Suspended timber floor with shallow joists: an indecent proposal

[tenovus]

Hello all!

 

I'm looking to insulate a Victorian suspended timber floor so I can stop heating the dirt. I'm planning to take most of the boards (not original) and replace with UFH chipboard panels. As my joists are 100mm deep (if I'm lucky - some are shallower), I don't think the Ecological Building Systems route alone is going to be quite good enough. I have some 100mm PIR to hand and plenty of crawlspace.

 

I've put together a bit of a proposal on Ubakus which I'd love some thoughts on. Essentially using woodfibre board to extend the joists, hanging a breathable membrane off those, filling those hammocks with loose fill or STEICOflex (or eq) then a layer of PIR at the top. I'd hope this would combine the hygroscopic and gap filling qualities of the natural insulation with the U value of the PIR. I've also looked at forgetting about using up the PIR and just going for 200mm of natural with the woodfibre board joist deepeners.

 

Would love to hear people's words of warning on this option. Would fixing the woodfibre directly to the base of the joists be more secure? Would anyone have any thoughts on the best way to do that?

 

Thanks in advance!

 

 

 

 

[Nickfromwales]

Hi.

Can you crawl under to execute the SF 036 and the membrane?

[tenovus]

5 minutes ago, Nickfromwales said:

Hi.

Can you crawl under to execute the SF 036 and the membrane?

 

I can, but I'll be taking the boards up anyway.

[Nickfromwales]

15 minutes ago, tenovus said:

 

I can, but I'll be taking the boards up anyway.

Totally get that, but if the membrane isn't absolutely perfect you'll get problems here. Eg the joints would need taping etc.

 

Have you not considered filling the voids and doing a solid floor? I assume this will remain, otherwise, a cold ventilated space, yes?

[tenovus]

Ah I see! Absolutely, it should all be accessible for making sure that membrane is airtight. I was thinking I could drape it over some of the joists too though that's not easily shown in the Ubakus. 

 

I'd read some people suggest it on similar threads but worried about messing too much with the house as it's managed the last 170 years fairly well! The space would remain cold and ventilated. We also have a small cellar that goes a little deeper than the crawlspace and, as a semi, next door's crawlspace is adjacent to ours - I'm not sure if that has implications for putting in a solid floor.

[Nickfromwales]

32 minutes ago, tenovus said:

I'm not sure if that has implications for putting in a solid floor.

Probably rules the idea out to be honest, plus there's a lot of depth then for 'infill'.

 

Maybe just add 50 or 75mm of treated sawn timber to the underside of the existing timbers, if access permits, and then use 140mm or 160mm PIR (respective to the additional timber) for all the insulation, cut in from above. Then just foil tape everything up top (running continuously from the foil of the PIR board, over the top of the joist, and onto the next PIR board) as the vapour barrier.

 

Then use Illbruck FM330 foam to all perimeter / gaps where cutting in the PIR isn't feasible.

 

The fact that draughtiness and cold air infiltration will be almost completely neutralised with this proposal will allow you to install thinner (high performance) insulation and still get a great end result.

[Iceverge]

 

My suggestion. U Value about 0.16. 

 

 

Rip some 9mm OSB, nail 2 x battens to either side of the bottom to create a flange. Nail it to the sides of the existing joists. 

 

Drop in a bottom piece of OSB to create a box. A/T tape the corners to make it completely wind proof. This is very important the performance as your insulation will get wind washed if it is not sealed.

 

Drop in the mineral wool/woodfiber insulation. 

 

Lay the Chipboard on top and fastidiously foam glue all the joints. This will be your internal airtight layer. 

 

 

This proposed arrangement can all be done without the bother of getting into the crawl space at all. If you add some glue to the OSB and the battens you would effectively be creating an I beam which would massively strengthen the floor too. 

 

 

I never like PIR between joists. Too many reports of it shrinking and coming loose over time. 

 

 

[tenovus]

Thanks for this Iceverge! Hadn't considered something so OSB based! Definitely allows me to make use of the massive bags of loftroll I have to hand!

 

Is there a risk of condensation with the OSB at the bottom? And do you think even when glued UFH chipboard will work as a vapour barrier or do you think I'd need to find one with a specific coating?

 

I'm tempted to try a version of the above but with a breathable membrane at the bottom instead of the box base. I guess it wouldn't have quite the same structural improvement!

[tenovus]

On 27/01/2025 at 22:03, Nickfromwales said:

Then use Illbruck FM330 foam to all perimeter / gaps where cutting in the PIR isn't feasible.

 

Thanks for this - will seek that foam out!

[Nickfromwales]

13 minutes ago, tenovus said:

Thanks for this - will seek that foam out!

The 330 is certified for use where you need to be ‘airtight’ and is closed cell, unlike cheaper open cell foams from the sheds. 
The open cell stuff can promote wicking of damp / moisture, which you don’t want  

It’s also far less friable and much more robust. Normal foams will allow you to simply scratch away at them with your fingertip, but the 330 is a much tougher product. 

