[greenbuild] Re: insulating old houses
- From: "joel gagnon" <joelpgagnon@xxxxxxxxxxx>
- To: greenbuild@xxxxxxxxxxxxx
- Date: Mon, 25 Feb 2002 13:30:51 -0500
I have a couple of points to add to the good points made by others.
First a bit about heat loss and insulation. If you graph heat loss (y axis)
versus resistance to heat loss or R value (x axis), you get a curve that
starts at infinity at 0 resistance, corresponding to no roof or open sky on
a clear night, dropping rapidly at first as a little bit of resistance cuts
heat loss dramatically. The curve flattens out and approaches 0 at very high
resistance. Picturing this helps one to understand that most of the gain
from insulation is had in the first few R's. Above R 25 or so, the return on
investment is much lower. In new construction it makes sense to create the
space in which to put the insulation so as to get the R's in, and it is
cost-effective to do so. In an existing building, it can make sense to
create the space if there is the need for extensive work anyway, or if the
insulation can be added inexpensively somehow.
With historic structures, historic preservation enters into the equation
(or should, in my opinion). I cringe whenever I hear of window replacement
undertaken for energy savings. Unless the windows are in very poor shape
this will never pay back in energy savings and nothing alters the character
of a building more than a change in fenestration. Better to tighten up
existing windows and add storms if they don't already exist. I have made up
storms with low-e glass to help get the benefit of new technology. Local
glass shops can order it for you.
Back to the roof. As has been mentioned by others, there are two schools of
thought on ventilation. One would have it that it is necessary to prevent
condensation and rot in the structure of the roof. The other has it that
ventilation is not needed if the structure of the roof is tight. I am with
the second school (the "hot roof" approach). Moreover, I have experimented
with my own house and demonstrated to my own satisfaction that tightness is
not required per se. I have a roof insulated with unfaced fiberglass 6
inches thick with no problems in the 15 years that it has been like that.
Moisture that diffuses in diffuses out as well. The only time you get
condensation with enough water to cause rot is if you have high indoor
humidity (high enough to get condensation on double-glazed windows) and air
movement that brings that moist air into contact with cold surfaces. That
can happen with or without insulation. The only rot encountered in this
house (built in 1870) was in the bathroom wall, which was uninsulated, and
in the sill beams at the corner of the foundation, a common spot for
summertime condensation due to the mass of the stone walls and their
connection to the cold ground. There is no problem blowing insulation into
rafter cavities or filling them with fiberglass batts provided there is not
a pathway for moist air to move from the house into the cavities and then
through them so moisture is deposited when it hits the dew point. Diffusion
will not be a problem, but air movement will. Adding ventilation to a roof
can actually exacerbate rather than solve moisture problems and compound
heat loss besides.
This last point deserves elaboration. In an old house, heat loss from air
movement (some would call it ventilation) can greatly exceed loss from
thermal conductance. It is generally much more cost-effective to tighten up
a house as the first attack before addressing the need for insulation. I
strongly endorse Jon Harrod's suggestion to check with a blower door,
although a careful examination of the house would probably catch most of the
problem areas without the benefit of this technology.
Once the major holes have been plugged, I would consider the construction of
the roof. Is there head space to lower the ceiling by adding foam boards on
the inside of the rafters? Do you have a good plaster ceiling or a mess that
needs to be taken out anyway? How deep are the rafters? The considerations
have been well covered by others. One thing to keep in mind is that the R
value of soft wood is only about 1.25/inch. A full-cut 2 X 4 is thus only
about R 5. If you look at insulated roofs on frosty mornings, you will often
see the "ghosts" of the rafters where the frost is absent due to the higher
heat loss through the rafters. Adding foam boards that cover the rafters
themselves addresses this heat loss in a way that is hard to achieve
otherwise. The foam can be either over or under the rafters, depending on
what works. It could even be both, although some people would cringe at that
suggestion since it puts a vapor barrier on both sides of the roof assembly.
You already have a vapor barrier on the roof side if you have asphalt
shingles. Adding foam will not make it worse. Putting it on the inside helps
reduce moisture migration into the assembly, but this does not matter
either, as I explained above.You can combine kinds of insulation in whatever
ways will work for you esthetically and practically.
Joel Gagnon
From: G DeClerck <gad14@xxxxxxxxxxx>
Reply-To: greenbuild@xxxxxxxxxxxxx
To: "greenbuild@xxxxxxxxxxxxx" <greenbuild@xxxxxxxxxxxxx>
Subject: [greenbuild] insulating old house
Date: Sat, 23 Feb 2002 12:48:19 -0500
Hi,
Love this list. I've been a subscriber for a couple of months now and am
impressed with all the great information being shared. I'm hoping you
can help me think about a problem i have...
I'm considering buying an old 1850's farm house in the ithaca area. The
house's insulation is described as being 'unknown'-- typical for older
homes I suppose. I would want to overhaul the house and make it as tight
as possible with new insulation (and drywall) throughout, high
performance windows and a high R-value roof (40+). Making a high R-value
roof may be a problem though. The second floor of the house is basically
the attic... it's a short-ish greek revival. There is no attic where
loose insualtion can be blown in-- the roof insualtion would have to be
under the sub roof itself. If I bought the place the roof would need a
complete overhaul right away (it's sagging and leaking at the moment).
Does anyone have any experience with this type of thing? Any
recommendations on a roof rebuild or thoughts on insulating old houses?
Any ideas would be much appreciated.
Thanks,
Genevieve
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