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.