[Greenbuilding] building floor on top of rigid foam... (basement/garage retrofit)

sat jiwan ikle-khalsa satjiwan_khalsa at hotmail.com
Sat Apr 16 12:53:26 PDT 2011

anyone have experience or thoughts on this?  (thought it might apply to retrofitting basements without slab insulation as well - provided sufficient head height.)

a client wants to energy efficiently convert an attached garage made into a living space.
and at the same time raise the floor up to be level with the adjacent room. there's no insulation under the slab. so what's a good floor assembly to get height and insulation. and in this case no need for bearing a lot of load.

we're aiming for about R20 floor insulation (rough passiv haus goal for our region - Mid Atlantic USA).  we're thinking to put 2 layers or two inch rigid foam on top of the existing slab then build a floor with 2x framing members sufficient to bring the height up so 3/4" plywood and carpet on top will meet the other room's height.

i know XPS is rated for under slab applications.  would the somewhat more point(linear) load of 2x material be a problem resting on the rigid foam? (note there's no walls planned on top of this floor and no other bearing weight than the floor and furniture.

would Roxul rigid foam work? is it specified for load applications?


a. lay down vapor barrier on slab, build appropriate height flooring and before applying plywood, fill joist bays with some kind of batt (roxul/denim) or loose fill (cellulose) material. there's plenty of room to get R20, but will loose thermal break with wood right on slab.

b. (crazy??)  build up more layers of foam insulation (6-8" total) - then lay plywood on top.  (a floating floor).  this seems a little loose to me, especially since you'd want to leave some gaps in plywood joints and edges of room to accommodate expansion.  (would using two (thinner?) layers of plywood, laid opposite directions and screwed to each other secure it enough??)

c. any other brilliant strategies?


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