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