How to Insulate Shed Floor: Existing and New Construction

Building a shed and want to keep it cool in the summer and warm in the winter? A friend of mine was turning his shed into a small bunkie for his kids. Since he lives in a cold climate, he wanted to know how to insulate a shed floor. I told him about several options, and he ultimately ended up lifting the entire structure up to install insulation.

When standing in your shed in the dead of winter, it’s impossible not to notice the chill coming up through your feet. Tons of heat is lost as the cold from the earth sucks whatever warmth your walls and ceiling managed to trap. Insulating your shed floor is the answer.

Insulating a shed floor is straightforward. Don’t get distracted by all the fancy insulation products out there – keep it simple and you can do the job in an afternoon. Here’s how:

• Remove the flooring of your shed
• Put wood blocks under each joist cavity
• Each joist cavity should have at least two support blocks
• Set up a table saw, or use a handheld jigsaw
• Cut XPS rigid foam insulation to fit between your floor joists
• Layer insulation flush to the top of joists
• Re-install flooring

Types of Insulation

When insulating a shed floor, it is hard not to be overwhelmed by the number of choices available. Eco-friendly products such as lamb’s wool and denim are gaining popularity, but the price point negates their viability for a shed.

Let’s take a look at the different types of insulation available for use in a shed floor, along with the pros and cons each offers when installing in a shed floor.

Fiberglass

Fiberglass insulation is still the most popular insulation on the market, despite some of the itchy drawbacks. It offers a high R-value and cuts well. On top of that, the fiberglass is not expensive.

Insulating shed walls and ceilings with fiberglass isn’t a bad idea, as some fiberglass insulation comes with facing. Facing covers both sides of the insulation with paper or plastic, which makes it much easier to handle.

If fiberglass is a good choice for ceilings and walls in your shed, then why not your floor? The floor of your shed is the most likely spot that you will encounter moisture.

Whether the moisture is caused by you, the ground, or outside elements, the floor will get wet at some point. Fiberglass does not like water and it causes it to lose R-value. As well, rodents have been known to make nests in fiberglass and I’ve known raccoons to eat it!

If you choose fiberglass for your floor, then you’ll want to seal it tightly on both sides. If you do, you’ll still run the risk of little critters getting in – it’s inevitable. Once they do, it’s hard to get them out.

Pros

• Cheap
• Offers high R-value
• Comes with optional facing

Cons

• Does not like moisture
• Rodents will dig through it
• Difficult to handle

Rigid Foam Board

Rigid foam board looks a lot like styrofoam – and some of these products actually are styrofoam. However, the type we are looking at is an XPS foam, which stands for extruded polystyrene.

Rigid foam is great for floors because it doesn’t lose its thickness over time. While batts typically have some elasticity that wears off over time, XPS does not. When cut to fit snug in a space, you can bank on that foam staying there forever.

Foam is also moisture repellant – it will not absorb water. If it gets wet, it will not lose much, if any, R-value. This makes it a great choice for floors.

As well, rodents have a hard time making nests in XPS. It can happen, but you won’t find extensive networks of mice tunnels like you would in fiberglass or Rock wool.

Pros

• Easy to cut
• Moisture resistant
• Rigid for life of the product

Cons

• More expensive than fiberglass
• Requires more cutting than batts

Spray Foam

Spray foam insulation is an effective, but more expensive, solution for floor insulation in your shed. One of the drawbacks for spray foam is that you need backing for it to adhere to the surface.

If you are installing insulation on a pre-built shed, you cannot simply drill a hole in the floor and pump it full of spray foam. Unless you installed sheathing or plywood on the underside of the joists, the foam will have nothing to adhere to.

However, on a new build, spray foam is the best choice for R-value. Spray foam offers the highest efficiency, as there are zero gaps around edges that would allow cold air or moisture in.

You can do spray foam yourself, with a kit from a big box store. They are expensive, however, and likely not worth the cost unless you are making your shed into a tiny house or sleep shed.

There are two types of spray foam: closed cell and open cell. The more expensive, two canister spray foam kits at your local hardware store are usually closed cell.

As the name suggests, they have encapsulated air pockets, which resist moisture absorption and provide a vapor barrier for your shed. Use this for your shed floor as the open cell is not durable enough to withstand exposure.

