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Community and Q&A

Old house – insulating basement ceiling vs rim joists?

dan_s_d | Posted in General Questions on

Hi all,

Thanks to you all i have a nice spray foamed conditioned attic space now supporting my AC equipment up there. It’s working out great.

Now on to the basement. The house is 125 year old foursquare type in the Boston area. It has a field stone foundation and partially finished basement. The basement floor is level and epoxied and the walls are recently pointed, lime coated and look nice. There are no water or moisture issues. We use it for a gym and a workshop. The first floor flooring is original hardwood with no subfloor. It’s mostly in great shape, but there are a few spots where if you shine a light, you can see it from the basement. Eg, the floor isn’t air tight.

From the previous owners, the basement ceiling is packed with fiberglass batting and covered in tyvec. Mice have been in it for awhile so you can see mouse poop and urine stains on the tyvec. Occasionally you can hear a mouse running around. It’s nasty and all has to go.

My goal is to get rid of the funky old insulation, maintain the first floor temperature, seal the house better against mice and air, and use as little labor/materials/effort as reasonable. I’m ok if the basement continues to get down to around 55F, but of courser warmer the better.

The question is, do i reinsulate the basement ceiling or can i eliminate all of that and just insulate the rim joist around the perimeter with 2″ rigid board and spray foam in a can? It never gets below 55Fish in the basement in the winter, the steam boiler does put off some heat.  I don’t really want to insulate the entire walls as that seems to be quite involved and we’d lose floor space. I guess worst case i could try doing the rim joists and always add the ceiling insulation again later?

Thanks a ton,

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  1. PAUL KUENN | | #1

    Glad you have a rim joist at that age instead of the joist buried in the foundation like I run up against. That 55F still has a big delta T if you keep the upstairs at 70ish in winter so heat is going down as well as up with air leakage. Insulating the band (rim) "boxes" will certainly help. I usually drywall basement ceilings and blow in cellulose to within 2' of walls around the perimeter. I'll use fiberglass batting at the outboard perimeter so that we can get to plumbing and wiring along edge if needed (or new mice holes).

    1. dan_s_d | | #4

      That's an interesting idea. So you think insulating the basement ceiling is still the way to go? I suppose even if I did that, I'd still want to seal/insulate the rim joists? I need to make sure I seal up any mice entry holes or else they'd have a happy home in the blown cellulose.

  2. Expert Member
    BILL WICHERS | | #2

    There is no problem insulating the rim joist with rigid foam panels and canned foam. I like EPS for this purpose since it allows a little bit of drying since EPS is more vapor open than XPS and especially foil faced polyiso. Cut the rigid foam panels a little small, leaving about a 3/8-1/2" gap all the way around. It helps to cut the edges with a bevel too, which makes the canned foam push the panels against the rim joist instead of trying to pop them out. You want to avoid any air space between the rigid foam panel and the rim joist. A hot wire cutter is the best way to cut the foam.

    I like to use Loctite's Tite Foam to seal in these insulating panels. Tite foam is a little more dense and tougher than the regular Great Stuff product. Tite foam also costs quite a bit more, so keep that in mind if you want to use it.

    I find it helps to cut pieces of wire to hold the panels in place while foaming. I cut pieces of 12 or 10 gauge copper wire an inch or two longer than the gap between joists. I place a foam panel against the rim joist, then a I press in a wire so that it locks against the joists (the ends bite into the wood a little), and holds the insulating panel in with spring force. I then adjust the position of the panel until the gaps are fairly even all the way around. I set all the panels like this, then make a second pass to foam them all in place. Doing it this way lets you do the foam rapidly so that you don't have issues with foam setting up in the straw. It goes fairly quickly once you get started -- cutting all the foam panels takes most of the time.


    1. dan_s_d | | #3

      Great info, thanks. What is your take on how the insulated rim joists will perform compared to the old basement ceiling insulation in terms of heating efficiency of the house (1st/2nd floor) itself?

