Unvented roof with only exterior insulation
WillLyle
| Posted in Green Building Techniques on
I am building a new home with all cathedral ceilings and will have a well vented attic space to mitigate moisture and circulate air. I would like 100 % of the insulation to be on the exterior of the roof. On this website I only see where the insulation is on top of the rood and also between the rafters. Is there a condensation issue if its 100 percent above the roof?
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Replies
In general terms, yes, you can have 100% of the insulation on top of the roof.
Your comment is a bit unclear because when we talk of a "ventilated attic" it is in combination with insulation on the ceiling below the attic. In that case there is no insulation in the roof or above it. The attic is ventilated and UNinsulated.
If you are doing an insulated roof, then your attic needs to be connected with the house so that it is heated, ventilated, and air conditioned along with the rest of the house.
I meant conditioned attic. Sorry
Thanks for responding
Will,
Yes for sure. The reason most assemblies also have permeable insulation below is that thick foam can become both expensive and difficult to install.
Yes 100 external insulation works but it is generally a poor choice because the foam insulation is so expensive and you likely need a second layer of sheeting over the foam to attach the roofing.
Walta
Here's a good start point for all of the insulation above the sheathing plane:
https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing
Just curious, are you nervous to place any insulation within the roof framing?