Hi -

I have a 2BR townhouse condo with some heat issues. I believe the problem is radiant heat coming from the roof. This is in a warm, dryish climate (Los Angeles.) The roofs and ceilings are sloped but there is no attic space above the upstairs rooms. So I imagine that there is fiberglass stuffed between the rafters above the ceiling. As far as I can tell, the roof is un-vented. I looked along the roof overhangs but couldn't find any vents. Looked on the roof too but didn't see any venting. I doubt there is a radiant barrier since the building is from 1978.

I'm looking for a cost effective way to add insulation and reduce the flow of radiant heat. I'm not allowed to modify or penetrate the roof and there is no attic, so any insulation I add would have to be from the inside in. Also, I'd like to leave the current insulation intact to save on costs (assuming it's in okay condition).

Based on some things I've read, I thinking of using polyisocyanurate boards (AKA polyiso) to accomplish this. I believe these sometimes come with foil on both sides. Here is my thought about how this would be done:

-Remove the existing drywall ceiling, but leave the existing insulation, vapor barrier, etc. in place.
-Install 3/4" deep furring strips perpendicular to the rafters. This is to create an air gap to make the polyiso's radiant foil work better
-Attach the polyiso boards to the furring strips
-Seal between the polyiso boards with some kind of vapor barrier tape (aluminum tape? house wrap tape?)
-Attach another set of furring strips to the polyiso boards (on the inside side of the room this time). Again to create a small air gap
-Attach drywall to the furring strips and finish the drywall.

Is this a sound idea? Would there be problems with moisture getting trapped?
Is it overkill to have two air gaps around the polyiso? I'm guessing that for me the air gap on the outside side of the polyiso is the more important one since my problem is heat coming into the living space more than heat going out. (Keep in mind that this is a warm climate.)

Thanks

Yes you would be creating a vapor lock between the ceiling poly and the foam board.
The way to redo this would be to gut the ceiling and use 2 inch xps foam cut tight between the roof deck truss sides. More insulation where you see fit varying upon design. There should be almost 14 inches of space above your ceiling sheet rock but there could be less. Vaulted ceiling trusses vary but there should be a slight gap a human cannot crawl though from attic non gable end.
This gap is supposed to have insulated on ceiling and then air gap and then roof sheathing along with air vents going out the roof and soffit inlets on below edges. This is required in almost every state i can think of. I would call city and find out.
Then poly ceiling with 6 mil clear vapor barrier.
Red tape the seems, sheet rock, mud-tape, sand, texture and paint.

However i would consult an hvac specialist or someone whom specializes in this subject.
Get a free bid or two and you will learn a lot about how to fix it then you will be certain all this work is required.
You might just need a required roof vent or two in which case if this is a rental or association fee'd property. I would ask the city building dept.

This message has been edited. Last edited by: JB Builder,

I found some info, source at bottom.
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Every state except California and Hawaii has adopted some version of the IRC. And California is expected to adopt it in 2008.
Until recently, the IRC required all attics and enclosed rafter
spaces to be vented. But the latest version allows unvented attic
assemblies if certain conditions are met.
According to Section R806.4 of the 2006 IRC, unvented assemblies are allowed if “no interior vapor retarders are installed on the
ceiling side (attic floor) of the unvented attic assembly” and if
“air-impermeable insulation is applied in direct contact with the
underside/interior of the structural roof deck.”
There is an exception that allows air-permeable insulation
(fiberglass and cellulose) to be used in unvented assemblies in
certain parts of the South (climate zones 2B and 3B).
It has long been possible to get an unvented assembly approved
by the inspector as an “alternate construction method.” But once
states update their codes to the 2006 IRC, it will no longer be necessary to get special approval for unvented assemblies.
In the meantime, the fact that the 2006 IRC allows unvented
assemblies should make it easier to get special approval in states
that have adopted earlier versions of the code.
Do not build an unvented attic assembly without first talking to
the local building inspector. Unvented assemblies are new in the
IRC, and your state might be using an older version of the code.
Also, the committee that wrote this section is still working on it,
so more changes may be on the way.

http://www.greenhorizon.com/fi...h%20Spray%20Foam.pdf

This message has been edited. Last edited by: JB Builder,

If it were me...
I would add a ridge and soffit vent