      1. Expert Member
        BILL WICHERS | | #5

        The two areas being insulated aren't entirely comparable. Basement ceiling insulation is there to insulated the conditioned living space from what I would assume to be an unconditioned basement. That does work, but is not idea.

        Insulating a rim joist is a good idea, but when done INSTEAD OF insulating the ceiling, you should really also insulate the basement walls to seperate what is now a conditioned basement from the unconditioned outdoors. The idea is to insulate all surfaces that are interfaces between conditioned living spaces and the unconditioned outdoors. Insulating the rim joist alone leaves the basement walls to lose heat.

        You can certainly insulate only the rim joist for now, and gain the benefits of a better insulated and air sealed rim joist -- and remember that rim joists are notoriously air leaky so sealing them DOES have a benefit. You just won't have insulated everywhere you need to for a fully insulated space.

        I would personally insulate the rim joist regardless of if you plan to insulate the basement walls at some point or not. Insulating the rim joist and foundation walls is usually superior in terms of performance compared to insulating the basement ceiling.


        1. dan_s_d | | #6

          Hi Bill, It's been over a year, but I do have the basement ceiling insulation out. I've been doing a lot of electrical work now that it's out and things are looking good.

          It hasn't been too bad temp wise in the basement, but now that it's finally getting colder out, I'm seeing 53-55Fish. I also just got a higher than normal heating bill.. time to insulate.

          Looking at Lowes and HD's offerings, I'm only finding 2" EPS at Lowes, but it is faced. Should I keep hunting for unfaced for maximum breathability?

          Also, is there a difference in all the types of canned spray foam? There is Locktite foam for doors and windows, cracks, big cracks, etc.

          I've also considered hiring out a spray foam contractor to just spray foam it all. Would 2" of closed cell work well?

          Thanks again

          1. Expert Member
            BILL WICHERS | | #8

            Unfaced is a little safer for the rimjoist. You should be able to order it through any commercial insulation supply, possibly even from the pro desk at Home Depot. If you're in an area with a Menards (the "green store"), I know they carry 2" EPS, but it's in their you-pick lumberyard area behind the store. Note that you can use the Safe'n'Sound version of mineral woold batts to provide fire protection to the EPS, and also add about R12-13 or so of additional insulating value to the assembly. You MUST use impermeable insulation first, directly against the rim joist, though. Batts alone are a problem.

            I like to use Loctite's Tite Foam for sealing in rigid foam panels here because I've found it to be a more durable materail which I trust to last longer. You could use Great Stuff too, but it's not as dense, so possibly more likely to fail over time. I typically use great stuff to seal penetrations in top plates, things like that, and Tite Foam for where I want the foam to hold something in place. OSI's Quadfoam is also a pretty good product.

            You will want to insulate the basement walls. You can do this with EPS, or with polyiso. I would use polyiso, for more R per inch. You'll need to protect EPS with a thermal barrier (usually 1/2" drywall), but with polyiso you can get one of the versions rated to be exposed in this type of application. The two such products I'm familiar with are Dow's Thermax and Johns Manville's CI Max. Both can be installed on a basement wall and left exposed, so you can put it up quick with the tap-in insulation anchors and be done.

            Spray foam on a rim joist can be a good option, but it's expensive. I would go with spray foam if I had a complex rim joist area with many things I'd have to cut around if using foam panels. You need to use closed cell spray foam if you go this route, and at least a 2" thick layer. If you go this route, I would insulate the basement walls first, then have the spray foam tie in the top of the foam panels to fully seal the entirety of the rim joist area.


  3. nynick | | #7

    Your problem isn't just the rim joists. It's the basement walls and floors. Bill's suggestion of EPS for the rim joists is a good DIY idea. Extending that to the basement walls would be even better. That's the way to go.

    Spray foam is an easy, lazy and expensive solution for the rim joists and walls. Choose this if the budget allows. You can achieve the same result with EPS foam boards on the basement walls.

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