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Cathedral ceiling insulation for a summer cabin

Need help with weekend summer cabin roof insulation, and best way to avoid moisture problems - zone 4, non vented roof.
Metal roof over plywood over 2x4 rafters, then plan to install exterior siding on interior for finished walls.
I currently have extra 2" rigid insulation sheets that are R10.

Option 1: Cut and cobble rigid insulation against bottom side of roof sheathing, rigid held in place with spray foam. (1A add radiant barrier paint to interior wall panels which will be exterior siding)
Option 2 : Install rigid foam flush with interior rafters leaving 1.5" airspace between roof sheathing and insulation. Roof is not vented, there are no soffit or ridge vents. (2A would be paint radiant barrier on interior side of roof sheeting)
Option 3: Install batt fiberglass insulation 3.5" thick with VB on interior side.
Option 4: Install spray foam - lost of $.
Option 5: ?

Summer cabin primary use with AC.

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Asked by ericski
Posted Jun 28, 2018 11:09 AM ET
Edited Jun 28, 2018 12:48 PM ET


3 Answers

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Of course it's always better to plan your insulation details before the roofing is on -- because now your options are limited.

If you can't vent the roof, and you can't install continuous rigid insulation above the roof sheathing, your only choice is closed-cell spray foam on the underside of the roof sheathing.

If you can retrofit soffit vents and a ridge vent, you'll have more options.

The cut-and-cobble approach is unsafe for unvented roof assemblies -- it is associated with failures and rot.

More information in these two articles:

"How to Build an Insulated Cathedral Ceiling"

"Cut-and-Cobble Insulation"

Answered by Martin Holladay
Posted Jun 28, 2018 12:38 PM ET
Edited Jun 28, 2018 12:40 PM ET.


With your limited use, I expect that you would get away with option 1. Three continuous air barriers (the sheathing, the sealed edge foam and something under the rafters) would help (air movement is by far the primary mover of moisture).

In the unlikely event the air sealing isn't enough, you can control air movement with building pressure (not very expensive for a well sealed building with limited use).

Answered by Jon R
Posted Jun 28, 2018 4:51 PM ET
Edited Jun 28, 2018 5:54 PM ET.


Eric and Jon,
There are two problems with Jon's suggestion.

The first problem is that in most parts of the country, you have to follow the building code -- and according to the usual interpretation of most building inspectors, cut-and-cobble insulation isn't allowed for unvented roof assemblies.

Of course, Eric may be building in an area that isn't subject to building codes.

The second problem is than any insulation strategy that depends on "limited use" of the building to avoid moisture problems is risky. Buildings last a long time, and a building that is used occasionally in 2019 might become a full-time home in 2030. That's why our insulation strategies need to be robust.

Answered by Martin Holladay
Posted Jun 29, 2018 7:06 AM ET
Edited Jun 29, 2018 7:07 AM ET.

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