Countertop Bolts for Stair
In the article "Installing an Over-the-Post Handrail" (4/06),
Jed Dixon uses an interesting rail bolt I've never seen before.
Is it possible to get the supplier's name?
North Haledon, N.J.
Author Jed Dixon responds: Those are Knape & Vogt
tight-joint fasteners, usually sold for fastening countertops
together from underneath. I've used them for years as rail
They're available from cabinet-hardware suppliers who carry
the KV line of drawer slides.
Which Comes First, Membrane or Drip Edge?
In light of the numerous listed standards (such as the "NRCA
Roofing and Waterproofing Manual" and ASTM) and of the fact
that all major roofing-shingle manufacturers disagree with your
opinion concerning the proper placement of the underlayment and
drip-edge flashing, how do you justify the January Q&A
on this topic?
Author Paul Fisette responds: I believe I explained my
reasoning for using the detail I described, although there are
definitely different opinions on how this should be done.
The Asphalt Roofing Manufacturers Association, for instance,
provides details in its technical bulletin "Preventing Damage
From Ice Dams" and advises roofers to extend underlayment over
the drip flashing at the eaves by 1/4 inch (see
the product specs for Grace Ice & Water Shield tell you
If you feel bound to a particular manufacturer's warranty, you
should follow the specific guidelines provided by that
Also, I encourage builders to follow the locally enforced
code. Where a local code indicates a specific eaves detail,
builders should follow the code. But based on roof performance,
I stand by the details I outlined in the Q&A.
Theory vs. Practice
I am a college professor who teaches courses in engineering
technology and technical physics to students in several majors,
including Construction Management. Therefore, I was pleased and
impressed that Dave Yates used Boyle's law and provided
numerical values in his response to the question about
protecting water heaters with expansion tanks (Q&A,
3/06). Overall, the content was valuable and informative.
I do want to note one correction. When pressure is used in
equations like Boyle's law, absolute pressures — not
gauge pressures — are required to be mathematically
correct. Therefore, the P2 gauge pressure for the first example
is 184 psi, not 168 psi, as stated. Likewise, the P2 gauge
pressure for the second example is 283 psi, not 250 psi. These
differences don't affect the outcome of the examples in terms
of the concepts, applications, and advice Dave Yates provided.
However, there may be situations encountered by readers in
which using the correct pressure values in calculations
involving Boyle's law and similar equations is of critical
Ohio State University
Author Dave Yates responds: Professor Zimmerman is
absolutely correct regarding the use of absolute pressures in
the classroom or for technical papers. There is, however, a
stark contrast between classroom theory and real-world
conditions. The gauges hvac technicians use don't register in
absolute pressures; I wanted my numbers to reflect what they'd
be seeing in the field. As a teacher at a local technical
college, I use both gauge and absolute pressures in discussing
Boyle's law and how it relates to thermal expansion.
Thanks for bringing this to the readers' attention.
Acoustically Dead Ducts
Good article on sound control ("Innovations in Sound Control")
in your March issue; Bonnie Schnitta did a great job covering a
However, when it comes to ducts, she's a little off-base. The
purpose of duct wrap is thermal, not acoustical. Because the
internal metal surface of ductwork transmits noise, you should
use duct board or duct liner for acoustical purposes.
Valley Forge, Pa.
Author Bonnie Schnitta responds: You are probably confusing
the duct wrap shown in the article with standard thermal duct
wrap. The material in the photos, as stated in the text, is
actually an acoustic barrier — a dense material
designed to stop sound.
Insulation of any type may absorb sound, but it does not stop
sound. This is illustrated in the sound transmission class
(STC) chart on page 111 of the article; note that adding
fiberglass insulation to a 2x4 wall assembly increases the STC
rating by 2; that is, one can count on insulation to provide an
additional 2 to 5 dB of transmission loss.
The material shown in the article has an STC of 29; that is,
it stops roughly 29 dB. If a duct is located in the ceiling of
a media room, but runs only from the boiler to a bedroom,
wrapping it with a barrier will inhibit sound from entering it.
Wrapping the kitchen exhaust duct will also prevent kitchen
noise that enters the duct from entering the bedroom via the
wall the duct passes through.
You are correct that metal ducts and other similar conduits,
due to their reflective hard surfaces, will carry noise for
long distances. The acoustic principle is the same one that
allows whales to transmit their songs for long distances under
water. Thus it's a common problem for a duct to carry sound
from a media room or kitchen (through the exhaust duct) to a
That's why we strongly recommended in the article that the
inside of the duct be lined with an absorber. This allows the
sounds that enter the duct at a vent to be absorbed —
if the duct and absorber are long enough — before they
reach a quiet room like a bedroom or study.