Pros

• Best possible R-value
• Moisture barrier
• DIY installation

Cons

• Most expensive option
• Labour intensive
• Not ideal for pre-built sheds

Mineral Wool

Mineral fiber insulation is similar to fiberglass in that it comes in batts and handles much like fiberglass. The benefit of mineral fiber is that it is much easier to handle.

Mineral fiber does not have the itchiness of fiberglass and is also easier to cut. A simple hand or drywall saw makes nice, straight cuts.

Much like fiberglass, mineral fiber does not appreciate moisture. While mineral fiber is made up of crushed rock and other materials, moisture can get trapped in the batts causing them to compress or sag. Compression and sagging result in air gaps around batts and compresses gaps within the batts, reducing R-value.

Pros

• Easy to handle
• Cost-effective
• Provides snug fit between joists

Cons

• Not moisture repellant
• Rodents can dig through batts

How To Insulate Shed Floor

Insulating a Pre-built Shed Floor

Chances are you already have a shed that doesn’t have an insulated floor. Whether you bought one from Home Depot already built or you made it yourself, insulating a shed floor often gets overlooked. How do you put insulation in your pre-built shed?

There are a couple of methods to install insulation in your pre-built shed. If you can take the floor off, then insulating from the top is easiest. However, if you can’t remove your floor, because it is nailed in or has a finished floor, then you’ll have to go underneath. We’ll take a look at the various methods, below.

How to Insulate a Floor from the Top

Insulating a shed floor from the top is by far the easiest method to insulate your floor. I recommend XPS rigid foam for this installation, as it is the best option for use at or near grade. Here’s how to insulate your shed floor from the top.

1. Remove the subfloor in your shed. This requires moving everything out of your shed and will be the most labor-intensive part of your install.
2. Before you install the rigid foam, you are going to need some wood blocks beneath each space between the gaps to ensure the rigid foam doesn’t fall out beneath the shed. While it does fit snug between joists, movement from above may jar it loose.

Alternatively, you can nail small scraps of wood to each corner of the joist cavity, at the bottom. These will function as shelves for the rigid foam to sit on.

3. Next, it is time to measure each joist cavity. An exact measurement will ensure your insulation fits tight for optimal R-value.
4. Cut your pieces of foam. Use a table saw or handheld jigsaw to cut. A cordless jigsaw is ideal as you’ll be cutting from 4×8 foot pieces. Remember, you can always cut smaller, but you can’t make a piece bigger if you cut too much.
5. Install the foam. XPS comes in 2”, 1.5”, and 1” thick pieces. The thickness you get depends on the width of your joists. If you have 2×6 or 2×8 joists, then using 2” panels and layering them is ideal. Each XPS 2” panel is R10, so if you have three, then you have a value of R30, which is excellent.
6. Re-install your floor. If you have a slight gap between the top of your foam and the top of the joists, it’s fine. An enclosed air gap also serves as an insulator.

Other Methods for Installing from the Top

Insulated floor tiles are also an option for insulating from the top. If you do not want to pull up your floor, and can’t get under your shed, then floor tiles provide some insulation.

They usually lock into place and are very easy to handle. They will not provide as much R-value as batts or XPS, but they offer a sturdy alternative.

If you don’t want to put foam between your joists, why not put it over the top of the joists? If you have plenty of headroom in your shed, then layering a few sheets of XPS over the joists is possible. You’ll have to adjust your door openings and attach your subfloor through the insulation into the joists.

Finally, you can install fiberglass or mineral fiber batts instead of rigid foam between your joists. This isn’t ideal because the batts are not as moisture resistant as XPS. Rodents are also more fond of batts, and having batts exposed to the outside on one side isn’t ideal.

How to Insulate a Shed Floor from the Bottom

Insulating a pre-built shed from the bottom isn’t always the easiest, but it can save you from removing your floor and emptying all the gear from your shed. Unless your shed is raised enough so that you can get under it, you’ll need to jack the entire structure.

Jacking a shed isn’t as hard as it sounds, with the right equipment. A farm jack, or automotive floor jack, will easily lift a shed enough so that you can fit under it. Do not use the jack alone – once jacked, be sure to put blocks underneath the shed to support the jack in case of jack failure.

We recommend using batts for this installation. Rigid foam is ideal, but batts are much easier to handle underneath a structure. Let’s take a look at the best way to insulate a pre-built shed from the bottom.

