What to do with 13″ of space between insulated slab and finished floor?
My client has an insulated slab on grade (4″ gravel, 2″ XPS, 6-mil vapor barrier, slab edge insulation 2′ to frost depth) on an addition. The existing home finished floor is 13″ above the finished slab.
The plan had been to step down out of the existing home onto the slab. The designer now wants me to build a suspended floor system to bring the floor levels in plane without any step down. Essentially a conditioned crawl, but I only have 13″ to work with from finished floor to insulated slab. The space is only 8×12 ft. Can I build a 2×8 floor system with 4 inches of open space underneath? Can I put another layer of XPS on top of the slab- taped and leave 2″? and then install two floor vents? Is that enough air? Any ideas on how to approach this would be helpful.
Patty
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It's unlikey that adding another layer of foam would pencil out on lifecycle cost basis unless your deep subsoil is permafrost or something. (And if you were to add foam XPS would be the least-green material possible. )
Where is this house located?
Patricia, I recall a Passivehouse project here ion GBA which had a similar situation. The designer had poured a slab, but also included a framed floor system above and filled the resulting space with cellulose. Unfortunately I can't remember what project or really comment intelligently on whether it's a good idea. Perhaps someone else can chime in.
I think your situation might fall foul of our code, which wants concealed spaces to be a certain minimum height. It might be worth checking with your BI first.
The code is what I am concerned about. What if I fill the whole thing with foam so there is no air movement. 2' of EPS taped and then open cell foam for the remainder. Could I do dense pack?
I'm only familiar with our codes here in BC and even so I'm not sure how the space would be interpreted. If it's a crawlspace then you are hooped. But if, as you say, you filled the whole thing with insulation maybe that negates any concerns. A first step is to see where you stand with your BI.
That aside, I'm interested (as I was with the passivehouse I mentioned) on what posters think about the risks of dense-packing from concrete to subfloor. If moisture does get into the cellulose, where does it dry to?
Malcolm,
I certainly wouldn't install cellulose insulation in this location, because of moisture worries.
If all you have is a slab at this point -- no framed walls -- it might be possible to build pony walls at the perimeter with CMUs or poured concrete (assuming that the slab has decent footings at the perimeter), and then fill the whole space up with gravel, topped with a new slab. That might solve the code problem (avoiding a non-conforming crawl space).
If I go Charlie's route which is my inclination, how is this any different from a basement floor retrofit of foam then subfloor. The only difference seems to be the thickness of the foam. The cost of the foam is negligible compared to raising the slab in such a small space.
EPS foam is sometimes sold as a light weight fill material for civil engineering projects and the like. For that purpose it comes in giant blocks. I'm sure it's more expensive that gravel, but by the time you factor everything in, maybe it's not that much more expensive, especially considering the fact that the space is small. You could then pour another slab on top, or just lay plywood subfloor "floated" on top of the foam. Tongue and groove to keep it flat, or if you are nervous about that, double layer of sub-floor with the second layer offset so the seams don't line up. 11" of foam can't be justified economically based on insulation value, but if you buy it as a spacer to hold the new floor up above the slab, and consider the insulation as a bonus, it might be justifiable.
Give the high climate impact, and high price, of XPS, XPS is definitely not justifiable.
(Edit...changed "just justifiable" typo to " not justifiable ")