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

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

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.)
Craftsman XSP 20 Gallon 6.5 Peak HP Wet Dry Shop Vac Blower
Craftsman XSP 20 Gallon 6.5 Peak HP Wet Dry Shop Vac Blower
GREAT STUFF PRO 14 Dispensing Gun
GREAT STUFF PRO 14 Dispensing Gun
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
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
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 – 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
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
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 4x8 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.
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
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.
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.
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
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.
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 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 light. Get the highest lumens you can for $25 or less
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.

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