6. Insulation & Flooring
You can skip this step and come back to it later once the rest of the structure is finished.
The big benefit of doing this stage now is that you’ll have a lovely flat surface upon which to build your walls before raising them.
The drawback is that you’ll need to keep the finished floor reasonably dry until your roof is finished. It also means ordering insulation and sheet material now rather than all-at-once when you need it for the roof and walls as well.
It’s a bit of a tough call but if you have reasonably long walls like mine you’ll almost certainly want to build them in situ. The other option is to buy the sheet material, lay it down and then store it away until your build is weatherproof.
Supporting the insulation
As you saw, I used short pieces of battens on the sides of the joists. By making a quick jig I got the tops of the battens the same depth as the insulation. This gave me a flat surface to which I could apply aluminium tape for a vapour barrier.
Another way of doing this would be to have you battens attached under the joists and you could run several metres of battens this way. The insulation sits on top. If there’s a gap between the top of the insulation and the top of the joists which makes taping it difficult you can instead use a polythene sheet (I’d recommend a minimum of 500 gauge/0.125mm). This can be cheaper than aluminium tape too but you will need to tape the joins. You’ll see my using it when I insulate the walls.
Should I use some kind of sheet under the joists?
I get asked this a lot, mostly because of a worry about vermin. Certainly don’t use a polythene sheet or damp proof membrane, that will trap moisture in the floor. You can add a breathable membrane but I don’t see a big benefit from doing so.
Insulation choice
We’ve learnt about U-values. It’s something we’ll come back to later in the series but I want to stress that you don’t have to go overboard. Older houses generally don’t have any insulation and those built before the 1920s don’t even have cavity walls, and they’re…relatively liveable. I know, I live in one and although I have been adding insulation, we didn’t succumb to frostbite before. That said, with higher energy prices now (writing in 2023) good insulation is an investment.
Your choice should be dictated by what you will use the room for. A cinema room will need to be more toasty than a workshop for example.
Rigid vs Flexible insulation
PIR is more expensive. Wool insulation is cheaper and will achieve a decent U-value BUT to use it between the floor joists will mean supporting it somehow. You could use netting or a breathable membrane but that risks rodents pulling chunks out for nesting material so you’ll probably want sheets of 9 or 11mm OSB fixed to the bottom of the joists. However, that increases the costs so I think PIR or polystyrene is your best bet for the floor and if you are wanting to reduce costs by using wool insulation it’s best saved for the walls.
Here’s a video of me insulating our office floor with wool insulation for comparison. The difference is that the house’s foundations are wrapped by walls with air vents so mice can’t get under the floor which is what makes wool suitable here but probably not for a garden room.
Note: For rigid insulation I highly recommend purchasing a foam gun to control the spray foam – a lesson I learnt during this stage! You can leave the can attached to the gun without it getting blocked up for a few weeks by placing it upside down (can down, gun up). When you do detach the can make sure you use gun cleaner to clear the gun of gunk.
Plywood vs OSB vs Chipboard
The title above lists these sheet materials from most expensive to cheapest. All will do the job for you and are dimensionally stable so there is no right or wrong choice.
Ply is made up of layers of wood shaved off the circumference of a tree, flattened and then glued together with alternating grain directions. If you are leaving the floor bare, it’s certainly the most attractive of the three and deals well with moisture or rain, drying out quite quickly.
OSB is made up of flat, thin shards of wood all compressed together at random orientations, giving it a similar strength to ply at a lower cost. It takes longer to dry out and there is some research showing that permeability and strength may be slightly compromised even after it dries.
Chipboard is essentially compressed particles, cheap to manufacture but doesn’t fair well when wet, and can crumble over time. However there is ‘P5’ chipboard which is water resistant and might make a good choice. I’ve used it in a bathroom remodel. Another option is something like Egger Protect which has a waterproof upper layer. If I were doing this project I’d probably opt for chipboard due to lower cost, just not the loftboard variety! If you do this stage later on once your building is watertight, chipboard will definitely be a good bet. Chipboard also comes in tongue and groove which I’ll talk about lower down.
Thickness
If your joists are on 400 or 16″ centres then you’ll want to use 18mm thick sheet material. For larger spacings (450 or 600), it’s worth going for 22mm to reduce any bounce.
How to make a really solid floor
Noggin Positions
You can go the extra mile and ensure that every edge of your sheet material is supported by a joist or noggin i.e. the long side of the sheet. This entails some planning when putting together your floor and for your noggins to be in a straight line rather than offset like mine. You’ll see this advice given for plywood under tiled bathrooms where you want very little movement. I didn’t bother but if you’re putting in a grand piano, may be it’s worth it!
Glue
I used grab adhesive under the ply but there is now a better product for this – D4 foaming adhesive. Your joists won’t be perfectly flat and this adhesive expands into any gaps giving you uniform support throughout the length of the joists. It is more pricey though and grab adhesive will do the job well enough. Here’s a pic of the D4 under my bathroom chipboard. You can also use in the tongue and groove to really lock it in place.
Screws
If you’re going to use expensive screws in any place in your build, this is it. You don’t want squeaking floors. Most screws will do the job just fine but if you like to do things right I highly recommend Spax chipboard flooring screws.
Do I need the expansion gaps between sheet material?
I get asked this quite frequently. Given that the sheet types are designed to be dimensionally stable, it’s probably okay if you just butt the sheets together, especially if you are laying them down in summer as they’ll more likely contract and than expand.
However, if you plan to lay flooring (laminate etc) over the top you may as well leave the gaps as you won’t see them.
Again, if you’re confident in keeping things dry, they shouldn’t expand much. The multiple screws per sheet stop a lot of the movement.
The other side of this debate is that it’s the timber floor itself which moves most rather than the sheets so if they shrink in winter, the boards may be forced closer together.
Tongue and groove
If you’ve used sheet material that has T&G, this helps lock the sheets together while providing some space for expansion, no gap needed here.
There’s a weird size perception element as you go through the build, sometimes it will look huge, other times much smaller. When I’d built my floor it looked massive but as the walls close things in, it looks smaller – just something to be aware of.
Task: Select your insulation and sheet material and you’ll have yourself a very fancy deck!