Expert: Bruce E. Johnson - 10/8/2009

Question
Help - PLEASE!

Description:
There is an addition on the house we just purchased which has about a 2 foot high crawlspace.  The area of the addition is ~300 square feet and it has no foundation - just pylons at the corners.   The front and the right side of the crawlspace are covered by plywood and siding, the left side attached to the house and the back is open (you can see under and gain access from my back yard).  The joists for the floor of the addition are exposed and are filled with rolled insulation held up by insulation wires.  I also stapled  bird netting over the whole space to keep out animals and help hold up the insulation.

My question: we just had a home energy audit and the person performing the audit suggested we put polyisocyanurate boards in the crawlspace.  The suggestion was to screw them into the joists and cover the exposed rolled insulation.  My question is: will this create a moisture barrier?  Are we at risk of trapping moisture in the space between the foam board and the floor - making a nice home for mold?  If it is a good idea - should the silver side go up towards the home or face down?

Thank you in advance!!!

Answer
Hi Chad, If the area between the joists is dry at the time of the insulation install you shouldn't have a problem.  If it is damp and musty under there when you install the polyiso then you may trap moisture and create a future problem.  I use polyiso a lot on roofs directly on top of plywood roof decks and don't seem to have any moisture build up between the layers.  Dead air space is being used more and more for insulated spaces and ventilation is being required less and less.  Always refer to the manufacturers recommendations for installation.  In this case I would think the silver side would face down but verify that with the manufacturer installation requirements.  I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com