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Extending R-value for cathedral ceiling Chris 12-11-2006
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Posted by on December 11, 2006, 1:27 pm



>>>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


Posted by DK on December 11, 2006, 2:38 pm


On 11 Dec 2006 13:27:57 -0500, nicksanspam@ece.villanova.edu 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.




Posted by on December 11, 2006, 3:09 pm



>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


Posted by Goedjn on December 11, 2006, 4:55 pm


On 11 Dec 2006 15:09:45 -0500, nicksanspam@ece.villanova.edu 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.

Posted by on December 11, 2006, 5:25 pm



>>>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


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