Why Sealing Up Your Rim Joist is Smart (Not Just Because it Saves a Surprising Amount of Money)

I live in the Midwest, and it gets very cold in winter, and very warm in summer. If you’re not insulating well, you’re losing more energy dollars than you should. This year I pulled down a ceiling in my basement due to water damage, and then decided to pull down all the gross old fiberglass batts from the band joist bays (also referred to as “band sills,”) and I’m glad I did.

I found ancient mouse nests, insects, ice (!) that formed on condensation created by temperature differential on either side of the band joist that the fiberglass batts weren’t alleviating, and even places where I could see a sliver of daylight between the sill and the band joist. Who knows what creatures and elements found their way into my living space due to that!

This will be an all-around How-To, and the 4′ x 8′ board cutting info will be toward the end.

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Basic tasks:

• Cleaning out Band Sills

  • So I pulled those old fiberglass batts and threw them away, and then shop vac‘d everything very well.
    • Wear dust masks, gloves, and long sleeves (keep the irritating fiberglass away from skin).
    • Some of the batts had disgusting mold, ice, moisture, mouse droppings, etc. in them, so be prepared.

• Calculate Your Rim/Band Joist Size & Which Material to Buy

  • I attached a calculator spreadsheet (somewhere below) and some instruction how to use it.

• Re-Insulating

  • Then I put 4 inches (2 plies of 2″ thick foam board) in each bay and spray foamed around their perimeters.
    • I wasn’t going to put a whole 4 inches of foam thickness in each bay originally. But after doing some research on appropriate R-values for my climate zone, I eventually realized to do this right, I needed more than just 2″ of foam.
    • It also may have been a requirement of the utility’s reimbursement policy to have a certain R-value. So look up your utility’s policy on efficiency reimbursements, and find out how to meet their qualifications.

• File Your Utility Rebate

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Animation:

 

Next Page: Supplies

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Supplies

If you notice the links under “Materials” and “Tools,” it’s because I linked to Amazon searches for those things, in case you want to do this project but need to load up a cart.

Materials:

• 4 x 8ft Foam Boards (Polyisocyanurate or Polystyrene)
  • You’ll probably need to go to your local home store for these as I don’t find them online for a reasonable price.
• Spray foam insulation – might as well buy a case of cans at least
• 3/8″ flexible plastic tubing for pushing over the tip of your foam gun, so you can get to the hard-to-reach nooks and crannies.
  • Hopefully a 25′ roll will do it.
• Work gloves that are tight enough to work in, but you don’t mind ruining with spray foam
• Work pants, long sleeve shirt, shoes, hat, etc.
  • DON’T GET SPRAY FOAM ON YOUR SKIN OR HAIR, it’s a pain.
  • Foam WILL GET EVERYWHERE, especially on your body. It’s weird stuff, it’s on the move, and has a mind of its own.
• Drop cloth or plastic
  • If you have a concrete floor and don’t care about foam falling on it, fine. But it WILL fall on your floor, and it DOES NOT come out of carpet.
• Dust masks
  • Especially for the cleaning part.
• Ear plugs
  • You may be logging a lot of loud shop vac time.

Tools:

• Shop Vac
• Spray foam Gun
  • I finally broke down and bought one of those $50 spray foam insulation guns, WORTH EVERY PENNY.
  • I left a can on the gun and turned the knob closed–9 months later I scraped off the old stuff from the tip with a utility knife, shook the whole thing for a while, opened the valve, and pulled the trigger. It came spraying right out. Now I’m a believer–spray foam on tap FOR EVERYONE! Not just for billionaires anymore!!!
• Reciprocating saw, with PVC, metal, or multipurpose blade
• Pen
• Tape measure
• Utility knife
• Chalk line, long level, yardstick, or other straight-line device or straight-edge tool
• Shims (can be made from scrap foam)
  • Why shims? Sometimes you want to prop up the blanks and create a gap so you can spray under them while having both hands full working the gun and articulating the flexible tube. Especially true on the long runs on the ends of the basement/crawl space. You can pull them out later, or not.
  • I also used regular door jamb shims as a tool to pry up some of the blanks, so think about sticking some of them on your step stool tray, or maybe a paint scraper…
• Work lamps and/or head light
  • The basement may seem lit well enough, but when you’re poking your head into a bay and blocking all the light, that when you’re glad for point lighting.
• Step ladder (Cosco brand folding “Work Platforms” highly recommended, again: WORTH EVERY PENNY, and will serve you in many other jobs to come.)

