Suspended floor insulation + floorboard sanding: order of the work?

[lizziecupcakes]

Hi all
I am now planning my suspended floor insulation project. We currently have untreated original floorboards (pine). They're in decent condition. After much thought, we think we should do the project in the following order:

1. sand and varnish floorboards first
2. add floor insulation under the floorboards
3. nail all the floorboards back down securely

The reason we think we should sand and varnish the floorboards first is because we would have a membrane sheet at the bottom end of the joists and within this would be the insulation. If we sand after, all the dust from sanding will fall into the insulation below, with nowhere to escape except back into the house.

I do plan to use cotton rope for the gaps between the floorboards too after cleaning them thoroughly.

What do you think?

I should mention that we will be using the electric hand sander for more control. We know this project will take a long time, but we're okay with that. So long as it gets warmer, and we'll do it room by room. Our target date for completing the ground floor insulation is September 2025

Thank you

 

[Cubist]

Sorry but that gets a definite NO! No offence but your plan sucks and here's why.

1. Theres a considerable risk of damage to the floorboards when you lift them. Almost inevitably much of any tongues in the boards will be lost or damaged beyond being saved and you will also lose some of the boards altogether owing to old damage, splits and breaks due to weaknesses in the grain structure, nail extraction damage and the list goes on.
2. You will inevitably need to replace some boards and therefore there may be a need to colour match the timbers after they're laid.
3. All the old nails will have to be removed and new ones used in their place when the boards are re-laid, You cannot use larger nails, and nor should you use a round bodied nail where a rectangular profile was previously used. Doing so risks damage to joists, the boards themselves or - worse still - a failure to achieve a firm fix between boards and joists.

If your wooden floors are mostly in good order and you have no other means of accessing the underfloor void to install insulation then I would strongly advise you abandon that effort. Your plan in my opinion should be as follows: -

1. Sand the boards - after driving any proud nail heads back below the surface.
2. Stain to colour match.
3. Fill the gaps and make other minor repairs to the boards.
   1. Colour match the gap filling materials.
4. Varnish.

BTW you could hire, and I would recommend you do, a drum sander and an edge sander, to do the bulk of the work. A hand sander will do the job but it will take considerably longer and more damaging to your knees - and the tool itself.

 

[Lacan07]

Hi all
I am now planning my suspended floor insulation project. We currently have untreated original floorboards (pine). They're in decent condition. After much thought, we think we should do the project in the following order:

1. sand and varnish floorboards first
2. add floor insulation under the floorboards
3. nail all the floorboards back down securely

The reason we think we should sand and varnish the floorboards first is because we would have a membrane sheet at the bottom end of the joists and within this would be the insulation. If we sand after, all the dust from sanding will fall into the insulation below, with nowhere to escape except back into the house.

I do plan to use cotton rope for the gaps between the floorboards too after cleaning them thoroughly.

What do you think?

I should mention that we will be using the electric hand sander for more control. We know this project will take a long time, but we're okay with that. So long as it gets warmer, and we'll do it room by room. Our target date for completing the ground floor insulation is September 2025

Thank you

If your going to use a sander like the Festool Rotex (which is really the only hand sander you should be using for a floor) then there won't be any dust. There is literally no dust if you connect it to a half decent vacuum. When I bought mine I managed to work out a way to connect it to my Nilfisk Vacuum and I was amazed. The Festool vacuum is even better but flipping expensive.

 

[fernicarry]

Are you doing the insulation from below? As @Cubist says its really the only way.

You should have a membrane above and below the insulation. One reason for the one above i.e. directly below the floorboards is to stop debris and spills falling through and contaminating the insulation and reducing its effectiveness. The lower one is to prevent wind-washing. So on that basis sanding before isn't essential but if you have a choice over the order then it would always be preferable to do the messy steps first.

With the membranes you shouldn't have any drafts so the cotton between the boards isn't necessary from that pov. If you want to disguise the gaps then have at it but I've never felt the need. In one or two places where I had silly wide gaps I hammered a sliver inbetween.

I've been making do with a 5" disc sander. Its not fast and you really don't want to do more than about an hour at a stretch but its small size means you can respect the edges of the boards particularly if they aren't at the same height rather than just planing the whole surface down to a uniform level. Any deep divots I sand by hand or clean them out with wire wool and meths.

