Insulation in renovation, Suspended timber floor

[andreas]

We have to replace some rotten floor joists and floorboards in a downstairs room on a renovation.

If I am reading correctly, if you take up all the floorboards you have to install insulation in that room? The joists are 100mm depth on sleeper walls, the distance between the bottom of the joist and the concrete oversite is 120mm.

What would be a good option for PIR install and at what thickness, balancing cost and performance? I have read multiple conflicting measurements, some say leave at least 150mm between underside of PIR and concrete, so would be limited to 70mm if correct?

[Super_Paulie]

as long as your airbricks are clear id just fill that with with 100mm PIR if it was me.

[andreas]

21 minutes ago, Super_Paulie said:

as long as your airbricks are clear id just fill that with with 100mm PIR if it was me.

Top of airbricks sit level with top of joists, I was thinking of retrofitting a scoop somehow to direct it under, but it's a bit tight to the first sleeper wall. 

Any PIR in particular, or just "Any PIR"? I see so many different types even though they all seem near identical

 

[Super_Paulie]

all seem the same really. Most times ive bought "Celotex" what actually turns up is Recticel or something else quite random, cant say ive noticed anything different between them.

When i had a similar situation i chamfered the PIR going up to the vent and foil taped the exposed end. I replaced the vent with a bigger flow version as well. Seems to be fine, howls a gale underneath the floor but zero drafts above it. 100mm PIR foamed and taped, but my joists were 120mm as i have UFH on top of it.

[andreas]

3 hours ago, Super_Paulie said:

all seem the same really. Most times ive bought "Celotex" what actually turns up is Recticel or something else quite random, cant say ive noticed anything different between them.

When i had a similar situation i chamfered the PIR going up to the vent and foil taped the exposed end. I replaced the vent with a bigger flow version as well. Seems to be fine, howls a gale underneath the floor but zero drafts above it. 100mm PIR foamed and taped, but my joists were 120mm as i have UFH on top of it.

I've just discovered that my joists are actually 91mm. I'm thinking 80mm PIR now, because I cannot be bothered to trim pockets out every metre so that the PIR can sit down over the sleeper walls!

[marshian]

58 minutes ago, andreas said:

I've just discovered that my joists are actually 91mm. I'm thinking 80mm PIR now, because I cannot be bothered to trim pockets out every metre so that the PIR can sit down over the sleeper walls!

 

I was in a similar situation and 75mm ended up as the sweet spot cost wise but it's always worth working out cost per m2 for various thicknesses as well as pack sizes.

 

Pound for pound it's the best thing I've ever done to the house from an energy reduction perspective

 

Did mean I had to add a PIV into the mix as humidity levels sky rocketed due to the reduction in uncontrolled air ingress but it's still been the best thing I've done you may need to consider additional ventilation options/solutions

 

 

[andreas]

2 hours ago, marshian said:

 

I was in a similar situation and 75mm ended up as the sweet spot cost wise but it's always worth working out cost per m2 for various thicknesses as well as pack sizes.

 

Pound for pound it's the best thing I've ever done to the house from an energy reduction perspective

 

Did mean I had to add a PIV into the mix as humidity levels sky rocketed due to the reduction in uncontrolled air ingress but it's still been the best thing I've done you may need to consider additional ventilation options/solutions

 

 

Thanks I'll look for 75mm as well. What brand / product did you go for?

[marshian]

On 20/05/2026 at 18:13, andreas said:

Thanks I'll look for 75mm as well. What brand / product did you go for?

Kingspan K70 from memory. 
 

ordered it thro insulation4less

[JohnMo]

If I was doing it I would do something like this with mineral wool.  Easier to work with than PIR, use a breather membrane and you end up with all the ventilation drafts eliminated also.

 

 

 

[saveasteading]

I would maintain the air gap, however draughty, unless this was seriously analysed as a whole.

 

Ang PIR is OK.

Celotex us a trade name which became the generic term.

The name has been disappeared because of their part in Grenfell.  J belive ig us now called sopratherm so you may want to bear that in mind. Not that kingspan is clean either.

Recticel and Unilin appear to be blameless.

 

I got good prices from 'Seconds' but slightly better from the local merchant.

 

It is difficult to fit it snuggly. Either tight or with gaps. Ii you had 100mm ice say 50mm pir and 50mm rockwool  bat.

 

So i think you should try rockwool bat/slab before buying lots... it must fit snugly for performance and to stay put.