[Onoff]

On 27/01/2025 at 23:06, Iceverge said:

I never like PIR between joists. Too many reports of it shrinking and coming loose over time. 

 

Agree about the shrinkage. What about if you were to deliberately cut the boards under width by say 20mm and foam fill the 10mm gap either side with a high performance, flexible foam such as the aforementioned Illbrück FM330?

 

Would that not allow for shrinkage? 

Edited yesterday at 06:46 by Onoff

[saveasteading]

Pir between joists 

I favour half pir and half mineral wool.

The thinner pir can be cut more accurately and the wool is squeezed in and completes the seal.

Doesn't expanding foam shrink too?

[Iceverge]

I think all foams shrink calling @SteamyTea? 

 

PIR is never a good choice between timbers in my opinion. 

 

The headline lamda value sucks people in and theres little regard given to the performance and overall cost of the system.

[Iceverge]

12 hours ago, tenovus said:

Is there a risk of condensation with the OSB at the bottom? And do you think even when glued UFH chipboard will work as a vapour barrier or do you think I'd need to find one with a specific coating?

 

Airtighess is the key. Not vapour diffusion. 

 

Hundreds and hundreds of times more moisture is carried by air leaks than by diffusion. 

 

You have to have a complete airtight layer. The chipboard floor will be fine but ensure you seal every joint and seal it to the walls too. 

[SteamyTea]

2 minutes ago, Iceverge said:

I think all foams shrink calling

Generally yes, for the exothermic chemistries, and shrink the most the newer they are.

 

[TerryE]

One point that hasn't been discussed here is ventilation of the void under the insulation.  You will have air bricks in the peripheral walls to allow free ventilation of the void.  It is absolutely essential that these are not blocked by the added insulation so you will need to provide a clear ducted path to the void so that the added insulation does not impede ventilating air flow.  You might also need to upgrade the wall ventilation.

 

If you don't do this then you could get a humidity / moisture buildup in the joists leading to dry-rot.

 

Edited 11 hours ago by TerryE

[Iceverge]

2 hours ago, SteamyTea said:

Generally yes, for the exothermic chemistries, and shrink the most the newer they are.

 

 

Where does EPS sit in this equation?

 

 

[SteamyTea]

2 hours ago, Iceverge said:

Where does EPS sit in this equation

They are a thermosetting polymer (polystyrene), but the E is for expanded, so the air will expand and contract after the polymer has fully cured.  This is not normally a problem as the movement is quite small.

 

If you look at a SIPs panel, you will often see that they are bowed, this is because some are basically two sheets of OSB, with a relatively lightweight pine frame, that is then injected with expanding polyurethane.  Initially the resin is a mixture of two parts, that then react, expand rapidly, then off gas, then contract as they cool.  Over the next few weeks, this contraction will continue.  It is possible to buy aged PU foams that are machined to a fixed size, but they are more expensive.

You can get SIPs panels that have expanded polystyrene in them that are more dimensionally stable, but they are usually more expensive and generally have a lower U-value.

[Iceverge]

Of all the foam products, EPS is my favourite. 

 

Not great in a fire, but then I don't suppose any foams are. Form personal "laboratory" fire testing it seems to give the least funky gasses too. 

[Iceverge]

23 hours ago, tenovus said:

I'm tempted to try a version of the above but with a breathable membrane at the bottom instead of the box base.

 

Sketch it for me please what you're thinking. 

 

You really need to have the bottom of the membrane quite taut or else you'll end up with baggy droopy bottoms empty voids in the insulation which isn't good. 

 

The deeper you go the harder this will be to execute in practice. 

 

 

[Nickfromwales]

11 hours ago, Iceverge said:

I think all foams shrink

I have to say, the Illbruck FM330 stuff is very good, a totally different beast to the regular build-shed foams.

 

I've had the pain benefit of being around some very elongated full new builds from day 1 to day Z (thanks to Covid) and have used this stuff for many different applications, from fixing things, general sealing, through to airtightness work and more; I've used it thick & thin! I've then been around it for anywhere up to a year, sometimes more at the plant rooms etc, after it was first squirted in, and I cannot recall any notable shrinkage with this stuff.

 

I do make a point of spraying a mist of water, if adverse I'll do a 50/50 water/PVA mix, and this does help the curing process no end.  

 

It's also massively hardier than regular foam, when poked / scraped / cut into etc.

 

16 hours ago, Onoff said:

 

Agree about the shrinkage. What about if you were to deliberately cut the boards under width by say 20mm and foam fill the 10mm gap either side with a high performance, flexible foam such as the aforementioned Illbrück FM330?

 

Would that not allow for shrinkage? 

Yup, I install the PIR with angled cuts, so there is friction for fitting, and a gap to get some foam into.

 

Gaps aren't the killer here though, addressing airflow up between the boards is, hence me saying to foil tape over the tops of the joists from PIR board to PIR board (foil backed obvs) as the VCL.

