Underfloor Insulation in Post and Pier foundation - Contractor Talk - Professional Construction and Remodeling Forum

confluencebuild · 01-03-2009, 09:09 PM

This 700 square foot house is perched upon a post and pier foundation.

2x t&g over 4x6's 4' OC. There plenty of ventilation with out the perimeter foundation!

My thoughts this far would be to install rigid insulation tight to the floor and seal the edges of the rigid with spray foam in a can. Im a little conserned about sealing it up though. Anybody think not sealing it with the foam and stapling up a moisture barrier(tyvec) would be alot better. It'll be a pain with all the piers and utilities down there.

mics_54 · 01-03-2009, 09:45 PM

This structure is in Santa Cruz?
What are you trying to accomplish?
I live in a colder climate so the usual practice is to insulate the walls of the crawl space and the rim joist rather than the floor.
Vapor barriers should always be installed on the warm side of the insulation. A vapor barrier should also be installed on the ground. The idea is to control the temperature and humidity in the crawl space. We close off the ventilation openings in the crawlspace walls in colder months and allow the living space heating to heat the crawl space also.

BTW tyvek is a water barrier but it isnt a vapor barrier.

maybe you could explain a bit more about what it is you wish to accomplish.

Chris Johnson · 01-03-2009, 10:07 PM

Check the specs on your ridgid foam. Ridgid foam is normally both an air and vapor barrier. Sealing it up with low expansion foam is fine, you can actually use the low expansion foam as glue and literally glue the sheets of foam to the underside of the sub floor, may need to use a couple fasterners to temporarily hold it until the foam cures (maybe 1 hour) than remove.

Insulating the floor system is the 'normal' practice in California

SLSTech · 01-03-2009, 10:15 PM

Demilic closed cell foam sprayed in? no cutting sheets, no air gaps, no problems working around mechanicals

confluencebuild · 01-04-2009, 03:29 AM

Im trying to get my wife to stop ^*@#!%@*(!!!!!
Its cold in our new house. We're upgrading our heating system and want to maximize it effect.

This 700 sf remodeled cottage is with out a perimeter foundation, just posts on piers, so I can't insulate the perimeter foundation.
The 4x6 girders or joists which support the 2x tnG subfloor are 4' on center, they dont make fiberglass that wide and itll be semi exposed so I think rigid would be the way to go.

My question is if I us the rigid and seal it so that it has an air barrier to reduce washing the heat out of the conditioned area above the insulation, will I shot myself in the foot by trapping incidental moisture in. If I did not seal the insulation and used tyvec as the air barrier, that would allow the moisture or vapor to escape. Id rather just push the rigid up to the subfloor and seal for simplicity and cost effectiveness.

Spray foam is outa my budget.

rock-it man · 01-07-2009, 10:23 PM

I think you will be fine sealing the joints and joist cavities with spray foam. You are worried about "trapping" moisture in, But your not trapping anything. The subfloor can still "breath" through your living space, and the joists from underneith. Condensation isn't an issue because the XPS doesnt allow the warm air to contact the cool air. Over a four foot span you may want to consider strapping it or something so it isnt disloged from seasonal shifting, deflection, critters or anything else.

confluencebuild · 01-08-2009, 02:05 PM

Thanks Rock,

Thats what I wanted to here.

Ill strap it one way or another, possibly screw straight through it to the sub floor.