I would like to replace the insulation in my crawl space. It is underneath an addition on my house, has a dirt floor and the only access/vent is a 2ft by 3ft wood panel on the side wall. I see that I should only insulate between the joists (floor of the addition). My 1st question is do I need to purchase insulation with vapor retardent?
I also noticed in the DIY video that I should lay a 6mil sheet of plastic along the dirt floor. Is it correct to do this or would it make more sense to fasten the plastic directly to the bottom of the floor joists and tape the seams shut?
Thank you for your question and welcome to the community.
How to insulate your crawl space depends on whether it's ventilated or unventilated. Having said that your crawl space has only one access panel I’m assuming that you have an unventilated crawl space. When insulating unvented crawl space you want to insulate your walls rather than the subfloor, the advantage of this approach is that you are eliminating problems associated with venting of your crawl space. Also in addition to not having any more problems with cold air coming inside your crawl space you are also saving on insulation due to walls usually being only about 3 feet high.
The US Department of Energy recommends three different options on how to insulate walls inside of the crawl space. Your first option would be to use rigid foam board on the outside walls; this is usually implemented with a new construction. The second option would be to use rigid foam board on the inside walls, when using this application joints between the boards should be taped and vapor barrier applied. The last option is to use faced (vapor retardant) batt insulation on you walls and to raise vapor barrier ( 6mill plastic) along the perimeter of the walls. The attached diagrams explain these three options in detail.
"I also noticed in the DIY video that I should lay a 6mil sheet of plastic along the dirt floor. Is it correct to do this or would it make more sense to fasten the plastic directly to the bottom of the floor joists and tape the seams shut?"
Vapor retarder barrier use depends on the climate and location of your home,please refer to attached picture or visit this link for more information. If your home does need a vapor barrier you will need to lay a 6mil sheet of plastic along the dirt floor and continue with 6" of plastic along the perimeter of your wall.
Hope that helps and good luck with you project.
We recently called a company to get rid of mice around the house. They came as a tag team. stating our crawl space needed new thermo venting system, new 6 ml plastic and new insulation. Where do I get the thermal vent system to put in my self? They quoted about $4000 to do what I believe would cost less than $500 ?