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Posted: Mar 25, 2009 06:53 PM	unvented porches So I sprayed a house top to bottom with a kneewall separating the conditioned space from the porch roof. I didn't plan on venting the porch roof because it isn't above any conditioned space and the sprayfoam is an air barrier from the house. Now the inspector is telling me it needs to be vented to 1/150 or 1/300. He says if you don't vent it, then you can sprayfoam the underside of the porch roof. That solution seems a bit odd to me. What's the point of just spraying the underside of the roof??
travis fails Posted: Mar 25, 2009 07:17 PM	It seems to me if you spray the roof deck you also must spray the porch ceiling fot the foam to do its job.
Gerry Wagoner Posted: Mar 25, 2009 08:35 PM	It sounds like he (inspector) is trying to eliminate a condensate surface (the underside of the roof). That is sensible in areas over a conditioned space, but meaningless in an exterior cavity. The presence of ventilation will introduce more moisture into the porch attic then it would normally get. The inspector seems to be treating the porch attic like an attic over a living space. Apples to oranges. best of luck, og
Posted: Mar 26, 2009 07:11 AM	Could it be that he is trying to prevent the heat buildup in the roof from contacting the underside of the porch roof? I don't know, somehow trying to extend the life of the shingles above.
Michael Fusco Posted: May 24, 2009 09:33 AM	I think gooch is on to something....some years ago I went to a job site where a retro attic was having moisture issues. After a few days of brainstoming and testing...here's what we found. The applicator had blocked the eve from the inside leaving an unvented eve (maybe 18" under the hang). This was on the east side of the house in a relatively cool climate. The sun went down, moistures (dew) formed in ground below the wall. Sun came up...heated the air...grabbed moisture...hot air rises....roof heated up (black shingle on roof) eve acted like a mini attic. When the sun went down, temp fell below dew point, air in eve gave up moisture, after a cycle or two, we had stains under the eve. Solution...filled eve with foam. Not quite as easy in this situation, but you have to look at the proch assembly on the merits of what is there. The code official might not have had the best solution, but in a round about way, you're on the right track....seal it against infilatration of air and moisture or vent it.

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Comments

Posted: Mar 25, 2009 06:53 PM

unvented porches

So I sprayed a house top to bottom with a kneewall separating the conditioned space from the porch roof.
I didn't plan on venting the porch roof because it isn't above any conditioned space and the sprayfoam is an air barrier from the house.
Now the inspector is telling me it needs to be vented to 1/150 or 1/300. He says if you don't vent it, then you can sprayfoam the underside of the porch roof.
That solution seems a bit odd to me. What's the point of just spraying the underside of the roof??

travis fails

Posted: Mar 25, 2009 07:17 PM

It seems to me if you spray the roof deck you also must spray the porch ceiling fot the foam to do its job.

Gerry Wagoner

Posted: Mar 25, 2009 08:35 PM

It sounds like he (inspector) is trying to eliminate a condensate surface (the underside of the roof). That is sensible in areas over a conditioned space, but meaningless in an exterior cavity.

The presence of ventilation will introduce more moisture into the porch attic then it would normally get. The inspector seems to be treating the porch attic like an attic over a living space. Apples to oranges.

best of luck,

og

Posted: Mar 26, 2009 07:11 AM

Could it be that he is trying to prevent the heat buildup in the roof from contacting the underside of the porch roof? I don't know, somehow trying to extend the life of the shingles above.

Michael Fusco

Posted: May 24, 2009 09:33 AM

I think gooch is on to something....some years ago I went to a job site where a retro attic was having moisture issues. After a few days of brainstoming and testing...here's what we found.
The applicator had blocked the eve from the inside leaving an unvented eve (maybe 18" under the hang). This was on the east side of the house in a relatively cool climate. The sun went down, moistures (dew) formed in ground below the wall. Sun came up...heated the air...grabbed moisture...hot air rises....roof heated up (black shingle on roof) eve acted like a mini attic. When the sun went down, temp fell below dew point, air in eve gave up moisture, after a cycle or two, we had stains under the eve.

Solution...filled eve with foam. Not quite as easy in this situation, but you have to look at the proch assembly on the merits of what is there. The code official might not have had the best solution, but in a round about way, you're on the right track....seal it against infilatration of air and moisture or vent it.

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