 

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See NEXT PAGE for help on figuring how much foam board to buy.

Figuring How Much Foam Board to Purchase

I like to figure material purchases out beforehand, because when it comes to getting bulky items from a home center, I either have to clear out my minivan (child seats and whatnot), borrow a pickup truck, or buy enough stuff to get the home center to deliver. So if I buy too much and want to take some back, or didn’t get enough, it’s a pain.

Download: Band Joist Insulation Calculator

Attached is a spreadsheet for calculating How Many Sheets to buy. Here’s how to use it:

Figure Square Footage

1. Figure out the perimeter of your foundation

• Write each wall length (in feet) in the left column, name them if desired in the right..
• If you have more than 6 walls, right-click on one of the the row numbers 3 through 8 and click “Insert”. (example: Row Numbers are on the left, you right-click right on a number and choose “insert”).
  • If you have less than 6 walls, just enter 0 or leave blank the extras.

2. Enter in a joist bay height (in feet)

• I put in 1ft to add a cushion. If you really want to get it more on the nose, try 9″ (convert to “0.75” for ft), if you have 2″x10″ joists.
• I ended up with 2 extra whole sheets because I wasn’t sure how much scrap there’d be, so take it from me and be more accurate with your inputs.

3. This cell doubles the square footage

• Since you’ll probably need more R-Value than one 2″ thick sheet will afford.

Product Data: Find some sheets of products on Home Center websites, start entering in their specs

4. Sq Ft per each sheet

5. Name of the product

• (I hyperlinked to the home center’s product page for printing out later)

6. Price per sheet

7. R-Value per sheet thickness

8. This cell tells you the cost per square foot of the material

9. Here’s your Approx Total Cost

• Total Sq Ft x Cost per Sq Ft (#3 X #8)
• The green cell is the lowest cost per unit of feet

10. Cost per R-Value

• Takes total cost divided by R-Val per Sheet Thickness (#9 / #7)

11. The number of sheets to buy

• (#3 / #4)

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Red Arrow shows the product I chose. Why?

Why not the 16″ wide sheets?

• Frankly, I don’t know what these are for… do they fit between the ribs on pole barn roofs? My joists are 16″ on center, meaning there’s actually about 15″ between the boards. So I would have to trim each of these foam sheets down from 16″ wide to about 14.25″ or so (to leave a gap for  spray foam). A big long cut for nothing.
• And they’re the most expensive total cost (the red cell under “Total Cost / R-val”, red equals worst value.)

Why not the Laminated XPS sheets?

• No idea. Now it looks like a decent option. I’m guessing the lowest R-val per joist bay for the same amount of work as the others convinced me. It has the lowest TOTAL COST to cover the sq.ft., but it would cost about the same as Foamular 150 4×8′ when R-val is factored in. And it may not even had enough R-val in 2 ply to qualify for the rebate program…
• I also don’t think this material holds up as well to moisture.

Why the Foamular 150 2″ thick?

• I think I ran into a big stack at the home center with a “$23.50 per Sheet” sign on them, and made my little spreadsheet go out the window.
• I also think it had to do with ease of working with the material.

Why not the Poly Iso Roof Decking?

• I really wanted to use Poly Iso material, because I’m under the impression it deals with condensation and other moisture better than Poly Styrene, and has a better R-val profile. .
• And I didn’t know about the idea of cutting through the fiberglass layers–is that going to wear my blades down faster? Do I have to wear a respirator? Seems like this stuff is made for putting on large flat roofs so it doesn’t need to be cut into little pieces 35 times per sheet–could be a real pain in the ass…

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See Next Page for Foam Board Sheet Efficient-Cutting Diagram

DIAGRAM – EFFICIENT CUT OF 4×8 FOAM BOARD SHEET, FOR BAND SILL INSULATION

Not Every Blank is a Short Rectangle!

This will be obvious as you do this project, but not every blank is a short rectangle that fits between 16″-on-centers joists. On the ends of your foundation you’ll have the spans where the floor joists run parallel to the band joist, and you’ll have to cut LONG pieces. I staggered my plies, if it made any difference in ease of install or insulation value, I don’t know. But the point is, some of these sheets you’ll want to cut long strips.

