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Community and Q&A

Insulating floor of mudroom before closing it off

hobbitrock | Posted in General Questions on

I’m building 5′ by 8′ mudroom addition where there previously had been a dilapidated porch-like structure. I’ve poured the foundation and secured the sill. The concrete foundation is about 5 inches above soil, then 6 x 6 sill. I will be framing the floor with 2x 6. I’m stuck with these dimensions because of grade etc. This “crawl space” is NOT connected the the main house’s crawl space and will be 100% inaccessible once the subfloor goes down. Also, foundation is non vented.
Anyway- the plan is to-

air seal between sill and foundation with canned spray foam, then apply rigid foam around perimeter in a cut and cobble fashion.. The goal is to fully air seal the perimeter

then…..6 mil plastic on soil weighted down with gravel and rocks

then mineral wool between floor joists with some sort of cleats or pipe strap to keep it from drooping down over time.

What am I getting wrong? I don’t want to make it more complicated than it needs to be. It’s a 150 year old house and this is a tiny addition. But I also don’t want to create a tropical eco system under my floor.
Thanks in advance.

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  1. hobbitrock | | #1

    Additional info- I'm in Climate zone 5. There will be no plumbing, hvac, or elec buried in this floor space. I will be be heating the mudroom on an as-needed basis with some sort of in-wall electric heater on a timer.

  2. GBA Editor
    Martin Holladay | | #2

    I wouldn't have chosen your details, but it's probably too late to make changes. The best approach would have been to install a concrete slab between your frostwall foundation. The frostwalls would be insulated on the interior with vertical rigid foam, down to the footings. Then, under the slab, you would install a compacted layer of crushed stone; then one or more layers of horizontal rigid insulation; then a layer of polyethylene; and then a 4-inch-thick concrete slab.

    It's probably too late for that.

    Inaccessible crawl spaces that aren't high enough for human access are usually illegal -- and they are illegal for a reason. If you can't get in there, you can't inspect anything or fix anything. Once the mice set up nests between the floor joists, what are you going to do?

    Assuming that it's too late to pour a slab, you should probably just go ahead with your plan and cross your fingers.

  3. hobbitrock | | #3

    Thanks for the reply Martin. Sounds like a call to the mason is in order. I've got other areas of the house that are inaccessible in this same way but that was just how the old timers built it. Kind of frustrating.

    So, going forward, even if I retrofit the assembly with a slab up to the level of current foundation, then attach sleepers, I still have a 5 1/2 inch joist bay to contend with just due to it's relationship to the main house. Should I fit rigid foam into the bay towards the bottom of the joists, spray foam the gaps then fill with mineral wool? Or should I just leave the joist bays uninsulated because I already air sealed the perimeter?
    Or....if I sealed off the bottom of the joist bays with pressure treated ply wood for critter control and then insulated the rest of the bay with mineral wool or rigid foam or combo of both.....?

  4. GBA Editor
    Martin Holladay | | #4

    I'm not necessarily advising that you install a slab -- although it is certainly worth considering. Now that the concrete foundation has already been prepared with joists in mind, switching horses in mid-stream gets complicated.

    If you do end up installing a slab, you would probably need to raise the foundation walls with poured concrete or CMUs. I'm not sure whether it's worth it. You would also need to protect the existing rim joist of your house with some type of peel-and-stick product (like Ice & Water Shield) where the new slab butts up to the rim joist.

    The final height of your slab would depend on whether you wanted to install 2x4 sleepers. Lots of ways to do this ... or you could just go ahead with Plan A, because it's too complicated to do something different at this point.

  5. hobbitrock | | #5

    Thanks for that. I wouldn't really be able to raise the height of the existing foundation walls without it being a pretty big deal. I used PT 6x6 as a sill instead of traditional sill and rim joist. This was at the suggestion of the mason because it tied in with the 6x6 white oak sill on the main wall of the house. I bored out, used deep well socket and got a good solid connection on the foundation pins which had been set high to facilitate this design.
    Looking back I realize I could have done it differently in a few different ways but there was a lot going on at the time. And the plans had been approved by the municipality which is probably worth mentioning. So here we are.
    Anyway, let me ask you this- since I already have a good connection for floor system framing to the foundation surface, wouldn't the retrofit slab be there just to fill up the air space? If so then it's not that intimidating because I could just pour it to be level with the perimeter foundation and it wouldn't be any higher than any of the other concrete. Sleepers could go in or not. I wouldn't really need them unless I wanted to tag the floor joists into them in a couple of spots.
    I could still stick with the 2x6 floor joists which I'm committed to because of the design but it seems like sealing them off with rigid foam directly on top of the slab cut and cobbled and spray foamed into each bay then filling the bays with mineral wool (or not)......would be a better design for an inaccessible space.

  6. GBA Editor
    Martin Holladay | | #6

    I don't see any advantage to the slab unless the slab allows you to eliminate the floor joists.

  7. hobbitrock | | #7

    Got it. Thanks Martin

  8. hobbitrock | | #8 one more question. Forget the mineral wool. I'm just going to air seal the perimeter with rigid foam and great stuff and also insulate between the joists with rigid foam boards as well. I was hoping to install 2 layers of 2 inch for greater r value but I'm also realizing this might be overkill. I'm aware of the need to stagger seams. I have foam board adhesive on hand as well as caulk, tape etc. Question is.....
    Reflective Polyiso or pink/blue xps? If I used 2 layers of the polyiso would I need to rip off the one or more of the reflective facings so I didn't have multiple vapor barriers? And if I use 2 layers of XPS would I need to install a 6 mil plastic vapor barrier under the subfloor?
    I've read several threads on GBA that seem to sort of apply but I'm still unsure about which product to use.
    As for environmental considerations.....I already have a fair amount of 2 inch XPS in my garage that has been cut and is no longer returnable so there's that incentive to use it up. Thanks.

