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I have a gable roof with a gable vent in front, but there is now a cathedral addition

In south Mississippi, I have a gable roof with a gable vent in front, but there is now a cathedral addition in the rear, so there is no longer a rear gable vent. No ridge vent, no soffitt vents. It is an unconditioned attic with HVAC ductwork. The air handler is in a closet below the ceiling. The fresh air intake is located in the ceiling of the front porch which is a cantilever.

I have two registers going from the original rear gable wall into the new addition which is split into two equal rooms, each 14'x16'.

How do I create a satisfactory envelope?

Do I do a floor, wall, ceiling in the original structure, and leave the HVAC ducts unconditioned, or can I use open cell, or 1" polyiso sheets on the undersides of the rafters for a roof envelope?

Asked by roger steinbrink
Posted Jun 19, 2014 8:56 AM ET
Edited Jun 19, 2014 9:37 AM ET

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Roger,
If you have attic ductwork, the best approach is to move the insulation to the sloped roof assembly, so that you convert your vented unconditioned attic to an unvented conditioned attic.

This article should answer your questions: Creating a Conditioned Attic.

For more information on why it is important to address this issue, you might want to read Keeping Ducts Indoors.

Answered by Martin Holladay, GBA Advisor
Posted Jun 19, 2014 9:40 AM ET
Edited Jun 19, 2014 9:42 AM ET.

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