Even 50mm would help a lot.

 

On 19/05/2026 at 19:32, andreas said:

have to install insulation

I don't think you have to but it is wise.

[torre]

On 20/05/2026 at 13:58, andreas said:

I cannot be bothered to trim pockets out every metre

 

Honestly that will be the least of your issues compared to the effort of cutting rigid insulation to retrofit tightly between old joists. Much easier to do something such as @JohnMo's suggestion.

 

Pretty sure this will fall under building control renovation of a thermal element.

[Redbeard]

On 19/05/2026 at 18:32, andreas said:

If I am reading correctly, if you take up all the floorboards you have to install insulation in that room?

Yes, you are reading correctly. You are 'renovating a thermal element' (adding or replacing a layer of something which gives on to the 'outside' - the sub-floor void being 'outside' for these purposes). You are unlikely to be picked up on it but yes, the obligation 'kicks in' because it is internal work - thus although it is only part of the ground floor, it is 100 percent *of the floor in the room you are looking at*. Doing it is good (Ecological Building Systems offer this Best Practice advice (https://www.ecologicalbuildingsystems.com/post/best-practice-approach-insulating-suspended-timber-floors?srsltid=AfmBOopVdS5DMUhiXLkltolwyE688i09EDVP0y4fzzJw_IZRi9jJ5hON) and UK gov't offers comprehensive advice too (https://assets.publishing.service.gov.uk/media/5f05d211d3bf7f2be6e0217a/suspended-timber-floors-underfloor-insulation-best-practice.pdf)).

 

To echo other comments 'fluff' is more likely to take up dimensional variations and not fall out.

 

 

[saveasteading]

I think we could easily argue against this  being required improvement to regs. 

1. Not 25% of the thermal envelope. 

2. The concrete slab below is part of the thermal envelope, not this raised floor.

 

It's a good idea to do it though.

[Ferdinand]

The last one of these I did we discussed on here.

Rather than microfitting rigid boards between wonky beams I used rock wool and a staple gun to take the insulation to the bottom of the joist level, leaving the underside exposed to help prevent damp. That is flexible enough to cover the variations simply. The rock wool was out of the roof, so the roof would be under 100mm and I got the full 300mm from the ECO programme because it was then uninsulated.

I also used "snorkel" vents in one or two places to adjust the level of air inflow to be below the insulation (the airbricks were at ground level when I wanted them one course up to prevent water ingress).

Then I also put celotex on top of the floor, with a click-fit Quickstep laminated floor on top of that. I (just) had the height to trim my doors.

We were able to do all that without replacing the entire floor by my handyman raising one floorboard every couple of feet.

I would give some thought as to which point you let Building Regs see it - I'm not sure what the current required standards are exactly, but for me it was quite tightly constrained. They were not happy with me doing the "fill it all with polystyrene beads" in a waterproof envelope thing. It's had tenants in it for nearly a decade now, and they all love it.

There is a thread called "little brown bungalow" about it, and we discussed the floor setup over in the Boffin's corner, here:
 

This was my buildup:


 

[saveasteading]

Another thought. You can buy netting again a BM. We havd some blue kicking around. I think it is to keep debris contained. It's not expensive. 

Debris netting,: that's it.

 

You could lay this across the joists, tucking down the sides and stapling. Then any wool material would stay put permanently. 

[Redbeard]

2 hours ago, saveasteading said:

I think we could easily argue against this  being required improvement to regs. 

1. Not 25% of the thermal envelope. 

 

@saveasteading, I agree it is not totally clear at first sight but I have queried and 'tested' this on many occasions. The impression I am left with is the same as this link suggests:

 

https://www.carmarthenshire.gov.wales/council-services/building-control/building-regulation-applications/thermal-elements-application/ says:

 

"Renovation of a Thermal Element means adding a new layer to a Thermal Element or the replacement of an existing layer. When the renovation is more than 50% of the surface of the individual element or 25% of the total of the building envelope, Building Regulations approval will be required prior to carrying out the work and the Thermal Element may require upgrading to provide more insulation.

When assessing this area proportion it should be taken as that of the individual element, not all the elements of that type of building. The area of the element should be interpreted in the context of whether the element is being renovated from inside or outside, e.g. if removing all the plaster finish from the inside of a solid brick wall, the area of the element is the area of external wall in the room. If removing external render, it is the area of the elevation in which that wall sits.

