INSULATING AN OPEN CRAWL SPACE

QUESTION FROM HENRY FROM SEVERNA PARK, MD:

 I have an open crawl space (by that I mean there is a vent to the outside) under my family room that I want to better insulate. 

After I put insulation between the floor joists can I then put insulation foam boards across the joists? 

I was thinking that would serve two purposes 1) insulate the joists and 2) make it more air tight. 

I do not have any experience with insulation so I don't know how that would affect the insulation between the joists.  If that is not a good solution, do i then have to cover my vent and then insulate the walls? 

Thanks for any help.


ANSWER FROM THE INSULATION DOCTOR:

Hello Henry,

First it is not legal to install any type of foam board that is not covered by a non-combustible product. Foam board sets off a toxic gas in the event of a fire which is deadly to the fire fighters.

I would recommend that you close the air vents in the fall and reopen them in the spring. It is not necessary to insulate the outside walls of the crawl space if you have already insulated the floor joists.

If you still have a cold floor, I would recommend removing the insulation from the floor joists and re-installing it onto the outside walls and opening one heat run into the crawl space. This is a much more efficient method to insulate a crawl space.

Hope this helps.

Bruce Jones
The Insulation Doctor