2019 Idaho Real Estate Blog

Are your rim joists wet? Main The Durable Green Home
the rim joist is the end band or board that the joists terminate at the exterior of the stem wall ( foundation ) this is usually a neglected are of insulation and air sealing and can be a huge durrability issue down the road. a homeowner called this week because he is selling his home and the home inspector called out that the rim should be insulated.....

True statement  but he was worried that this could lead to wet rim joists he has heard about in other homes with insulated rims. This  statement is true as well, especially when the insulation usually applied is not the best detail. The problem with not insulated is that it should really be done from an energy viewpoint and the reason the rim is dry now is that there is enough air flow through the crawls space to dry out the humidity collecting on the rim, and the rim is as warm as the rest wood in the crawls space as well.  IF the crawl space is kept at a low humidity level meaning the vapor barrier is properly placed (covers all dirt, seams lapped or taped, lapped over footings) then this works fairly well and I have not seen problems with wet rims (although the energy loss is is substantial over time) but  here is what usually happens, the crawl space is fairly humid, maybe not  all the time, but for sure when the home is built, water coming in from  downpours, or other infiltration, the rim  is insulated with batt fiberglass, when the temperature drops  outside the insulation keeps  the rim cold ( insulated from the warmer crawl space, the humid air travels through the fiberglass  or around it, and condenses on the rim and over time can cause rot especially in engineered rim  joists that we build with now. while there are many ways that we can keep this from  happening a great detail came  out in the current issue of www.greenbuildingadvisor.com dated October 20,  2009 in this detail it  shows insetting the rim enough to allow a rigid foam strip on the exterior. we would still insulated the interior (crawl space side  as well with batt or other) but the rigid  foam  will keep the rim  warm not  allowing condensation to form on the rim. One should air seal (caulk) the connections as well to not allow air from the exterior to infiltrate, but this is a great detail and will keep the rim from rotting. Every remodel project should be using this detail, you don't have to inset the rim to accomplish this, and exterior insulation is still the best way to insulated rim, crawl or basements if you have the ability to do so.

 
Posted by Tad Duby at 10/25/2009 4:24:00 PM
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