Suspended timber floor insulation

[Del-inquent]

This is possibly a really stupid question seeing as I couldn't find anyone else asking it with a search, but... 

Our ground floor is suspended timber floor. The joists are 100mm deep. The void below is 100 - 150mm. I've got to take up several floors for other reasons and as there is no insulation under there I thought I'd put some in. I am looking at using rockwool or similar under there using a membrane draped over the joists.

If I made the drapes deeper, or tacked some timber underneath the joists, I could put 150mm insulation in. But would that actually achieve much given the joist depths there currently? 

[Super_Paulie]

id probably go for 100mm PIR if it was me, held in place with pins hammered into the joists. PIR is roughly double the effectiveness of wool so 100mm PIR is equivalent to 200mm of wool, seems to make sense.

[Del-inquent]

12 minutes ago, Super_Paulie said:

id probably go for 100mm PIR if it was me, held in place with pins hammered into the joists. PIR is roughly double the effectiveness of wool so 100mm PIR is equivalent to 200mm of wool, seems to make sense.

I've had mixed messages on this. Originally I was going to go for PIR, but several people with far more knowledge on this than me (not hard, my knowledge on it could be written on a postage stamp...) have said that whilst the base figure of PIR is much more effective, movement, gaps, difficulty in effectively sealing in a retrofit with warped and twisted joitss etc drop the efficiency quickly, and it's not breathable so can introduce problems with moisture retention against the joists, which does concern me as last thing I want to do is bring more problems to a property that has plenty already lol

 

[Super_Paulie]

as long as you dont cover the joists at the bottom then thats not a problem, they are exposed to the air underneath to take away moisture.

Ive retrofitted my entire ground floor with PIR and its not without its challenges but its nothing a tight fitting cut wont sort with some air-tight foam and aluminum tape. 100mm wool or 100mm PIR, its a no brainer in my eyes as its double the effectiveness. 

[larry]

On 04/10/2023 at 10:02, Del-inquent said:

I've had mixed messages on this. Originally I was going to go for PIR, but several people with far more knowledge on this than me (not hard, my knowledge on it could be written on a postage stamp...) have said that whilst the base figure of PIR is much more effective, movement, gaps, difficulty in effectively sealing in a retrofit with warped and twisted joitss etc drop the efficiency quickly, and it's not breathable so can introduce problems with moisture retention against the joists, which does concern me as last thing I want to do is bring more problems to a property that has plenty already lol

 

I'm just doing mine with 100mm rockwool, decided against celotex for reasons you mentioned. And also I had a shedload already...

Ended up being a far bigger job than anticipated, boards were impossible to lift so cut out, then joist ends rotten and sleeper walls crumbling so have replaced whole lot. And of course not enough ventilation. Did mean I've put slightly thicker joists (4" Vs 3"). 

Need to get it back down before it gets too cold!!

[Sparrowhawk]

On 04/10/2023 at 10:02, Del-inquent said:

whilst the base figure of PIR is much more effective, movement, gaps, difficulty in effectively sealing in a retrofit with warped and twisted joitss etc

 

Let me introduce you to... Gapotape!

 

In all seriousness I'm making the same decision for our suspended floors. Probably going Rockwool but whatever's cheapest (so long as it's not glass fibre) as the airtightness will have a bigger impact on our comfort than more insulation. Loose wood fibre is appealing because it can fill all gaps and is easier to push into hard to reach areas.