Kris Bunda Design

How To: Efficiently Cut 4x8ft Foam Boards & Insulate a Band Joist

BAND SILL FOAM BOARD INSULATION - MOST EFFICIENT CUT OF 4FT X 8FT SHEET

BAND SILL FOAM BOARD INSULATION - MOST EFFICIENT CUT OF 4FT X 8FT SHEET

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.


Band Sill Insulating

Basic tasks:


Animation:

 

Next Page: Supplies


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:

3/8″ ID clear plastic tubing – for giving your foam gun the reach in awkward places
GREAT STUFF PRO foam – You’ll be needing more of this than you think.

Tools:

Craftsman XSP 20 Gallon 6.5 Peak HP Wet Dry Shop Vac Blower
GREAT STUFF PRO 14 Dispensing Gun
Cosco Pro Tech II Work Platform Folding Step Stool – I own this and HIGHLY RECOMMEND it for this job, because you can put so many tools and supplies on the shelves while working overhead. But they do not sell it anymore
Max Work Platform Project Ladder by Cosco – Because of the shelf and large platforms, I recommend one of these while doing so much overhead work as foaming your band sills
Reciprocating saw – THIS is the tool for the job. At first I thought I was going to do it all with a PVC hand saw. THAT would’ve been dumb.
Reciprocating saw blades – For foam board, the small-toothed metal blades worked best for me
straight edge saw tool – Honestly, you do not need to be this accurate, but in lieu of chalk line, you will want something straight to draw cut lines
Tape measure – Draw marks on either sides of 4×8 boards, then connect them with chalk line or straight edge and pen. You also use this a lot when dry fitting bays with protrusions or sister joists that need adjustment.
I didn’t use one of these for cutting boards, but maybe it would help draw lines to cut – Strait-Line 6041101CD LL120 Manual Level Interior Line Laser
long level – Or shorter 3-foot long one like I used – helps connect your marks and draw straight lines.
Chalk line – You can make your own 8ft-long marks by cutting into the side marks, putting the chalk line end into the cut, and pulling to the other end of the board and snapping. NOTE: Chalk does NOT transfer well to foam board, but should be good enough guide.
Utility knife
shims – These help you position a piece of foam board while in an awkward joist bay, so you can spray foam around it.
LED Work lamps – Light is your friend in this job, and you never get too much of it when you are sticking your head into a dark joist bay. Trust me on this, get a good adjustable tripod LED light AND wear a good head lamp.
LED head light – You will be glad you have this when your fat head is up in a joist bay blocking all the light from your tripod lamp. Get the highest lumens you can for $25 or less

 


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.

BAND SILL FOAM BOARD INSULATION CALCULATOR

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

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

3. This cell doubles the square footage

BAND JOIST FOAM BOARD INSULATION CALCULATOR – MEASURING FOR SQ FOOTAGE

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

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

10. Cost per R-Value

11. The number of sheets to buy

BAND JOIST FOAM BOARD INSULATION CALCULATOR – FIGURING OUT BEST COST PER R-VALUE PRODUCT

Red Arrow shows the product I chose. Why?

Why not the 16″ wide sheets?

BAND JOIST FOAM BOARD INSULATION CALCULATOR – FIGURING OUT BEST COST PER R-VALUE PRODUCT

Why not the Laminated XPS sheets?

Why the Foamular 150 2″ thick?

Why not the Poly Iso Roof Decking?


See Next Page for Foam Board Sheet Efficient-Cutting Diagram

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

PDF DIAGRAM – MOST EFFICIENT CUTTING OF 4FT X 8FT FOAM BOARD SHEETS

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:
LASER LINE:
 MARKER AND STRAIGHT EDGE:
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.

BAND SILL INSULATION WITH PINK RIGID FOAM – ON CUTTING AROUND PROTRUSION, DIFFERENT FOR EACH PLY
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.

Example of Utility Insulation Upgrade Rebates

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.

BAND SILL FOAM BOARD COMPOSITE BLUEPRINT GRAPHIC

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).

Insulate synonyms

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.

 

 

 

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