Question regarding rigid foam over paper-backed insulation

[redcar]

We have an old building that we have been "modernizing" while trying to keep it looking original. The crawl space under the building is wide open to the elements, and in addition to insulating the walls/attic we have added paper-backed insulation between the floor joists to help hold heat/cool inside the building. As it is a very old building, we still have some air leaks in the flooring material. My question is: can I nail up a solid or mostly solid layer of rigid foam insulation to the floor joists (this would put the rigid foam between the ground and the current paper-backed insulation) -- or will I end up with a massive trapped moisture problem?

Thanks for any/all help.

Bob

 

[walrus]

We have an old building that we have been "modernizing" while trying to keep it looking original. The crawl space under the building is wide open to the elements, and in addition to insulating the walls/attic we have added paper-backed insulation between the floor joists to help hold heat/cool inside the building. As it is a very old building, we still have some air leaks in the flooring material. My question is: can I nail up a solid or mostly solid layer of rigid foam insulation to the floor joists (this would put the rigid foam between the ground and the current paper-backed insulation) -- or will I end up with a massive trapped moisture problem?

Thanks for any/all help.

Bob

Where are you located? Depends is the answer. What kind of floor is in the heated space, can the insulated cavity dry to the living space. In other words you don't want a vapor barrier on both sides of the insulated space. If you are going to do it, I'd caulk or canned foam any cracks in the rigid foam, I'd use 2" minimum and I'd cover the dirt below with at leats 6 mil poly to keep moisture issue down

 

[jklingel]

read on buildingscience.com about crawl spaces. or greenbuildingadvisor.com. they recommend bringing crawl spaces into the conditioned space, ie, insulate the walls, seal/vb the (dirt?) floor, and get the insulation off of the floor between house and crawl. (that can be very problematic.)

 

[burleymike]

The best thing would be to put foil faced isocyanurate board on the underside of the joists. As long as you don't have any type of a vapor barrier on the interior you will not have moisture problems.

The foam board will act as a vapor and air barrier. Tape all the joints and use spray foam or acoustic sealant to seal the foam board around the edges.

If it is in your budget closed cell spray foam would work best. The foam should coat the bottom of the floor and the joists. Buildingscience.com as some good info on this very subject.

 

[redcar]

Thanks for the help. We are in South Carolina -- and we have put 6 mil poly on the ground to help with controlling ground moisture. Historical design of building won't allow crawl space to be closed in, so can't incorporate it into to the living space. But I will spend some time on the Buildingscience.com and Greenbuildingadvisor.com sites -- thanks for the leads.

Bob