1. If you need to jack your shed to get under it, then do it now. If you have a particularly large riding mower, consider removing it to make the structure lighter and jacking easier.
2. Once jacked, get under the shed and measure the joist cavities.
3. Use fiberglass or mineral fiber batts and cut them to size to fit each cavity. Use batts sized to fit your joist width. If you have 2×8 joists, you could opt for two layers of 2×4 batts.
4. Install batts – make sure they fit snug but are not compressed. Compression removes internal air gaps and reduces R-value.
5. Once batts are installed, you’ll want to enclose the joist cavity with OSB sheathing, treated plywood, or hardware cloth. I recommend hardware cloth, as it only requires a stapler and you can overlap layers. Hauling a large sheet of plywood underneath a cramped space is difficult, at best.
6. Staple hardware cloth onto joist bottoms. Use lots of staples. Overlap hardware cloth by at least an inch to ensure all cavities are protected from rodents.

Other Methods

Spray foam is another method to use when insulating from the bottom. If you have a tractor or other machine that can lift the shed, then you can use spray foam.

Although expensive, spray foam is by far the best insulating option for a structure. Once the spray foam is on, install hardware cloth or treated plywood.

Rigid foam is another option, and much cheaper than spray foam. If you can jack your shed up enough, then XPS foam is ideal. Use cheap metal strapping or run several strips of treated strapping across the bottom to hold rigid foam in place.

Another option is using a radiant barrier. Radiant barrier looks much like foil bubble wrap. It provides decent R-value at minimal size. It installs with staples and is easy to handle.

It doesn’t offer much insulation value, but when installed in joist cavities, provides a barrier between ground and shed.

Insulating a Shed Floor: New Construction

Installing insulation in a shed floor is simple when done during initial construction. When framing your floor, consider adding sheathing or treated plywood underneath your joists. This will allow you to safely add any type of insulation, without worrying about excessive amounts of moisture or rodents getting in.

If you are like my friend and wanting to make your shed habitable for people during the winter, then you’ll want a vapor barrier between the batts in the floor and the structure. When you heat the inside of the structure, you’ll want to ensure the moisture in the shed does not get into the batts below.

I recommend the use of closed cell spray foam for new construction. Closed cell foam is ideal if you have a larger budget.

You can raise one side of the shed and apply spray foam to one side, then do the same to the other side. If you cannot afford closed cell spray foam, rigid foam or fiberglass batts are a good second option.

Installing closed cell spray foam in a cold climate requires a minimum of R-30 in your shed floor, for a heated space. Each inch of spray equals roughly R-6. Therefore, you’ll need at least 5 layers to achieve R-30. Here’s how to apply spray foam to the floor of your shed construction.

1. After framing your shed floor, install your flooring.
2. Lift one side of shed structure, high enough that you can access at least half of the structure to spray. Begin spraying your foam.
3. When finished with one side, slowly lower and lift the other side. Spray the other side.
4. Enclosing each side with hardware cloth is a good idea. Once your spray foam sets, lift each side again and staple hardware cloth over the entirety of the underside of the shed. Make sure you overlap each row by an inch.

Other Methods for New Construction

Rigid Foam between joists is also an option. However, batts offer a higher R-value and since the cavities are enclosed, you won’t have to worry about moisture or rodents like you would if the cavities were open at the bottom. For this reason, rigid foam should be a secondary option in new construction.

Using fiberglass batts and sealing the bottom with treated plywood or OSB sheathing is a great option for a small shed. Since you can flip the floor frame of a small shed over, you can easily screw sheathing into the bottom after you install the batts. The low price of fiberglass negates the cost of using sheathing on the bottom of your shed.

If your new shed is on a concrete slab, you’ll want to ensure your slab is over rigid foam panels. Depending on how you construct your shed on the slab.

If you raise your shed on joists, then you can use the above methods to insulate joist cavities. If the shed is directly on the slab, then you can put insulated floor panels over the concrete.

Conclusion

However, you decide to insulate your shed, remember that moisture is your greatest enemy. Moisture negates R-value and introduces issues like mold and rot to your structure. Ensuring a secure joist cavity is critical when installing any type of insulation.

As always, I hope you’ve found this guide helpful. Please share any comments or suggestions below and best of luck with insulating the floor of your new, or old, shed.