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I live in a twenty year old Florida condo quadplex. The upstairs ceilings are two by four trusses of a mansard roof.

I have replaced the ac, wall insulation, ducts. An IR thermometer shows huge heat gain from roof itself. I want to fir down the trusses to double the insulation to the new high r rated 2x6 insulation. Can I simply bolt 2x6s to each truss or do I need to build out the edges with strapping? Do you have an alternate idea?

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2 Answers 2

You're really better off with external solutions: some form of cool roof. Presumably your three neighbors face the same unwanted heat gain, and perhaps could be roped into a common project. Solid foil faced rigid foam is used in cases like this.

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Absolutely cool roof. Always stop the unwanted stuff as close to the source as you can, before it can get a chance to accumulate. In this case, the "unwanted stuff" is heat energy. The cooler you can keep the roof, the cooler will be the ceiling, and the easier the rest of the job will be. If you just insulate, you may cause the roofing to buckle from the trapped heat. –  TDHofstetter Aug 18 at 1:14
    
Condos often have restrictions on exterior changes; typically the association owns the outside of the building. –  TomG Aug 18 at 2:45
    
In this case the association is just four families: they can collectively solve this problem for all the units. –  Bryce Aug 18 at 5:38

Besides simply extending the roof trusses, you could

1) Reinsulate the 3.5 inches between the roof trusses with something of higher performance, like spray in polyurethane. That is about twice the insulation value of fiberglass and contributes substantially to roof/joist solidity.

2) If twice the insulation is not enough improvement, instead of extending the trusses, add a layer of 2x4s (or 2x6s, etc.) so that the result with the roof trusses are a cross hatch pattern. Lumber is not an excellent insulator (it ranges anywhere from 35% of fiberglass to 90%), so minimizing the amount of lumber conduction from the roof to the ceiling is a worthy goal. I am unsure of how to tie those together, but maybe something like this (bottom second from right) would work. This approach may require consulting with a structural engineer to make sure the roof is strong enough.

3) Insulate the new layer.

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