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

On 11 Dec 2006 15:09:45 -0500, wrote:

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

You don't stop water-vapor on the far side of the insulation,
you stop it on the near side.

wrote:
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


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
the summer?

Goedjn 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.

You don't stop water-vapor on the far side of the insulation,
you stop it on the near side.

You stop it on the warm side. Plastic film under insulation would do that.

Nick

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.

Because that's how cathedral ceilings are built base on years of
construction experience...

You might say the same about shark repellent in Kansas :-)

Nick

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
air/moisture barrier.

wrote:
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.

Because that's how cathedral ceilings are built base on years of
construction experience...

You might say the same about shark repellent in Kansas :-)

Nick

And you might say that you're an idiot, giving out advice based on
equations, without knowing anything about the real world, construction,
or how roofs are properly ventilated.

marson wrote:

where I'm from, Duluth, MN, the airspace is required by code.

IIRC, ERVs are also code, and a big waste of money.

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.

"Chutes" encourage "windwash," no? I suggest that you
ignore the code and take control of your own life.

Nick

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.

See
http://www.north-rthn.org/french/f-f...fs-french.html

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
final say?

Cheers,
Paul

On 11 Dec 2006 14:20:34 -0800, wrote:

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
the summer?

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.

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 :-)

Nick Pine wrote:
marson wrote:

where I'm from, Duluth, MN, the airspace is required by code.

IIRC, ERVs are also code, and a big waste of money.

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.

"Chutes" encourage "windwash," no? I suggest that you
ignore the code and take control of your own life.

Nick

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.

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

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

marson wrote:

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?

Get an architect or PE to sign an "unvented" drawing?

... 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.

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.

Nick

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

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
houses value.

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.