Expert: Bruce E. Johnson - 7/12/2010

Question
I am working with a contractor to build an 15' x 17'addition to my home.  The addition will have about a 3' crawl space and the foundation will be wood post buried 4' down in concrete.  I need help selecting a product for closing in this crawl space and advice on how much ventilation is needed to avoid mold and moisture build-up.  The floor of the addition will be insulated and a moisture barrier will be installed with a covering of metal.  Can you help??

Answer
Hi Charles, it kind of depends on where you live.  If you live in a real wet climate you might need more ventilation than a dry climate.  If your existing house has a similar type foundation and has a vent every 12 feet then I would do the same on the addition or at least one vent per side wall. It sounds like you have the underside pretty well protected.  Be sure to put a good layer of visqueen on the ground under the room.  I always do a block or concrete stem wall with a crawl space foundation but you can also use pressure treated wood framing with whatever siding, brick or stone you prefer installed over it.  I am not a big fan of burying posts in concrete, they eventually rot out.  An elevated post base bolted or poured into a concrete pad will outperform a buried post over the years.  However if this ia common practice in your area and the building department allows this type of construction then I guess it's okay. I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com.