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

On 11 Dec 2006 08:11:12 -0800, "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

You can add rigid foam insulation crosswise over the joists,
as long as you cover it with sheetrock afterwards.
That's why sheetrock screws come in a 3" (75mm) version.

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.

Why not put up a good plastic film vapor barrier under 8" of insulation?

Nick

wrote:
Chris wrote:

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.

Why not put up a good plastic film vapor barrier under 8" of insulation?

You're supposed to leave space for airflow between the insulation and
the underside of the roof.

Chris

Chris Friesen wrote:
wrote:
Chris wrote:

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.

Why not put up a good plastic film vapor barrier under 8" of insulation?

You're supposed to leave space for airflow between the insulation and
the underside of the roof.

Chris

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.

Chris Friesen wrote:

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.

Why not put up a good plastic film vapor barrier under 8" of insulation?

You're supposed to leave space for airflow between the insulation and
the underside of the roof.

Maybe not, if there's no chance of condensation from moist house air
leaking up under the roof.

Nick

On 11 Dec 2006 13:27:57 -0500, wrote:

Chris Friesen wrote:

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.

Why not put up a good plastic film vapor barrier under 8" of insulation?

You're supposed to leave space for airflow between the insulation and
the underside of the roof.

Maybe not, if there's no chance of condensation from moist house air
leaking up under the roof.

Nick

Here in the southern US, not leaving a place for ventilation will cost
you a roof job about 10 years before it should.

DK wrote:

Here in the southern US, not leaving a place for ventilation will cost
you a roof job about 10 years before it should.

Why would you say that? Seems unlikely to me, if there's no chance of
condensation from moist house air leaking up under the roof.

Nick

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

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.

bowgus wrote:

... R20 conductance is .050 ... 95% efficient, and R28 conductance
is .036 ... 96.4% efficient or not much of a difference.

But a 2000 ft^2 R20 roof in a 10,000 DD climate will leak
24 million Btu/year, vs 17 million at R28.

Nick

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

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.

Well that's bizarre ... a cross posting to the same group :-)

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

L d'Bonnie wrote:

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.

How strange. Why would the ceiling be so hot with a ceiling fan going?
Is the stove red hot, with no insulation in the house walls? :-)

You want lots of R1/inch snow on the roof, no?

How can it melt with that 2" air gap?

Nick

wrote:
L d'Bonnie wrote:

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.

How strange. Why would the ceiling be so hot with a ceiling fan going?
Is the stove red hot, with no insulation in the house walls? :-)

You want lots of R1/inch snow on the roof, no?

How can it melt with that 2" air gap?

Nick

Nick, you seem to have no experience heating a house in a cold climate.
of course the heat rushes to the peak of his vault! ever heated a
house with a wood space heater? an air space alone won't stop snowmelt
either. that one has been proven time and again.

marson wrote:
wrote:
L d'Bonnie wrote:

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.

How strange. Why would the ceiling be so hot with a ceiling fan going?
Is the stove red hot, with no insulation in the house walls? :-)

You want lots of R1/inch snow on the roof, no?

How can it melt with that 2" air gap?

Nick

Nick, you seem to have no experience heating a house in a cold climate.
of course the heat rushes to the peak of his vault! ever heated a
house with a wood space heater? an air space alone won't stop snowmelt
either. that one has been proven time and again.


Don't bother Nick. He's busy trying to solve some partial differential
equations to calculate how hot it theoretically should get at the top
of a vaulted ceiling, instead of actually having one and knowing it
from experience..

marson wrote:

L d'Bonnie wrote:

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.

How strange. Why would the ceiling be so hot with a ceiling fan going?
Is the stove red hot, with no insulation in the house walls? :-)

You want lots of R1/inch snow on the roof, no?

How can it melt with that 2" air gap?

Nick

Nick, you seem to have no experience heating a house in a cold climate.

Wrong.

... of course the heat rushes to the peak of his vault! ever heated a
house with a wood space heater?

Sure. Ever studied basic physics?

Nick

wrote:
marson wrote:

L d'Bonnie wrote:

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.

How strange. Why would the ceiling be so hot with a ceiling fan going?
Is the stove red hot, with no insulation in the house walls? :-)

You want lots of R1/inch snow on the roof, no?

How can it melt with that 2" air gap?

Nick

Nick, you seem to have no experience heating a house in a cold climate.

Wrong.

... of course the heat rushes to the peak of his vault! ever heated a
house with a wood space heater?

Sure. Ever studied basic physics?

Nick

as a matter of fact i have...but what's your point? you were arguing
that it shouldn't get hot up there by your calculations.