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Rebuilding a troublesome Micky Moused roof assembly

climbing_carpenter | Posted in Energy Efficiency and Durability on

I’ve stumbled across a troubled 1960’s chalet.  Worse than carpenter ants, it’s been attacked by an amateur carpenter!

I’m contracted to replace the roof and add insulation in the process as there is currently very little insulation.  The original roof assembly is 4×8 rafters, 4′ OC, sheathed with full dimension 2×6 T&G which also serves as the finished ceiling.  This assembly had no insulation.

A homeowner added a layer of 1″ XPS years ago, strapped with 1.5″ strapping to vent the roof, and sheathed with 1/2 CDX.  My intent was to frame a 2×8 non structural assembly on top of the existing and insulate with R30 mineral wool, sheath with 5/8 ADV-T and finish with metal panels.

My concern is the venting as it will compromise the effectiveness of the mineral wool.  I’d like to eliminate the air gap by demoing back to original  but can’t because of too many odd plains coming together.  It’d cost more than the project is worth.  I need to build on top of the existing assembly as is (insulating the original 4×8 rafter bays will disrupt the character of the home).

I’d like to airseal existing sheathing with ice and water, spray foam or pack eave vents, and build the new roof assembly as planned.  I’ll use 5/8 zip or tar paper as an underlayment to allow drying to the outside- no closure strips in the metal.

My intent is to incorporate the existing roof vent into the heated envelope of the house as best I can-  although I don’t have access to deliver a high standard of work.  It’s a situation where we have to work with what we have.

Thoughts on performance?  Any simple ways to make improvements?

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Chris,

    Difficult retrofit. It's always hard to know how far to go, and what makes sense to do.

    If you are eliminating the existing venting, why not build a new vented-roof above? It's a lot safer than relying on the sheathing drying through a metal roof.

    A second consideration is that without sufficient foam above the mineral-wool it wouldn't meet any building codes I'm familiar with.

  2. Expert Member
    MALCOLM TAYLOR | | #2

    Chris,

    Difficult retrofit. It's always hard to know how far to go, and what makes sense to do.

    If you are eliminating the existing venting, why not build a new vented-roof above? It's a lot safer than relying on the sheathing drying through a metal roof.

    A second consideration is that without sufficient foam above the mineral-wool, it wouldn't meet any building codes I'm familiar with unless you included venting.

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