[greenbuild] Re: celotex
- From: "STEVE PAISLEY" <pppaisley@xxxxxxx>
- To: <greenbuild@xxxxxxxxxxxxx>
- Date: Fri, 6 Jun 2003 10:15:22 -0400
Genevieve--
I agree with Joel that applying wood siding as a vented rainscreen is a good
idea, particularly if the roof overhang is small (1 foot or less) or the wall
is tall. My experience has been that in our area 1 foot of roof overhang will
protect 3 to 4 vertical feet of wall from all but the most wind-blown rain (2
feet of overhang would protect 6 to 8 feet of wall, etc..). If you are going
to paint your siding, check out the Swedish "Falun Red" paint at
www.falurodfarg.com. I haven't tried it, but it sounds like an earth
pigment+starch+linseed oil paint that breathes much better than oil or latex
paints.
As far as the vapor barrier/condensation issue goes, older houses (perhaps
like Joel's?) rarely have problems with condensation on exterior wall
sheathing. This is usually because the house and its siding have enough air
leaks that moist air from in the house is constantly escaping and being
replaced by drier outdoor air. This means that winter interior humidity levels
will be very low. This is great for the sheathing but tends to cause dried-out
nasal passages as well as high heating fuel consumption.
One more thought--the only function of the OSB sheathing on a wood-sided
house is to prevent racking (i.e., keeping your rectangular walls from turning
into trapezoids when the wind blows.) You could probably omit the OSB
altogether if you used a couple of diagonal metal straps (made just for this
purpose and rated for racking resistance) on each wall. You could then apply
Tyvek right to the studs, followed by diagonally applied 1x strapping to
provide even more racking resistance as well as an air space behind the siding.
Eliminating the OSB will also allow you to have this air space without the
wall becoming so thick that it extends beyond your window and door trim.
--Steve Paisley
----- Original Message -----
From: joel gagnon
Sent: Thursday, June 05, 2003 1:29 PM
To: greenbuild@xxxxxxxxxxxxx
Subject: [greenbuild] Re: celotex
Jon's comments have been excellent (which is to say I agree with him). While
I agree that the vapor barrier is safest on the inside, I would note that
oriented strand board and plywood are themselves excellent vapor barriers.
Unless indoor humidity is managed and air movement into the walls prevented,
the potential for condensation in the walls is great. The only place I have
seen problems in walls was where air movement brought moisture into the
cavity.
There is one other good reason not to use celotex on the outside, however. I
have read of problems putting wood siding over foam board unless an air
space is created behind the siding (a "rainscreen"). Otherwise, if the wood
gets wet due to wind-driven rain, ice dams, or condensation from moisture in
the house getting behind the siding at the joints in the foam boards, it has
no way to get out. It will, at minimum, drive the paint off the siding. At
worse, it can rot the siding. So either use a rainscreen (spacers or one of
the new matt-like products) or put the foam on the inside. Incidentally,
felt would probably be better under the siding than Tyvec. It has a history
of performing adequately in this application. The most robust installation
of your siding would put the rainscreen under the siding and backprime the
siding to increase its moisture resistance.
In an interesting historical note, my house has the siding applied directly
to the studs, with no sheathing at all. It holds paint really well, even
after filling the stud cavities with insulation. It also holds paint well if
Tyvec is placed under the siding, as I did on one section, and if a
permeable sheathing is placed under the siding. I attribute that to the
triangular air spaces behind the lap siding. Perhaps the rainscreen can be
omitted if lap siding is used instead of bevel siding. If you can't get wood
lap siding, I would consider fiber cement lap siding. I details just like
wood and holds paint well.
Joel Gagnon
Joel Gagnon
From: "Jon Harrod" <insul8er@xxxxxxxxx>
Reply-To: greenbuild@xxxxxxxxxxxxx
To: <greenbuild@xxxxxxxxxxxxx>
Subject: [greenbuild] Re: celotex
Date: Mon, 2 Jun 2003 15:02:49 -0400
Hi Genevieve,
A couple thoughts on your inquiry--
The first is that, as far as I know, polyisocyanurate ("Celotex") and
extruded polystyrene ("Styrofoam" or "blueboard") are still being
manufactured with HFCs, which damage the ozone layer and contribute to
global warming. Which is not to say that these products should never be
used, but just that they should be used judiciously, where they can provide
benefits that outweigh their negative impacts. There are a couple questions
I would ask in making this assessment.
--How much insulation do you have in the wall cavities?
--What type?
--How much potential is there to increase the cavity insulation with less
damaging products (i.e. cellulose)?
--How leaky is your house? Would air leakage around the foam and into the
wall cavities reduce the effectiveness of the foam insulation?
With answers to these questions, an energy auditor can work with you to look
at the energy costs and benefits of adding the foam versus other options.
The second issue relates to potential moisture problems. Celotex is
completely impermeable to air and moisture (i.e. it is a great air/vapor
barrier). In cold climates, you want the air/vapor barrier on the interior
side of the wall cavity. By placing an impermeable barrier to the outside,
you create the potential for condensation, which would lead to mold on the
osb. Whether this condensation actually occurs depends on a number of
factors including the permeability and leakiness of the interior wall
covering, the type of insulation, the thickness of the foam board, and the
moisture level in the house.
On balance, my recommendation would be to steer away from exterior foam
unless it is part of a comprehensive plan to improve the building envelope
in a way that prevents future problems.
Good luck,
Jon Harrod
Performance Systems Contracting
-----Original Message-----
From: greenbuild-bounce@xxxxxxxxxxxxx
[mailto:greenbuild-bounce@xxxxxxxxxxxxx]On Behalf Of Genevieve DeClerck
Sent: Monday, June 02, 2003 1:53 PM
To: greenbuild@xxxxxxxxxxxxx
Subject: [greenbuild] celotex
In my readings of ways to improve energy efficiency on a house I came across
a few mentions of using
polyisocyanurate foam core panels (e.g. Celotex) on the exterior of house
under the siding... the
argument being that having a thermal barrier like this reduces thermal
bridging through the studs.
I'm considering using this material on my old house which will be fully
re-sided this summer. The
proposed idea is to sheath the house as normal with OSB, then cover that
with Tyvek, then fasten
celotex panels to the studs, then finally apply pine clapboards to the studs
(if we can find them!).
Does anyone have any experience with this? Any info, opinions, advice would
be greatly appreciated.
Thanks,
Genevieve
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