Insulation

[PHAT]

We are fairly new to DIY home improvement, and I'm looking for some advice on insulation/building envelope.  I've tried looking online, but I can't really find what I'm looking for for our situation.

We have a small section of our house that does not have a basement or crawl space - it is open and you can walk/crawl underneath.  Its great for storing bikes/lawn mower, etc, but we are getting some unwanted critters and its not properly insulated so it needs to be re-done.  The floor above in the house is slate tile with heated floor, so all the work needs to be done from below (outside).  Currently what we have is plywood underneath the heated tile, then pink insulation between the joists, then some clear poly plastic as the outside (bottom) layer.  The plastic is the only thing holding the insulation in place...the plastic is all ripped and the insulation is all falling out. 

We live in Canada, so it gets cold, and we get A LOT of snow, but this area stays pretty much dry since its underneath the house.  I want to insulate the floor better so we stop losing heat, and hopefully close things up a bit so the squatters (mouse? squirrel?) living underneath the floor can't get in.

So my questions...Do I need vapour barrier in this situation?  If yes, where do I put it?  Can I just replace the fiberglass insulation and cover it with something stronger to keep it in place? (plywood?  some other kind of sheeting?).  Should I be using blue SM foam? A few cans of spray foam?
Complicating factors:  There are some electrical wires running in the area, so I will need to unclip them and work around them.  There is  also one weird triangle-shaped section that will be a bit more awkward.

Photos of the space to give an idea of what we are working with (and the weird triangle-shaped area with sketchy electrical work)

[NaN]

Why not just seal off the sides of this area under the house (essentially making it a closed off crawl space with a few vents)? That will solve the critter problem. And the pocket of air below should help the insulation issue a little. The wires seem like a mess to deal with.

[PHAT]

Closing it off is a good idea, but a few problems...
1.  It doesn't solve the problem of the insulation that is already falling out (or completely missing).  It just puts an air gap, which isn't much use when its -25C and my electric heater comes on.  I guess we could insulate the whole thing (including the new walls we would build...which brings me to #2...
2.  (I think?) a lot more work and materials required.  If its actually going to be effective at keeping out mice, it needs to be tight everywhere without any little cracks.  And what to do where it meets the ground?  There is no foundation here, so how would that transition look?  We've got lots of other improvements we are working on as well, so I'm hoping for a weekend project here, not an all-summer thing :) 
3.  Most importantly, if its closed in, where will we keep our bikes!?  We already have a (unfinished) crawlspace, and this is actually much more useful!  Its accessible from the front of the house for bikes, and from the back yard for lawn mower and gardening supplies.  So to keep its current usefulness, it would need 2 doors

I'm not trying to cut corners and do a crappy job, but I'm looking for some easy low-hanging fruit that a beginner can actually accomplish in a reasonable time frame (while looking after a 1yo and 3yo)

[bacchi]

https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

3" of closed cell foam would work but it's expensive (figure 9).

Or rigid polyiso with or without batt insulation (figures 6 and 7). That's more of a DIY install as long as you tape the seams thoroughly.

[Mighty Eyebrows]

There are a number of excellent posts on GreenBuildingAdvisor, if you search "insulate a floor":

https://www.greenbuildingadvisor.com/article/the-best-way-to-insulate-a-floor

https://www.greenbuildingadvisor.com/article/insulate-raised-floor

There are a number of variations, but you can get an idea of what they are trying to achieve.

Yes, you probably want to have something to protect the insulation from below, both for critters and for airflow.

[Fishindude]

I'd tear the old stuff out and expose the framing and underside of floor deck, spray foam it, then install new soffit material.
Spray foam is great stuff because it seals everything up airtight as well as providing high R value per inch compared to fiberglass.

[PHAT]

https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

3" of closed cell foam would work but it's expensive (figure 9).

Or rigid polyiso with or without batt insulation (figures 6 and 7). That's more of a DIY install as long as you tape the seams thoroughly.

Thank you!  I was actually reading this website the other day, but google provided me with a different (less useful) crawl space article on the same site.  Elevated/open pier crawlspace is essentially what I have (although I didn't know the term, so my googling was less effective)

As clarification, as I didn't include in the original post - the total area that we need to do is approximately 10x12 feet, so not a huge area.  The joists are 9.5 inches deep, so I have some space to work with (except for the one spot with the questionable wiring).

