Replies

1. 
   davidmeiland | Oct 21, 2010 10:05am | #1

   You could consider

   sealing the crawl space and insulating the inside of the concrete walls. Generally you would first install a very high-quality cover over the ground and up onto the walls, sealing all edges carefully, then install foam insulation board on the walls (or I suppose it could be spray foam. Your crawl space would then be unventilated. There are a lot of factors to consider when doing this, and if it seems interesting I would suggest you read up on sealed crawl spaces at the Building Science website, Green Building Advisor, etc. 

   To further insulate the floor, yes you could use spray foam but I would probably suggest removing the existing insulation first and spraying the underside of the floor. Or, you could install foam insulation board on the bottoms of the joists, leaving the existing in place. Can you get 4x8 sheets into the crawl space?

1. 
   YNC_Johnson | Oct 21, 2010 04:31pm | #2

   I can get 4x8 sheets into my crawl space.  This space has a 4' x 5' opening that leads into the basement, which is the only door into it.

   How does foam insulation boards compare to fiberglass insulation?  I guess what I'm asking is what is the R value of 2 inches of foam board?

   Thank you for your reply.  Rex.

2. 
   YNC_Johnson | Oct 21, 2010 04:55pm | #3

   Conditioned crawl spaces

   I took your advice and went to http://www.buildingscience.com/resources/cond-crawlspaces and found a couple of great articles on vented vice conditioned crawl spaces - thanks.  It's a terrific web-site filled with good information.  But this does lead me to another question.

   After doing some reading up on converting to a conditioned crawl space (which isn't cheap), in the end will it be truly cost effective vice just adding more fiberglass insulation between my joists?  Or as you suggested, adding foam boards to the bottom of the joists?

   During the middle of the winter I see a large buildup of ice on the inside walls of my crawlspace and worry about that.  At that time of the year moisture inside the house isn't a problem because I run my 120,000 BTU basement wood furnace.  That drys the house out so have to run a humidifier anyway.

   Again, I appreciate the help your giving me.

   V/R,

   Rex

1. 
   davidmeiland | Oct 21, 2010 05:16pm | #4

   The foam board

   would provide an effective air barrier if installed tightly and taped at the seams. Fiberglass doesn't do much to stop air movement, and it's not a very good insulator unless it is completely enclosed on all six sides. Adding foam to the underside of the floor frame would help the fiberglass perform, and you could use foil-faced polyiso at almost R-7 per inch and get another R13 or so (2") in addition to the air barrier. It would probably make a noticeable difference. More fiberglass probably won't. A thorough closed cell spray-foam treatment would also make a big difference.

   The ice on the inside of the crawlspace foundation... is the moisture coming thru the walls and freezing on the inside, or is it moisture from the house contacting the walls and turning to ice? During heavy rain and non-freezing weather, do you get moisture in the crawl space?

   If you are going to make changes like this I encourage you to read thoroughly and understand the relevant building science issues. These types of things change the performance of the building, and while big improvements are possible it is also easy to cause problems. It is necessary to understand all of the factors affecting your specific house in addition to understanding the regional best practices.

2. 
   DanH | Oct 21, 2010 07:42pm | #5

   Ice buildup on the walls is obviously due to moisture.  Could be coming up from the ground (the ground should be covered with plastic to minimize this), or it could be coming from the house.   Remember that even though the humidity in the house may be only 35%, that equals 100% humidity at 40F, so if air is leaking from the house it likely will cause frost buildup in colder weather.