Insulation/Vapor Barrier ??? for enclosing a porch


Old 02-01-10, 08:46 PM
Thread Starter
Join Date: Jan 2010
Location: Lincoln, Nebraska
Posts: 35
Insulation/Vapor Barrier ??? for enclosing a porch

I'm going to enclose the screened in porch at our lake home. There's a deck above the porch, with a water tight floor, 2X8 joists. I plan to insulate with fiberglass, then use vinyl soffit for the ceiling.

The walls will be 2x6 (to match up with the existing 6X6 support structure, drywall on the inside. Most of the wall will be taken up by new construction vinyl windows, covering 90% of the wall above 24". I planned on insulating the wall below the windows with fiberglass batts, and using a plastic vapor barrier.

I'll have vinyl siding on the exterior - planning on using plywood for sheathing, with Tyvek as a wrap. Does this make sense?

When I last did framin(35+ years ago), we used plywood for sheathing, tar paper for the house wrap. I see there are foam sheets for use as sheating - is that a better choice? (I don't need the plywood for stiffness, since the existing structure is adequate).

So, in order from inside to out, it would be:
vapor barrier
sheathing (plywood or foam)
house wrap

Does this make sense?
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Old 02-02-10, 02:47 AM
Join Date: Oct 2008
Location: New England
Posts: 9,997
With the vapor barrier on the inside, any foam sheathing on the outside needs to be permeable enough to allow some drying to the outside. Pink or blue, but no foil faced or plastic faced.

I assume you will be heating this in some way. With warm moist air inside, the soffit material fot the ceiling will allow moisture into that cavity. The vapor barrier and air barrier (plywood, sheetrock, other) need to be in contact and continuous, no leaks.

With no obvious way to vent the ceiling area, this becomes an application where a "hot roof" as they call it might be a good choice. The typical configuration is to foam the cavity between the rafters forming the ceiling and create a solid insulation barrier that allows not air to leak through.

So, all looks ok for your approach, except the ceiling. Need something different for that.

Old 02-02-10, 03:04 PM
Thread Starter
Join Date: Jan 2010
Location: Lincoln, Nebraska
Posts: 35
It will only be heated when in use - I'm putting in a wall furnace.

This room will be more of a "3 Season" room - we won't use it when it's really cold outside.

I'll rethink the ceiling.

What's your opinion of sheathing - plywood or foam? I'm thinking plywood, since there's not much wall area anyway.

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