I'm looking to see if there is an option of spray foam rentals in my area, but I don't think there is.  As a possible DIY alternative...could I use rigid foam cut to fit between the joists, then a couple of cans of hardware store spray foam to seal around the edges? (sorry, this might be a stupid question... but I don't really see how its much different than the expensive spray foam that covers everything)

I also like the idea of the open space with insulation below and then OSB to cover it - I think I could probably manage that on my own.  Question from a beginner...how do I get the rigid foam to stick to the joists so it doesn't fall down before I get the OSB up?  And how do I get the OSB to stay up if the joists are covered in foam?  Can I screw through the foam into the joists?  Talk to me like I know nothing! :)

[Abe]

You can use glue (gorilla glue or equivalent, they’ll have something for you to use at the hardware store) to stick the foam board in place. avoid putting nails in it to maximize the R value. Osb however should be nailed through the foam boards into the joists. Will probably need a nail every 12-18 inches. There is some “thermal bridging” from nailing but it’ll be insignificant, and nailing is necessary to avoid the boards falling off over time.

[Fishindude]

Rather than renting and spraying foam yourself, there is likely a spray foam contractor in the area that would come do the insulation work for you pretty reasonably.

You could do rigid board insulation as noted above but it won't be as easy to get all the little gaps and cracks sealed and filled with insulation.   If you go this route, spend extra time sealing all the little gaps or you will still have a cold room above.

[zolotiyeruki]

I'm looking to see if there is an option of spray foam rentals in my area, but I don't think there is.  As a possible DIY alternative...could I use rigid foam cut to fit between the joists, then a couple of cans of hardware store spray foam to seal around the edges? (sorry, this might be a stupid question... but I don't really see how its much different than the expensive spray foam that covers everything)

I also like the idea of the open space with insulation below and then OSB to cover it - I think I could probably manage that on my own.  Question from a beginner...how do I get the rigid foam to stick to the joists so it doesn't fall down before I get the OSB up?  And how do I get the OSB to stay up if the joists are covered in foam?  Can I screw through the foam into the joists?  Talk to me like I know nothing! :)

I've actually done the foam-board-plus-canned-foam approach, and it works pretty well.  There *is* a foam-specific adhesive (PL300) you can get to adhere the foam sheets to things.  Since you're in a heating-dominated climate, I would suggest that you put the foam insulation up against the underside of your subfloor as a vapor retarder, then fill the remaining space between the joists with batt insulation, for the extra R-value.  You'll need something to hold the batts up, and there are specific products for that, but you can use any sort of poly fabric, like landscaping fabric.  You just need to staple it to the bottom of the joists after installing the batts.  It's easy to cut and fit around things like electrical boxes and wire.

So it'd look something like the attached

[PHAT]

Great to hear that rigid foam + spray can foam is not a terrible idea!  I think I might use it in a couple of tricky spots where the "proper" way might not work so well - either an awkward space, or electrical boxes in the way that I probably shouldn't cover. 

This week I will be calling around for some costing info to hopefully get a solid plan!

[ChpBstrd]

Attach a series of 2x4s perpendicular to the existing floor joists using metal hangers. Close off the new framework on both ends with more 2x4's to make a box. This will do 3 things:

1) Support the existing fiberglass insulation so it does not push down on / detach the panelling / siding.
2) Add almost 2 inches of space for additional insulation. Hell make it a 2x6 if you want uber-insulation.
3) Create a gap between the existing joist and the panelling so you can firmly attach the panelling flat against the 2x4s without messing with the wiring.

The cheapskate solution would be to just use narrow strips of wood to create the gap and not add any insulation.

Another insulation idea is to attach rigid foam to the bottom of the existing floor joists, cutting out only the areas where wiring crosses over a joist. Then attach exterior panelling or soffit material to the floor joists by screwing long screws through the foam. It's often a bad idea to create a vapor barrier toward the exterior of your insulation because moisture could condense between the internal and external vapor barriers, so make it squirrel proof but not airtight.

[NaN]

Good options by @ChpBstrd. I would caution of hanging too much weight from your existing framing unless you do the calculation on how much your spans can take. Since you have slate tile additional deflection could causing tile cracking. In that case the foam panel approach would be the best solution with enough cutouts for wires.