Dust extraction absolutely essential on all machinery IMO. The garage here was knee deep in wood chippings at one point recently because I'd moved the extractor indoors to do the floor....

 

[lizziecupcakes]

Hi all, thanks all for your input.

We are quite sure we need the floor insulation no matter what because it has been very cold in the house and rugs aren’t that helpful. We are prioritising comfort over the floorboards and will take the risk if we damage a few.

When our electricians came they managed to take up a few of the floorboards without damage (they only chipped one a bit on its edge). We also took up all the floorboards in the loft before we added the insulation there (about 30 pine ones) and managed without any damage. So we feel confident but know there is a risk, which @Cubist rightly mentions (thank you for your honesty!)

Thanks @Lacan07, it’s the Festool hand one we will be buying, more for our hands sake (minimises vibration). We are going to do this work slowly from March - September (4 different rooms, 2 hallways). Can you share how you attached it to your hoover ? I have a Henry one.

@fernicarry thank you for your post! We are quite excited to start the project . I will definitely be using membrane below and above the joist. May I ask which membrane brand you used and which insulation material? I don’t know which membrane to purchase, but we were thinking of rockwool for insualtion (or glass mineral wool that I used in our loft - Knauf brand).

I am working out thr solution for the dust extraction aspect now and finding ways to tackle this. Ideally, I’d prefer to sand and varnish after putting all the insulation down (with the money we are saving from doing it ourselves m, we might consider a proper dust extraction machine @Lacan07 , and maybe try to sell it after - do you think it would sell so we could atleast get some money back on it? I’m mot sure if people want to buy these things second-hand.

We won’t be able to go into the crawl space unfortunately. I wish we could but it won’t be possible.

 

[fernicarry]

I've used a few different brands of breather membrane. Last one was Thermafleece branded but I'd be surprised if they actually made it themselves. Application guides say to use a vapour layer for the top one, I'm not convinced that upsetting the balance of the building that much is advisable. Over on BH a lot of people recommend an intelligent membrane on the warm side, which just means one that acts like a breather membrane some of the time and a vapour control membrane the rest of the time depending on the relative vapour pressures. That would probably be an ideal compromise but I imagine there is a price premium associated with it. BH is worth a read for this sort of work, but there are some approaches on there that would definitely not find favour with the PPUK community so you'll have to draw your own conclusions. For insulation I'm using Indinature.

If you are lifting the boards you will presumably also take off the skirtings which means you can lap the membrane up onto the external walls to give you a perfect air tight tray. Its the air tighness that matters IMO. The insulation is just a nice to have for a suspended floor since you don't have the direct conduction path into the ground that you would with a solid floor. Make sure you get the details right around joist ends as well.

I still wouldn't do it from above though... At least not without a good supply of matching spare boards.

 

[lizziecupcakes]

thanks @fernicarry .

I'll look into matching spare boards in the meantime. I plan to sand a tiny section down and see the result. If we can't get hold of (or afford reclaimed boards), we will get the closest matching new pine board. It's a compromise we will just have to make. There's a couple like that in the house already from the previous owner and while it might be an unsightly sight for some, we don't mind things like that. To me it adds character, and it's just part of the story of the house. Part of our story is, we're so so cold lol.

is there anywhere you recommend buying reclaimed boards from, or even new pine floorboards?

Since this is our forever home too, we will take the risk of damaged boards. We're hoping it'll only be a few and the added timeline to get the work done will make us be super careful too.

I'll look into the Thermafleece. I did read about the vapour layer for the top/warmer end, so I'll do a bit more research before we make a decision. Our aim would be to maintain as much breathability as possible though. I haven't heard of Indinature - I'll check it out, thanks!

I also intend to take off the skirting and lap the membrane up onto the walls (I think there's some very sticky tape that will secure it to the wall) to get an air tight tray.

Btw, what is BH?

Also, I wanted to ask you about the difference you feel in your home since you put your floor insulation in? Is it much warmer? Are you glad you did it?

I'm hoping @Lacan07 finds time soon about attaching their festool to a hoover. If I can do this, I will be super happy.