Advice needed: Insulating and encapsulating unique crawlspace
I have purchased the materials to insulate and encapsulate the dirt floor crawlspace under our 3-season porch – we live in Massachusetts (zone 5A). I have done a lot of reading on the subject but the construction of our crawlspace “foundation” appears to be unique.
The walls are not concrete or block on 3 sides but rather expanded metal lathe and mortar. There are 2×4 studs inside the mortar wall and a sill plate/beam that supports the porch structure. The block wall and brick chimney are the outside of the main house structure. The porch floor is just wood boards (not tongue & groove) without any paper or barrier (you can actually see air/light between some of them). I am attaching pictures so you can see what we’re dealing with. We don’t have any major runoff issues down there so we aren’t putting in any drainage or sump pumps. We plan to remove all the old fiberglass batts between the joists.
We are having the main house insulated with blown-in cellulose from the exterior through a utilities sponsored program and they won’t insulate the upper part of the porch (which is heated) unless we treat the crawlspace ourselves.
We have purchased:
• 2″ thick rigid foam insulation – exterior grade, unfaced polystyrene, R10
• Spray foam insulation for filling gaps
• 20mil Silverback vapor barrier
• Felt 550 underlayment to protect the vapor barrier from punctures
• LocTite PL Premium Construction Adhesive for attaching and sealing
• Nylon expansion fasteners to help attach the vapor barrier
My questions are:
– Do we need to fit the rigid foam in between the 2×4 wall studs, against the metal lathe/mortar or can we apply it over the studs? Because of the mortar oozing between the lathe, we’ll never eliminate air gaps between the mortar and rigid foam boards… Over the studs would be so much easier.
– Do we insulate the ceiling (i.e., porch floor) with rigid foam too and if so, does the rigid foam have to go between the joists or can we apply it across the joists (below the cavity)?
– Will we need to put rigid insulation blocking along the rim joist between the joist cavities? If we need to insulate the rim joists, should we insulate the sill plate too?
– Do we also put vapor barrier under the porch floor (crawlspace ceiling)? If so, does the vapor barrier go between the insulation and porch floor or below the insulation?
We don’t plan to insulate the concrete block foundation wall or the chimney (our basement is on the other side) but we will extend the vapor barrier up those walls. I’ve read not to extend the vapor barrier all the way to the sill plate as that can cause rot and provide termites a route to the wood plus, leaving it open provides access for termite inspection.
I feel like I’ve researched and decided the right thing to do then read more and changed my mind at least 5 times. For background, I’m a furniture-maker, so I’m very handy but building envelope science is not my wheelhouse. I’d really appreciate some advice tailored to our unique situation.
Thank you in advance!
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