Flat roof insulation
myoon595
| Posted in Energy Efficiency and Durability on
Here is the Chicago area.
If I can’t ventilate the flat roof, then what happens?
what if I am using the fiber insulation and radiant barrier together..?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Mansig,
Here is a link to an article that will explain several options: Insulating Low-Slope Residential Roofs.
Q. "If I can't ventilate the flat roof, then what happens?"
A. If you can't ventilate the roof, then you will be building an unvented assembly. This is possible -- indeed, it is the most common way that commercial roofs are built. By far the most common approach to building an unvented assembly is to install a thick layer of rigid foam above the roof sheathing.
Q. "What if I am using the fiber insulation and radiant barrier together?"
A. If you want to use fiber (air-permeable) insulation, then you need either: (a) to build a vented assembly, or (b) to install enough rigid foam above the roof sheathing, or closed-cell spray foam below the roof sheathing, to prevent moisture accumulation in your "hybrid" assembly. More details can be found in the article I linked to.
Radiant barrier is of almost no value unless there literally isn't space for anything else. In a horizontal location with at least an inch of air space to the layers on both sides it'll do something- more during the cooling season than during the heating season, but in general you're better off using something else.
In the Chicago area (US climate zone 5A) you can use closed cell spray foam against the roof deck with fiber on the interior side, as long as the R-value of the closed cell foam is at least 40% of the total R-value.