Chris wrote:
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?

Thank you,
Chris


With a cathedral ceiling you get one chance to do it right.
Put in the max. Stuff in 6" of fiberglass then add as much
foam below as you can afford, then add a bit more.
I'm in Manitoba, Hydro is recommending R50 now.
I put in R60 two years ago. Yes R-Six-Zero.

http://www.mts.net/~lmlod/Cabinfront6.jpg

Temps. around here drop below -40 Deg F. in the winter.
With a good fire going in the wood stove the inside temp.
near the ceiling peak is probably 80 or 90 deg. F. even with
a fan going. A lot of heat to keep snow off the roof.

As for economy and the 95% - 96% boys, where were they years
ago when R12 was enough, then R20, R40.

The cost of heat is only going to go up. The bean counters will
whine today because "You won't get the return for your money"
then tomorrow they will cry "It costs so much to heat, put in
more insulation". In reality that bunch doesn't know their ass
from a hole in the ground.

If you are building to sell and make a quick buck then go bean
counter minimum, otherwise think 20 years down the road. If not
for yourself then for your kids if you want them to be able
to afford the place.

Lots of ventilation. I put in metal ridge vent right across all
three sections.

One added bonus of the metal ridge vent is that birds don't
like to land on the things. they stay away from my roof and crap
all over my neighbors roof (ridge vent covered with a shingles)
Seagulls, crows etc. the stuff covers half the top of his roof
and hardly a drop on mine .

By the way I built that place, I hired out the foundation,
shingles and HVAC, the rest I did with help from family and
friends, not a fricken bean counter among them.
and as they say "Put my money where my mouth is".

LdB