Tip: To Mark Boards for Cutting: Use a Chalk Line, Laser Line, or Marker & Straight Edge

CHALK LINE:

• I put my board on top of a little platform (sawhorses) and when along each edge and made pen marks according to the print above.
• Then I’d make a little cut with the utility knife at each mark, to hold the chalk line end.
• Take a chalk line, set it into the cut, pull it across to its corresponding cut or mark, and snap a line.
  • NOTE: I could not get chalk to stick to this foam worth a damn, but if I snapped it good and then cut right away, it worked alright.

LASER LINE:

• You kinda have to make sure your board is clamped down, OR if you have a laser that has those sharp needles on the bottom and can stick it to the board without getting in the way of cutting, try that.

 MARKER AND STRAIGHT EDGE:

• I started doing this more toward the end. I had a 6ft level and a 3ft level, and would just mark off my lines with a pen, then connect them with a level-as-a-straight-edge, and draw my line.
  • Certain pens mark on this foam well, you’ll have to find what works for you.

The tool I used for cutting was a reciprocating saw with a metal or PVC blade on it.

At first I tried a PVC hand saw, and that was a joke.

INSTRUCTIONS: Dry Fitting Workflow

It’s smart to dry fit at least a half-dozen bays at a time. Even after you’ve measured several joist bays and determined they’re all about the same width, & therefore you cut a bunch of blanks out of the 4×8 sheet, you’ll still have to do more trimming and cutting to make them fit around protrusions.

• Cut up a board and get your 30 to 35 pieces.
• Walk down a row of joist bays, and drop 2 pieces on the floor under each bay. (Let’s call these “BLANKS” from now on, so we’re not confused)
• Get your step ladder, equipped with:
  • A pen that marks legibly and easily on your foam board
  • A tape measure
  • A utility knife and/or drywall hand saw
  • A reciprocating saw
    • NOTE: I scooted a little trash can along everywhere my step ladder went, and rested my recip saw on it. Cutting a hole for a pipe is quick with that saw, but makes a lot of crumbs, so I did it over the trash can. Still makes a mess, but at least you can toss your scraps in as you go.
• Move down the line, dry fitting the blanks in the bays.
• If you have a bay with a protrusion, that protrusion will often require a different trimming for the 1st ply that goes in than the 2nd ply (See YouTube animation video or graphic below).
  • Measure to the protrusions in the bay and mark them up on the blank with your pen.
  • Cut them to fit, with enough room to get in later and squirt spray foam (about 1/2″ all around).
  • If one blank’s cutouts are different than the other blank in the same bay (happens unless a protrusion is totally perpendicular to the band, like a sill cock), then:
    • Write yourself a note on the blanks, like “1st”, when you put them back on the floor below their joist bay.
    • Or, put the first one in the bay and the 2nd one on the floor for later, when you’re moving through spray foaming.
    • Sometimes you need to get even more creative with cuts than the graphic below shows, like cutting a blank into 3 pieces to get them to fit around a pipe.
• Once you’ve got a line of at least 8-to-10 bays that have been dry fit (, you can confidently move through it again with your spray foam gun, and fill in the cracks.
  • NOTE: Why get at least eight bays’ blanks lined up before getting your foam gun ready?
    • Because that’s about how long a large can of foam lasts.
    • AND Because you don’t want to stop constantly when you’re foaming. Even with the gun version, you may have to push 3/8″ tubing over the tip to get into certain areas, and you don’t want it sitting there curing while you take ten minutes to get a couple blanks fit up.
  • Wear gloves, long sleeves, pants, shoes, and hair covering–all of which should be “trashable” garments. Also wear: a head lamp
    • (I’m not being a safety nanny here–trust me–I’ve cleaned enough of this stuff from my arm hair and fingertips to cover up.)
  • Get your spray foam gun and a can of “Pro” spray foam (it screws into the gun)
    • Be ready to push some 3/8″ ID plastic tubing over the nozzle of the spray foam gun tip, depending on how much structure is in your way. There were at least a third of my bays where I wouldn’t have been able to foam them without cutting an appropriate length of tubing to squirt the gun through.
    • TIP: Start on the part of your basement with the least protrusions and the easiest access, then get make it harder on yourself as you get the hang of it.

Animation I made to illustrate how each dry-fitted piece may be different, so cut ahead of time and either mark them, or put one in the bay and one on the floor, or put them in order on the floor… whatever helps you.

