowner and a safety measure
that builders and others involved in the home construction
process should not overlook. They are certainly well worth the
National Electrical Manufacturers Association