New construction crawl space design
Getting ready to pour my foundation with crawlspace. for a small building. Site appears well drained. This is in zone 6.
3″ XPS will be put on outside of foundation with drains installed at footings. Not planning on insulating the main floor of living space. 2″x 6″ construction with 3″ XPS all the way up the outside with blown in cellulose in stud wall. (24″ OC construction).
One of the biggest issues for me is working through the Energy Star advice and picking the best solution for the cost without spending more than is needed but knowing when to spend the extra $$ because it is well worth it.
My question is:
1. Do I put in 6 mil plastic for interior crawlspace attached up the side of concrete with furring? Will this be enough to control the humid hot summer days that do happen? OR
2. XPS under rat slab? If this is the way to go, what width XPS. Cost is a factor BUT is it worth putting that insulation in. OR
3. 6 mil plastic and rat slab?
4. Any other cost advise or practical advice?
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Even if you decide to put XPS under your rat slab, you should also include 6 mil poly. The 6 mil poly belongs between the XPS and the concrete.
At current energy prices, you may not be able to justify the cost of rigid foam under your rat slab. However, energy prices may rise in the future, and it will never be easier to insulate your crawl space floor than right now.
Mathematics alone won't be able to answer this question. The answer depends on your budget and your priorities.
Yes to the plastic. The rat slab will have no effect on humidity issues but you may wish to install one for other reasons - they are almost unknown in my area (Zone 4A). If immediate budgets are tight omitting the slab at this stage leaves the door open for insulation retrofit later on.
The purpose of the plastic (along with not having vents in the foundation wall) is to severely limit the entry of moisture in the crawlspace. In many cases this is enough to control crawlspace humidity levels. It's common practice though to either negatively pressurize the crawlspace with a small exhaust fan so that conditioned air is drawn in from the house above though air leaks in the floor or to provide a small supply register from the house a/c system (if you have one).
Does it go without saying that good exterior grading around the perimeter of the house is essential to crawlspace humidity management? If you have standing water or saturated soil directly adjoining the foundation wall it will find its way in sooner or later.