  9. user-4524083 | | #9

    Joseph - Would it be possible to build the floor outside of the foundation, and then set it in place afterwards? Or build it on its side ( 5' high) and drop it in place? If so, then you could seal the bottom of the joists with plywood and seal it very well, then drop it in, insulate it with mineral wool, put in your sub floor, and keep it dry until the roof is on. This would pretty much eliminate the potential mice issue. You may even be able to put 2" of foam under the floor joists and then the under side plywood. I'd still put poly on the dirt floor, and seal it to the foundation wall, to reduce ground moisture. Just a thought. Good luck!

  10. GBA Editor
    Martin Holladay | | #10

    Kevin raised some good points about buildability. Think through all of the steps before you decide how to proceed, so that you know that you have access to do what you intend to do.

    Multiple layers of foil facing in stacked polyiso are not a problem. The foil-faced polyiso is already vapor-impermeable -- one layer of aluminum foil is just as impermeable as four layers of aluminum foil, so more (or fewer) layers of foil don't change the permeance in either direction.

  11. hobbitrock | | #11

    Thanks for the responses. As of now the poly on the ground is installed, sealed to the foundation, gravel on top of that. The short sides of the rim joist are insulated with 2 inch XPS and the joists and hangers are in. At this point I can't really build the floor as a separate assembly but.....
    I did install 1.5 X 1.5 inch PT cleats on the bottom edge of each joist so I'd have something to attach the foam board to since I would be installing from above. I figured I'd have a good chance to make a perfect seal with the combination of foam board adhesive, caulk, tape and spray foam.
    This leaves a 4 inch deep cavity ( 2x6 joists) so 2 layers of 2 inch foam board would fill it up nicely.
    This idea actually sprung from a similar idea to yours, Kevin. I was thinking I'd install the PT cleats then glue and caulk 1/2 in. PT ply into each joist bay and then fill with Roxul. I could still do that but I think I'd be more inclined to stick with rigid foam and just reduce it to 3 1/2 inches rather than 4 because of the 1/2 inch ply.
    I had sort of let go of the idea of the ply since I was using rigid foam and it would be less cozy for mice. Now I'm thinking about going back to it just to make it like fort knox. Overkill?
    Martin, thanks for clarification about the polyiso. I thought I had seen a thread somewhere on GBA where multiple vapor barriers on the polyiso had come up as an issue and some guy was ripping the foil off of one side. Sounded like a total nightmare. I guess what's eluding me is this....if the perimeter is insulated and unvented and there's plastic on the ground running up the sides of the foundation and there's a vapor impermeable insulation situation in the floor does anything dry out?

  12. GBA Editor
    Martin Holladay | | #12

    Q. "How does anything dry out?"

    A. If you want to build an unvented crawl space, you need to include details to make sure that the crawl space isn't damp. In some cases, it's a good idea to run a dehumidifier down there for a few months after construction is complete, to make sure the air is dry. In most cases, codes require a little bit of air exchange and the introduction of some conditioned air.

    Which is a long-winded way of saying that you need access to your crawl space -- as I pointed out in Comment #2.

    For more information on sealed crawl spaces, see Building an Unvented Crawl Space.

    You're in a tough position, because you got quite a ways down a bad path before you realized it was a bad path.

  13. hobbitrock | | #13

    Great article. So the only thing it seems like I might be able to do is install a floor register for air exchange...? Or go back to the mineral wool idea to allow vapor to dry to the inside and run a dehumidifier in the mudroom as needed?

  14. GBA Editor
    Martin Holladay | | #14

    This is a judgment call. How damp is the soil in this area? Does your building have wide roof overhangs? Does the soil near this room slope away from the foundation? Are their gutters at the roof eaves? Are the gutters connected to conductor pipes that convey the roof water far from the building?

    Is the soil well-drained or full of clay?

  15. hobbitrock | | #15

    The soil isn't too bad but yes there is a lot of clay in this town. I didn't dig the 4 ft piers so I don't know exactly. Usually , out on the job, we'll hit clay when digging footers in basements on the low side of town. My house is up a little bit.
    The over hang on the main house is 12 inches. No gutter yet, but I'll install them this summer. The basement in the main house which only accounts for about 40% of the Sq. footage stays pretty dry even though the house is at the bottom of a sloped yard. French Drains are in the plans and I had thought about installing one close to the mudroom addition. There is still some grading to do as well to slope the soil away from the structure as much as possible. So, long story short, the war on water is being waged piece by piece.
    Honestly it seems like I might want to scrap the idea of insulating the floor or else insulate it with EPS since it's more permeable. Seems like drying to the inside is the only option and since I've insulated the perimeter really well, the floor shouldn't get too cold. Could I get by with that?

  16. GBA Editor
    Martin Holladay | | #16

    If the walls of your crawl space have an adequate layer of insulation, there is no need to install insulation between your floor joists. That said, I wouldn't expect much upward drying through your floor assembly. Your subfloor (plywood or OSB) is a pretty good vapor retarder, and most types of flooring have a low vapor permeance as well.

  17. hobbitrock | | #17

    Thanks for the response. So maybe a floor vent is the way to go?

  18. GBA Editor
    Martin Holladay | | #18

    Don't try to mix systems. If you create a vented crawl space, the insulation on your crawl space walls becomes (more or less) worthless, and you need insulation between the floor joists.

    You are overthinking this. If you are really worried, built a trap door in the floor so you can stick your head down there every three years to see what's going on.

  19. hobbitrock | | #19

    Thanks Martin. That sounds like the way to go.

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