Adding foam insulation to the roof on a manufactued home
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| Posted in Energy Efficiency and Durability on
I have a neighbor here in Vermont who needs a new roof on her 70’s doublewide manufactured home. The roof currently has asphalt shingles on OSB. The roof is vented. I thought this might be the perfect opportunity to add some foam insulation on top of the existing sheathing. I can do metal over strapping on top of the insulation or asphalt over another layer of OSB. Which will perform better? Is the payback worth the cost? Currently heating with propane. The roof only has moderate slope. What do I do with the existing vents? Any suggestions are appreciated.
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Replies
William,
In Vermont (Climate Zone 6), you want to aim for an R-49 roof assembly. You can install rigid foam above the roof sheathing if you want, as long as you have a plan to achieve the recommended total R-value.
This approach -- an unvented cathedral ceiling -- requires that the rigid foam which you install on the exterior side of the roof sheathing have a minimum R-value of R-25 (about 6.5 or 7.5 inches of EPS, 5 inches of XPS, or 4.5 inches of polyiso). You will also need an additional layer of R-24 insulation on the interior side of the roof sheathing; this insulation will need to be in direct contact with the underside of the roof sheathing.
Of course, since you are converting an existing vented unconditioned attic into an unvented conditioned attic, you will need to carefully seal up the soffit vents and ridge vents. You will also need to insulate the gable walls in the attic.
For more information, see these two articles:
How to Install Rigid Foam On Top of Roof Sheathing
Creating a Conditioned Attic
Thank you Martin. Insulating the gable ends in the attic had not occurred to me.