The consequence of this change is that much building work, previously exempt from the Building Regulations as it was considered a repair, may now require approval. For example:

• Replacement of a slate or tiled roof covering even if like for like  
• Re-plastering of a wall
• Replacement of felt on a flat roof
• Renewal of a ceiling below a cold loft space
• External rendering or re-rendering of a wall
• Renewal of cladding to a dormer
• Renovation of a ground floor involving replacement of screed or timber decking"

Something of even more interest (to the pocket at least) is that my LA used, many years ago, to charge me separately for each thermal element touched, whereas their final position was that one Bldg Notice application covered 1 *or more* thermal elements. Saved me (or rather the client) up to £360 on some jobs.

 

Re netting, it's one of the 'hammocks' I used to use, until I started using breathable membrane which, when taped, not only holds up the quilt but also limits 'wind-wash'.

[Redbeard]

I was looking for an English source (as the above is from Wales) and found this in https://www.tameside.gov.uk/buildingcontrol/guidancenotes/note24guide1.pdf  - it's at the bottom of p.1. It is the first time, as far as I know, that I have ever seen this:

 

For some reason I cannot cut and paste from that pdf. It's at the very bottom of p.1 and begins 'From 9th January 2013... and in summary says "the installation of thermal insulation to suspended floors..." and ends "...is not considered to be controllable under the scope of these regulations". 

It's not what I have gleaned from other sources, and it obviously hasn't been proof-read!

 

Edited 2 hours ago by Redbeard

[saveasteading]

37 minutes ago, Redbeard said:

found this

It won't open for me.

Anyway, between us we have found a few interpretations. Authorities can't make up their own rules so these are their interpretation and could be argued with.

I haven't done the important thing of looking at the actual regulations.

 

It's the sort of issue that tempts one to just do it, which is generally a bad premise. By which I mean that the OP intends to make sensible improvements but what if the bco refuses or demands something else?

And yet, it would be possible to cause harm and that should be controlled.

[Ferdinand]

1 hour ago, Redbeard said:

 

@saveasteading, I agree it is not totally clear at first sight but I have queried and 'tested' this on many occasions. The impression I am left with is the same as this link suggests:

 

https://www.carmarthenshire.gov.wales/council-services/building-control/building-regulation-applications/thermal-elements-application/ says:

 

"Renovation of a Thermal Element means adding a new layer to a Thermal Element or the replacement of an existing layer. When the renovation is more than 50% of the surface of the individual element or 25% of the total of the building envelope, Building Regulations approval will be required prior to carrying out the work and the Thermal Element may require upgrading to provide more insulation.

When assessing this area proportion it should be taken as that of the individual element, not all the elements of that type of building. The area of the element should be interpreted in the context of whether the element is being renovated from inside or outside, e.g. if removing all the plaster finish from the inside of a solid brick wall, the area of the element is the area of external wall in the room. If removing external render, it is the area of the elevation in which that wall sits.

The consequence of this change is that much building work, previously exempt from the Building Regulations as it was considered a repair, may now require approval. For example:

• Replacement of a slate or tiled roof covering even if like for like  
• Re-plastering of a wall
• Replacement of felt on a flat roof
• Renewal of a ceiling below a cold loft space
• External rendering or re-rendering of a wall
• Renewal of cladding to a dormer
• Renovation of a ground floor involving replacement of screed or timber decking"

Something of even more interest (to the pocket at least) is that my LA used, many years ago, to charge me separately for each thermal element touched, whereas their final position was that one Bldg Notice application covered 1 *or more* thermal elements. Saved me (or rather the client) up to £360 on some jobs.

 

Re netting, it's one of the 'hammocks' I used to use, until I started using breathable membrane which, when taped, not only holds up the quilt but also limits 'wind-wash'.

So if it is "50% or more", then you can raise 40% of the floor boards for access or repair and it does not apply !!!!

With a single bound you will be free !!!

(I think)

Though you may have already gone past this.

[Redbeard]

Yep, tackle 100% of the floor in 40, 40 and 20% tranches and you may have cracked it! But don't quote me!

 

My justification for spending more than I probably have to on Bldg Notices is to avoid the situation on a subsequent sale where a buyer's solicitor queries BC approval for specific works. If the answer is, or suggests that, that you don't have it they express 'outrage', which can be instantly dispensed with if you drop the price by £x000. I will have in my hand a sheaf of BC approvals and stand my ground.