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Question DetailsAsked on 11/23/2017

roof is installed over 1/2 inch celotex, installed over 2x6 tongue and groove deck,which is my ceiling.how do ivent

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You might try again with more detail because your question is pretty short - the first text box is for a short subject of the question, then there is a second text box which will allow for a fairly lengthy explanation of your issue.


If your question is how do you vent the attic - you don't if there is no airspace between the roof sheathing and the interior plaster or drywall - like in your case where the 2x6 T&G serves as interior finish (presumably on a cathedral or lodge type ceiling) and as the roof sheathing as well.


If you are getting condensation / mold in the high areas of the ceilings, then about all you can do short of replacing the roof with a new roof with adequate framing to allow for ventilation and insulation, is to improve ventilation on the underside of the T&G - commonly by use of circulating fans suspended from the ceiling (which in your case means having exposed conduit or having to conceal it to run wiring to it). Commonly, reversible large-diameter slow-speed fans set close tothe ceiling are used for this - running in the winter in "downcast" direction, forcing the air down at the fan so replacement air moves in along the ceiling to absorb the moisture - also in the process warming the underside of the T&G to reduce the condensation and frosting. And opposite direction in the summer to dissipate some cooler air into the peak area which then diffuses along the ceiling back to the rest of the room, preventing roof overheating.


Not energy efficient because you are increasing the convection (air movement) portion of the heat loss through the T&G, but can be necessary - especially on poorly insulated roofs like yours, where 1/2" of celotex (which should not be used as a roofing insulation anyway because it readily absorbs water, if you mean the cellulose board rather than Celotex brand polyiso foam board which gives you about R3.5 versus about R1.3 for cellulose board) is not much better than nothing as roof insulation goes, where numbers from about R24-R30 in mild climates and R48-R60 in cold or hot areas is more normal.


Come time to reroof, if this is a "forever" home for you, I would talk to an Architect about conceptual plans and cost estimate for a properly ventilated and insulated roof - commonly using (shudder) plywood joists like are used under most floors these days to provide airspace for insulation and overlying ventilation airflow. There are also now (though still pretty pricey) ventilated contour closed cell foam insulation board solutions with ventilation channels to remove moisture and maintain airflow under the roof. If something like that is done, removal of the celotex and putting a vapor barrier under the insulation would be normal - provided there is no vapor barrier already on top of the planks.


Another option - assuming you now have exposed beams/rafters in cathedral ceilings, is to use that between-framing area to provide outside airflow from eaves to ridge venting and then insulation under that ventilation area, sometimes "deepening" the framing to give adequate depth for both, then vapor barrier under framing and drywall under that as a new ceiling - changing to a peaked drywalled interior finish from exposed wood, though it is possible to use a T&G product for that finish if desired. Provided there is not a vapor barrier under the celotex this could put of reroofing till it needs it normally, though the celotex will be a problem (if cellulose type, not closed cell foam) if it ever gets wet from a roofing or roof penetration leak.


You can find some other similar questions with more in-depth discussion of the roof ventilation and insulation issues, in the Home > Insulation link,, under Browse Projects, at lower left.

Answered 11 months ago by LCD




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