[Onoff]

There's is of course another bonus of making suspended floors in old properties like this airtight. 

 

It allows a leaky gas pipe to fill the void up long before you smell it...

 

💣😂

 

Another reason to pay close attention to cross ventilation to the outside, underneath whatever you do. 

 

 

[Ferdinand]

Are you still here, @tenovus?

I did something similar back in 2018, which we discussed here - but I did it all from above raising each 3/4 floorboard, and used rockwool and a staple gun, plus 25-40mm PIR above the floor, not completely sealing the bottoms of the joists from the (dry) ventilated space below.

Gave me a u-value of 0.23 and the PIR taped gave me a vapour control membrane.

The big +ves were 1 - Easy to install, and 2 - I moved all my services into channels in the PIR dunning alonmg the inside walls, which meant no holes in the insulation layer.

This was the layering.


The thread is here, with discussion:

 

[tenovus]

Thanks everyone for all the wisdom - it's like winding up a series of clockwork experts and watching them go! Mega reply incoming...

 

20 hours ago, TerryE said:

One point that hasn't been discussed here is ventilation of the void under the insulation.  You will have air bricks in the peripheral walls to allow free ventilation of the void.  It is absolutely essential that these are not blocked by the added insulation so you will need to provide a clear ducted path to the void so that the added insulation does not impede ventilating air flow.  You might also need to upgrade the wall ventilation.

 

If you don't do this then you could get a humidity / moisture buildup in the joists leading to dry-rot.

 

On ventilation, this is something I've entirely internalised and become paranoid about so will be making sure of it! As it stands, it feels very ventilated under there but I'll be sure to keep it flowing! When it comes to wall ventilation under the floors, would this be done internally?

 

 

 

11 hours ago, Iceverge said:

 

Sketch it for me please what you're thinking. 

 

You really need to have the bottom of the membrane quite taut or else you'll end up with baggy droopy bottoms empty voids in the insulation which isn't good. 

 

The deeper you go the harder this will be to execute in practice. 

 

 

Apologies for the dodgy sketching - I'm yet to perfect the fast draw! The plan would be to sandwich the membrane between the OSB and the joist then staple and batten to the bottom of the OSB to keep the hammock taut across the gap.

 

 

 

 

 

11 hours ago, Nickfromwales said:

I have to say, the Illbruck FM330 stuff is very good, a totally different beast to the regular build-shed foams.

 

I've had the pain benefit of being around some very elongated full new builds from day 1 to day Z (thanks to Covid) and have used this stuff for many different applications, from fixing things, general sealing, through to airtightness work and more; I've used it thick & thin! I've then been around it for anywhere up to a year, sometimes more at the plant rooms etc, after it was first squirted in, and I cannot recall any notable shrinkage with this stuff.

 

I do make a point of spraying a mist of water, if adverse I'll do a 50/50 water/PVA mix, and this does help the curing process no end.  

 

It's also massively hardier than regular foam, when poked / scraped / cut into etc.

 

Yup, I install the PIR with angled cuts, so there is friction for fitting, and a gap to get some foam into.

 

Gaps aren't the killer here though, addressing airflow up between the boards is, hence me saying to foil tape over the tops of the joists from PIR board to PIR board (foil backed obvs) as the VCL.

Thanks, Nick! Just on the VCL - would there be an issue with using a polythene VCL sheet over PIR so that I don't need to get too exact with the foil tape or does that create issues by having two impermeable layers on top of each other?

 

 

2 hours ago, Onoff said:

There's is of course another bonus of making suspended floors in old properties like this airtight. 

 

It allows a leaky gas pipe to fill the void up long before you smell it...

 

💣😂

 

Another reason to pay close attention to cross ventilation to the outside, underneath whatever you do. 

 

 

Luckily no gas pipes in this part of the house (besides the long empty coal gas ones) and, fingers crossed, will be going to an ASHP in future. Much as the idea of a massive balloon of natural gas under the lounge sounds tempting...

 

 

[tenovus]

1 hour ago, Ferdinand said:

Are you still here, @tenovus?

I did something similar back in 2018, which we discussed here - but I did it all from above raising each 3/4 floorboard, and used rockwool and a staple gun, plus 25-40mm PIR above the floor, not completely sealing the bottoms of the joists from the (dry) ventilated space below.

Gave me a u-value of 0.23 and the PIR taped gave me a vapour control membrane.

The big +ves were 1 - Easy to install, and 2 - I moved all my services into channels in the PIR dunning alonmg the inside walls, which meant no holes in the insulation layer.

This was the layering.


The thread is here, with discussion:

 

 

 

Ah that would be very tempting! Sadly, I think we've decided to avoid options with much floor build up because the room is open plan with the dining room which has some much nicer boards that we're likely going to keep in place so we don't want the step up. A rod for our own backs...

 

Thanks for the inspiration though! I too have access to some free mineral wool - mine apparently cleared from a raided weed farm!