I recently purchased 100-year old Dutch Colonial with a gambrel roof
so that the outside walls on the second floor are sloped. There is no
insulation in the house and no soffit vents. It has a ridge vent and
two gable vents with fans both blowing out. (These are used on hot
nights, along with opening the pull-down stair case to vent the whole
house. The house is in Massachusetts.)
I plan to use fiberglass rolls or mat in the attic but am concerned
about the walls which include the lower portion of the gambrel roof.
Blowing in fiberglass or cellulose will assure that there is no
ventilation in this lower portion of the roof - will that create
problems? There may be some ventilation now through the walls but that
will be blocked by the blown-in.
The short version of this question: is it okay to use blown in
insulation on the lower portion of a gambrel roof?
(I haven't decided on using cellulose or fiberglass for this
application - that is a question for another day )
Proper venting of an attic (or basement) is important to prevent the buildup of
moisture. In addition, excessive heat in a poorly vented attic space tends to
diminish the life of the the roofing material, as well as introduce unwanted
heat in the living space below.
The sharply sloped portion of a gambrel roof is really more of a wall, that
just happens to be covered by roofing shingles. As long as that wall cavity is
completely filled with insulation, there will be no voids to trap or collect
moisture, and thusly, no need to vent it.
That only leaves the top, slightly sloped portion of your roof, creating an
attic void. As long as that area is adequetly vented, I don't see a problem.
Plastic foam chutes are sold for about 1$ you staple them to the roof
where the fiberglass will come up to , they are ridged to allow air
flow and have a good airspace, at box stores. I have the same type roof
and studded the interior and ran 4" R7.2 per inch foamboard then paneled
it in wood. That area is impossible to insulate outside-in the attic to
proper levels. Remember Code recommended insulation amounts are
minimums . Exceeding them by 50- 100 % is beneficial . Since allot of
the cost is labor do it right the first time.
Water entrapment is partially a function of the vapor permeability of the
walls. Because if vapor makes it past the wall, it will indeed be trapped
within the wall cavity, since shingles on the outside will prevent it from
going any further.
The way that insulation works is by creating voids of airspace, without any
path of airflow. The poster is wrong, IMO, in saying that there would be no
voids. The insulation has got to be able to release its trapped water, or be
I suggest asking the folks at Owens-Corning about this.
On 04 Jul 2004 14:22:13 GMT, email@example.com (HA HA Budys Here)
wrote (with possible editing):
That is correct and well said imo.
That is only partially correct. Vapor can permeate a wall as Mr Baugh
so properly stated. You can prevent moisture from entering from the
warm side in winter by using a vapor barrier (4 mil or thicker poly
film works ok), but it's very hard to protect the outlets effectively.
If the op's roofing shingles are dark, the wall will absorb heat and
should be vented if for no other reason than to convect heat upwards.
I would want venting for the inevitable moisture which gets trapped as
FWIW, I'd use fiberglass bats 1/2" less thick than the walls and
provide soffit ventilation. You can do this by simply drilling holes
up from the soffit into the wall cavities above and plugging them
screened vent plugs. You would also need to drill holes through the
bottom plates of the walls and ensure that there was a clear channel
for the plugs below; i.e., not covered by insulation.
Then, as others have suggested, I'd do the rafters with the fiberglass
using proper vent for ventilation. Keeping ventilation behind roofing
will extend the life of your shingles.
Also, I've never seen blown-in insulation that didn't settle. We only
use it on horizontal surfaces. In my home we use it in the attic over
12" of fiberglass.
I'd love to be able to use bats in the lower (wall) portion of the gambrel roof but
I have no way to get them in short of gutting all the plaster and lath which I'm not
ready to do. The plaster in this old house is in very good shape. I am considering
blown-in because it's the only way I know to add insulation to existing walls. (Is
it common to gut perfectly good walls just to add insulation?) I could drill holes
in the soffit to add vents but they would just be blocked by the insulation if I
blow it in. I would need a clear channel about 10 feet long from the soffit at the
bottom of the lower roof portion to the knee where the roof slope changes (which is
where the attic joists are attached.) I don't know of any way to get the clear
channel since I have no access.
Is ventilation so important that if I can not ventilate properly, I should not
consider adding the blown-in and just leave the walls uninsulated? Or should I just
go with the blown-in and paint my walls with vaper barrier paint and hope for the
best? Either way, I'll add plenty of mat between and on top of the joists in the
It may not be for very long IF you blow insulation in... read on...
It is getting more and more common, especially in this tight real-estate
market, to gut an entire older home and retrofit it with all the modern
Sometimes I think there's something very wrong seeing an old home with restored
raised panel mahogony wall panels, with an original gas antique gas lamp
(working) in it, and below, near the baseboard, a central vac inlet, and just
to the left of that, an ethernet port, and at wall switch height, a central
audio system LCD screen. But hey, I could get used to it...
I'm fresh out of ideas too.
The problem that will crop up if you go ahead with blown-in is that the life of
the roof will be diminished by excessive heat. It's the problem you MIGHT end
up with that's bone-chilling... mold.
I'm unfamiliar with the vapor barrier paint.
If it's effective, that may be a good solution.
On Mon, 05 Jul 2004 17:06:27 -0400, Scott Duncan
Well, there are actually two problems here with adding insulation -
the raised temperature of the roof shingles and trapping moisture
which will condense to form water.
There is another alternative, which will take care of the moisture
problem, but I don't believe it will help with the first, and I'm on
shaky ground here because I don't know that much about it:
Instead of blown in, there are supposed to be some new types of
urea-formaldehyde foam which do not leach formaldehyde. UF does not
expand, in fact it shrinks about 1% so there is no danger of popping
the walls and cracking the plaster. However, it is open cell so it
MIGHT be vapor permeable to some extent. How permeable? I don't
know, nor do I know how good it is against outgassing formaldehyde,
but if I were in your situation, I think I'd do a bit of research. It
is possible that a combination of vapor barrier paint and UF might
work the best.
FWIW, I'd have trouble removing lath and plaster too. It's beautiful,
but it's a real craftsman's trade. There are folks who do it in MA,
but it's likely to cost quite a bit.
I agree, however...
I happen to live in an area where gambrel roofs are very common.
Insulation-wise, in newer (well, newer than the plaster-and-lathe days the OP's
home was built) homes these roofs were treated exctly like walls. And,
incidentally, most end up with black roofing shingles.
The insulation of course included a foil vapor barrier, which the OP cannot at
this point install. But you are correct, he will need one if he's to introduce
insulation into a roof/wall cavity where none existed before.
firstname.lastname@example.org (Scott Duncan) wrote in message
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