Expert: Bruce E. Johnson - 7/21/2010

Question
I am building a three season room over a pre-existing deck which is essentially open on three sides. I would like to insulate under the deckI read your previous posts about using visqueen on the ground, foil bat insulation between the joists and then tyvak under the joists. The deck floor is tongue and groove ipe. I am concerned about water leeching under the ipe (capillary effect), between the boards and enough air circulation to dissipate the water? Seems like a mold problem waiting to happen. Thanks for the help.

Answer
Hi steve, you are right.  If the room will be open to the weather then you will need to waterproof the deck otherwise you are asking for water related problems.  A complete moisture prevention system would have to be in place based on what you are planning to do.  If the room is going to be fully enclosed then roper ventilation between the insulation and the visqueen covered ground is a must along with proper moisture control above the floor inside the room itself, i.e. air conditioning, dehudidifier, or other forms of air movement depending on your locale. How your existing home crawlspace is ventilated should be an example as to how your room should be designed providing your house crawlspace is not having moisture control issues.  I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson