How to Build and Install a Custom Crawlspace Ventilation System


Do you need to ventilate your attic, crawlspace, or unfinished basement? Would you prefer to build and install a customized ventilation fan as opposed to paying dearly for an overpriced expensive version? Here is a step by step process that can be easily replicated.

Things You'll Need

  • Jigsaw
  • Screwdriver(s)
  • Drill & Bits
  • Masonry/Wood Anchors/Screws
  • Bolts/Screws
  • Fan(s) sized properly to your Crawlspace/Attic/Basement
  • 12"x18" Poly Cutting Board(s)
  • Determine if you plan on using a pull, or push/pull system. If you only have one fan, then you'll be using a push system that pushes the air from your crawlspace/attic to the outside. If you want to use two or more fans, then you'll be using a push/pull system that pulls with one fan and uses another fan to push it out. This is important to know how to mount your fan(s) later. In this example I'm only showing a push system.

    TIP: Make sure your fan is sized properly to efficiently move air through your crawlspace/attic. To do this, you'll need to calculate the volume of your crawlspace/attic. Measure the square footage of your crawlspace/attic (length * width) and then multiply this by the height from the floor/ground up to the floor joists. This will give you the volume in cubic feet. Then divide the total by 15. So the formula will be

    CFM = (Length x Width x Height)/15

    My Crawlspace for Example: (61' long x 35' wide x 3' high)/15 = 433 cfm

    Make sure you purchase a fan with a rating of at least this value. It would also be recommended that you select a fan that can handle the moisture and temperature that it will be operating in.

  • Now we'll start to mount the fan to a faceplate that will be used to cover the ventilation port in the side of your home's crawlspace/attic. For my faceplate, I decided to use a non-absorbent poly cutting board that I picked up from my local hardware store. Whatever you decide to use, make sure it is 1) rigid enough to hold the fan you are using, and 2) able to withstand the extreme moisture and temperatures.

    Before drilling out the marks on your faceplate (polyboard), make sure you are mounting the fan correctly so that the airflow is in the correct direction. In my system, I'm forcing air out so I mounted the face of the fan to the faceplate.

    Set your fan on top of your faceplate aligning it exactly where you want it to be mounted and mark the mounting holes with a pencil or marker.

  • Now, select a drill bit that is the same size or slightly larger than the mounting holes on the fan itself and drill the holes.

  • Mount the fan to the faceplate to ensure you marked and drilled the holes properly. This will also allow you to mark the fan opening that will need to be cut out so that air can flow through the polyboard.

    Using a marker, stick it through the inside of the fan casing and trace the outer rim. Be careful not to cut your hand on the fan blades.

  • Secure the faceplate to something that will allow you to drill holes and cut out the fan opening. Drill a hole (or multiple holes if you're using polyboard like me - the more holes, the better) along the inside of your traced fan opening line. Keeping the drill holes on the inside of your drawn line will make your fan opening hole smooth.

  • Use a jigsaw to cut out the fan opening. If you want a good a good clean line, you may choose to file down any imperfections on your cuts. (Since mine won't be seen from outside, I don't mind the imperfections.)

  • Next, mount your fan to the faceplate making sure to include the opening hole fan guard. Again, make sure the fan is blowing in the right direction.

  • Now, that your fan is mounted and secured properly, take it to the location in your crawlspace/attic where it is to be mounted. Hold the assembly in place so that the fan opening will force/pull air without being obstructed. Mark holes on your faceplate where you plan to bolt/screw it into the crawlspace/attic wall. With it still being held in place, mark on the wall around the outside of the faceplate as a point of reference. (You'll use this line in the next step.) Now drill out the holes with a bit that's sized to your screws/bolts.

  • Holding the fan assembly back up to the same position (using the line you traced on the wall in the previous step) drill your mounting holes into the wall.

    Bolt/screw the assembly to the wall.

Tips & Warnings

  • Select an industrial fan that is rated for the conditions in which it is being used. Industrial fans last longer and require less maintenance.
  • Installing a ventilation system will be aided by installing a vapor barrier as well. These two systems in conjunction with each other will drastically reduce the amount of moisture damage caused in your crawlspace/attic.
  • Use a humidistat and thermostat to improve your ventilation systems performance. Typically, your system should only run in temperatures greater than 50 degrees F.
  • Install a manual switch in line with your fan so that you can enable/disable it at a convenient location.
  • Make sure your volumetric air flow calculations are correct.
  • Make sure your fan is protected by a fan guard on both inlet and outlets so no critters can crawl in the fan causing it to fail.

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