I have to add fiber glass insulation on my cathedral roof, for a small
cottage 16' x 24', new construction, in Canada.
The roof rafters are 2" x 8". For that size, I should use R20
(6") in order to keep 2" for ventilation on the top of insulation.
I though I could to be able to use add another 2" x 2" lumber in
order to extend the rafters wide to 2 x 10 (instead of 2 x 8) and to
use R28 (8") insulation instead of R20 (6"). Is that a big
difference for R-value? The cottage is used to spend weekends and some
vacation days and not (yet!) for permanent living.
What would you recommend to me?
And another question: In order to keep the 2" for ventilation I
though to use raft-R-mate. Is it wrong to put this raft-R-mate the
entire long of rafters or only at the beginning should be enough?
I would extend the rafters down so you can use the thicker insulation.
I'd also use the air channel baffle material the entire length, as that
is the best insurance that you will have a clear space for air movement.
Because that's how cathedral ceilings are built base on years of
construction experience. You provide a space between the insulation
and the sheathing so that air can move from soffit to peak, where it
exists via a ridge vent or other venting mechanism. I doubt you;d get
a building inspector to pass off on an unventilated cathederal ceiling.
Besides moisture, what do you think having no air flow under the
sheathing is going to do to the life of the roof when it gets to 150 in
where I'm from, Duluth, MN, the airspace is required by code. I think
it is important to use a chute the entire length, not only because it
ensures airflow, but it also reduces "windwash" which strips heat away
from your insulation. I like to use cardboard vent chutes (available
at good lumberyards). I also think adding a layer of foam to the
underside is a good idea. even a layer of 1/2" polyisocyanurate
(thermax is a good brand) would reduce thermal bridging and provide an
no, chutes don't encourage windwash because the chutes keep the cold
air from the insulation. it's easy to say "take control of your own
life" but when a building inspector is standing there telling you you
must have vent space period, what are you going to do? we have had
trouble getting hot roofs with 12" of icynene by our building
department, let alone fiberglass.
ironically, i basically agree with you. if i was building my own house
and i didn't have to deal with inspectors, i wouldn't vent the roof
either...but i would also be very anal retentive about my vapor
barrier. in practice, cathedral ceilings do fail (condensation raining
in the house), and unless someone is there who really knows what they
are doing, the safe bet is to ventilate.
Get an architect or PE to sign an "unvented" drawing?
Sure. And there are degrees of venting. With near-perfect vapor barriers on
both sides, a small vapor barrier or roof leak could make a lake. Perhaps
the roof should be slightly less airtight than the ceiling, with a ridge
vent but no wind-whistling chutes.
If I were building a new house, I might put insulation in the attic floor
and make the steep south roof clear Dynaglas polycarbonate plastic and make
the underside of the north roof white and collect warm air and light in
the attic in wintertime, with a few simple skylights in the attic floor,
with no venting in wintertime.
Research your code and recommended energy code and guidlines. R 20-R28
are substandard and inneficient. Your ceiling is where most your heat
loss is. R 35 in Zone 5 is considered not optimal, R 60 is. I dont know
your Zone but going to R 100 is getting more common in cold areas. You
need an airspace but I dought you need 2", 1" should do, Foam chutes are
cheap and easy to install. Furring out 2" is a good idea. For the best
performance spray on foam, there are types with R7 per " , twice that of
fiberglass. Also Fiberglass looses efectivness at extreme cold, maybe
near 50-60% at below zero temps. A vapor barrier is needed on
fiberglass, not foam, and inside next to the sheeting. 6.5" of closed
cell blown in foam is R 45, it may impress you but its still not an
optimal R value. Research, www.Energystar.com will help. Insulation
manufacturers Dow, Owens Corning etc will recommend maybe R60. I will
bet your Govs green sites might be even higher, R 70++, I dought your
code is under R35 unless you are on a coast with completly moderate
temps, but in canada? What you do now will be it, so if you can do it
go for the maximun as it will have a big payback and increase your
I don't wish to take sides in this argument and I certainly have no
practical expertise in this area, but the following article discusses
both "cold" and "hot" (i.e., vented and unvented) roof design in
colder climates; more specifically, Alaska.
As to whether or not one should follow code, that's a personal call
but it would seem somewhat foolhardy to do otherwise. If this should
become an issue with the inspector, who do you think will have the
On 11 Dec 2006 14:20:34 -0800, email@example.com wrote:
Fwiw ... R20 conductance is .050 ... 95% efficient, and R28 conductance
is .036 ... 96.4% efficient or not much of a difference. And you didn't
say but just in case, a ridge vent is the perfect partner for that
raft-R-mate (although I would think if the fiberglass is installed
carefully the raft-R-mate should not be required). And of course a
completely sealed vapor barrier.
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