Above roof sheathing insulation values
WillLyle
| Posted in Green Building Techniques on
I am building a new home with all cathedral ceilings. I am building a unvented attic and conditioning it as part of the home envelope. I am hoping to use the monopoly style roofing system I’ve seen on this website. I would like 100% above roof insulation to leave depth between rafter for HVAC wiring etc.
Is this possible? If so I’m in zone 4 in Tennessee and I have found conflicting Rvalues. Some say that R30 is all that is needed with 100 % above. The 2018 International code says total 38? Any insight from anyone?
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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?