Feels Like Much More Could be Said About Technique…

I don’t want to spend another thousand words here on little tricks I learned. By the time you get done with your band sill, you’ll be an expert… right when you don’t need to be anymore. And of course, it ain’t rocket science.

Tip: consider tracing and labeling your covered-up wiring on the face of the pink foam board.

One other tip I found out in retrospect: Consider using a good sharpie marker to label and trace out any Romex home wiring you may be covering up. When the radon mitigation specialist visited, he needed to cut a hole in the joist, and I couldn’t remember if wires ran down the middle of the joist or along the top (on one of the ends of the foundation where there’s long runs instead of 16″ bays). Having a black marker trace of the approx routing would’ve told me if we can just cut through it with a hole saw or have to yank a section of the insulation back to be sure.

Now this is a moot point if you come by later and spray a couple coats of fresh paint all over your basement, as was our fate. Not sure what to do then other than just deal with not knowing exactly where your wires are.

Next Page: The Economics of This Thing.

Save Money:

Make Sure to Get Your Rebate Filed!

As a last note, DO NOT FORGET to go on your utility’s website and see how wonderful progressive arm-twisting legislation has mandated the electricity producers to reward you (their customers) for finding ways to pay them less money over time through efficiency upgrades! Often, they’ve paid substantial portions of my energy upgrades. So if you have to spend 20 minutes filling out paperwork to get a $600 check that pays for most of your project, why not?

Just make sure you check the specs beforehand. For example, it may say you have to achieve a certain amount of R-Value to qualify for a rebate. So plan accordingly.

This Should Reduce Your Utility Bill Enough to Be Worth the Investment

It may be difficult to justify all this work for such a small, narrow strip of surface area compared to the rest of your home. But it makes a difference when done well. There are probably a lot of protrusions through your band joist for things like outdoor outlets, dryer vents, HVAC lines, utilities, etc. So when the various slivers and holes are sealed up, the fewer places for as much heat transfer to occur.

It’s not just the protrusions to the outdoors, but near your band joist there are likely many holes drilled up into the stud bays of the 1st floor’s walls for wiring and plumbing to pass through. When you shove 4 inches of insulation into the band sill, you cover those little pathways for insects, rodents, and especially air to move through. So you’re effectively separating the wall system from the floor system. Often when it comes to insulation, compartmentalization is your friend (insulation is another way of saying compartmentalization).

The air inside our walls can get very cold in winter, to the point where I could place my finger in front of the holes in the electrical outlets and feel little “pin points” of concentrated cold (or heat transfer, in reality). So I went around to all the outlets and spray foamed the insides of them, and that heat transfer pathway mostly went away. Same concept when you’re plugging all the holes the plumbers and electricians originally made through your band joist and through the sill plate, into the stud bays above.

Conclusion:

This was a weird project, but ultimately rewarding. I can’t use 2 winters’ utility bills with a straight face as a sample size to prove a savings bump (hey-I guess it actually works year-round to slow energy transfer, not just winter). But I really feel like the house is tighter and more regulated. It’s that same feeling I got when I dumped a truckload of cellulose in my attic years ago and went on a caulk and spray foam tirade of outlets and windows. Both projects produced results that I consider more than subtle, and I feel it’s been reflected in utility bills but also our comfort level.

SO Hot…

There are times when it’s SO HOT OUTSIDE! And our asphalt roof is just cooking our house and the AC is running hard. But since the cellulose upgrade and various other weatherizations, it seems like we sustain that conditioned air setpoint for longer, and the compressor/blower intervals are shorter and farther apart.

SO Cold…

There are times when it’s SO UNBELIEVABLY COLD AND WINDY outside, and our house is basically in a Reverse Blast Furnace, with the bellows blowing full bore at our house; just wicking, pulling, ripping the heat through the walls as though you could watch a mercury thermometer drop in real time. But after this band joist project, it seems like those days are fewer and less intense. That’s what I mean by “Our Comfort Levels Increased,”

I also got similar “efficiency bumps” (real and/or perceived) from a couple HVAC technology upgrades; when we got a new furnace and AC unit in 2009, and when we got a Nest “learning thermostat” in 2014. It really makes a difference when your thermostat understands no one’s home, and therefore doesn’t run the equipment to setpoint.

DO IT!

So yes, Go for it, retrofit your home for energy efficiency. It’s comfortable. And if you don’t spend too much money over budget on the projects, and get rebates, they may pay for themselves in your lifetime or even immediately.