Replies

1. 
   DanH | Feb 02, 2015 07:01pm | #1

   What do the rafters have to do with your crawl space?

1. 
   saulgood | Feb 04, 2015 09:43pm | #2

   I think he means floor joists - the "rafters" of the crawlspace.   

2. 
   skivilla | Feb 12, 2015 12:41pm | #4

   The use of a vapor Retarder over the floor joist in a crawl spac

   Sorry I meant floor joists vapor Retarder.  If this was used would it cause a problem?

2. 
   mark122 | Feb 04, 2015 09:57pm | #3

   instead of putting up more plastic, can you make sure your current moisture barrier is sealed properly at all seams, up piers and against the foundation wall? thick mill plastic wont make a difference if its not properly sealed .

1. 
   skivilla | Feb 12, 2015 12:52pm | #5

   The vapor barrier on the floor Joists

   This is an open vented Crawlspace.  The 6 mil vapor barrier on the floor is sealed up to the wall and piers.  But I believe I am still losing allot of heat from the floor because of the Batt R19 insulation in the floor joists. In some places i can see the bottom of the sub floor.  I am in a warm Florida climate.  I was thinking of putting in an Air barrier or vapor barrier with a high Permeability. So I would not introduce a mold environment.  But I am not sure if that is a good idea or not.  What do you think?

1. 
   mark122 | Feb 12, 2015 05:17pm | #6

   never been a big fan of fiberglass in crawl spaces. if your problem is draft, then closed cell foam would be the ablsolut best way to remedy your problem.

   You can get a single pass of foam (average 3/4'') and seal everything. Being in Fl. you shouldnt have to much to worry about with insulating your crawl space, but you can flash and batt if you are wanting to keep the r value on the bottom side of the floor. flash and batt is a thin coat of foam for air/moisture sealing and fiberglass for added r value.

3. 
   DanH | Feb 12, 2015 07:53pm | #7

   What I put together from your several posts is that you have a floor over a crawlspace.  The crawl is vented and there is also 6 mil plastic on the ground.

   The floor above is reasonably standard joist construction with R19 fiberglass between the joists.  You don't mention whether there is any sort of facing on the fiberglass.  This is in Florida.

   You are correct to want to do something.  The fiberglass, installed open and exposed as it most likely is, is virtually worthless.  And humdity from the crawl is rapidly making its way into the house (along with the heat).

   The minimum you could do is install plastic on the bottom of the joists.  This would prevent the infiltration of humid air into the house and also geatly improve the effectiveness of the fiberglass insulation.  There would be some hand-wringing re condensation with this approach, since the cold air from the house would sink down to the plastic and cool it, and then humidity from the crawl would collect on the bottom of the plastic.  So there almost certainly would be condensation, but it would occur in a location where the moisture would not be reasonably expected to cause any damage.

   You could use other approaches:  1) Enclose and "condition" the crawl, placing foam insulation on the ground and the crawlspace walls.  2) Replace the fiberglass with foam of some sort, between the joists.  3) Install maybe an inch of foam on the bottom of the joists (instead of the plastic). 

4. 
   Perry525 | Feb 15, 2015 10:49am | #8

   Mold, wood rot, insulation, heating, crawl space, floor.

   Wood is hygroscopic, as such it absorbs  moisture from the air, when the wood is below dew point.

    If the amount of water in the wood gets too high, then mold can take hold and wood rot can start.

   Ascertain the air temperature  in your crawl space over night compared with the daytime temperature to see if it is dropping to dew point  or below.

   If it is, then you either need to raise the temperature of the wood above dew point (probably impossible if you have air conditioning) or stop the air/moisture from getting to the wood. You can of course fit a humidistat controlled heater to monitor the humidity, and  raise the temperature when required. But this will involve ongoing running costs. Another way is to fit a dehumidifier which will present a lower surface temperature that will attract and condense the moisture, also with ongoing costs.

    Adding open cell insulation between the joists (if you have air conditioning) will only make matters worse by lowering their temperature.

    The best and cheapest solution with a limited one time cost, is to encapsulate the joists and floor with a plastic barrier. Or to completely enclose the ground and walls with plastic sheet. Keep in mind that water vapor molecules are very tiny, small enough to enter wood, therefore all holes and joins need to be water vapor proof. Trying to water vapor proof a large area is quite a challenge.

    

5. 
   skivilla | Mar 02, 2015 01:40pm | #9

   Type of plastic sheething

   If I decide to go with the plastic on the floor joists. Would it make a difference if I use a high or low permeable plastic?