DPC above *and* below insulation?

[Tim2021]

Getting ready to pour the floor slab here.....

 

Plan is: 100mm polystyrene in the cavity, cut flush with the outer wall.

 

75mm PIR inside, flush with the top of the inner wall. (Inner wall is 7N aircrete block for thermal bridging)

 

Radon barrier above the PIR, out over the polystyrene, extending right through to exterior.

 

A142 mesh out to the slab edge above the inner wall on 30mm mesh-men, UFH pipe tie-wrapped to it.

 

Does that all sound OK? 

 

Do I also need another polythene layer between the 75mm PIR and the compacted sand/gravel beneath?

 

Thanks,

Tim

 

[Mr Punter]

If the pir is your floor insulation, 75mm is not much.  If you have UFH you will lose lots of heat.

[Tim2021]

36 minutes ago, Mr Punter said:

If the pir is your floor insulation, 75mm is not much.  If you have UFH you will lose lots of heat.

Thanks for the input.

 

The online calculator says that 75mm will give a u-value of 0.19 (target is <0.2).... what u-value would you suggest for this please? Is my target wrong?

[Marvin]

Goodness! I have 200mm PIR in my floor. I must have been too enthusiastic again...

[markc]

I would always put a DPM between blinding layer and insulation

[Moonshine]

22 minutes ago, Tim2021 said:

Thanks for the input.

 

The online calculator says that 75mm will give a u-value of 0.19 (target is <0.2).... what u-value would you suggest for this please? Is my target wrong?

 

Where is your target of 0.2 coming from? 75mm may well achieve that target bit its not going to perform very well (i am putting 150mm in my floor)

[Tim2021]

1 minute ago, Moonshine said:

 

Where is your target of 0.2 coming from? 75mm may well achieve that target bit its not going to perform very well (i am putting 150mm in my floor)

I found it in Part L I think, I will email my inspectors (Stroma) and ask. They've had the plans for months and haven't questioned it.

[Mr Punter]

The regs have some flexibility.  The worst you can have for a floor is 0.25.  The suggested is 0.13.  In reality if you are at 0.19 you will need to do a hell of a lot elsewhere.

 

We were putting in 75mm Celotex 15 years ago, but the regs have tightened up a lot since then.

 

Ground floors are an easy win as the insulation is simple to fit and you don't lose any floor area or have a larger footprint.  I think you should look at 150mm PIR or 250mm EPS.

[ADLIan]

75mm PUR sounds to be the very minimum to get through Building Regs - would expect at least 100mm and as above 150mm+. I'm not aware of U=0.20 being against any flooring requirement, 0.25 is the worst acceptable. What does your SAP report say?

 

Normally PUR in a floor requires a dpm below and another membrane above - check with manufacturer instructions and BBA certificate install instructions

[joe90]

1 hour ago, Marvin said:

Goodness! I have 200mm PIR in my floor. I must have been too enthusiastic again...

No, I have 200mm also ?

[Tim2021]

Thanks for the assistance.

 

BCO has just emailed and confirmed target 0.2 and that >70mm will do it. (I wish I had checked here earlier and added insulation but I really don't want to dig it all out again.)

 

Heating is by GSHP / Solar PV so little carbon impact long term.

 

I will put another polythene layer below the insulation as advised on this thread.

 

Tim

[ADLIan]

The gshp and pv will keep the co2 emissions down but there is also a fabric energy efficiency standard and if other U-values are not up to scratch you may have problems. U=0.2 in a floor is not good. Have you had a SAP assessment done as this will give insulation requirements and fabric efficiency numbers. Does your bco have the sap numbers to confirm compliance with Regs?

[Moonshine]

9 minutes ago, Tim2021 said:

BCO has just emailed and confirmed target 0.2 and that >70mm will do it. (I wish I had checked here earlier and added insulation but I really don't want to dig it all out again.)

 

Heating is by GSHP / Solar PV so little carbon impact long term

 

sorry to say but i think that you are going to get cold floors, as the GSHP / ASHP relies on having well insulated houses

[Tim2021]

I did a little basic maths to give me some confidence....If I just calculate at the heat loss into the floor and ignore the power heating the house for a moment.

 

The extension floor is 25m^2, if I wanted it to be at 35 degrees C in January (when the ground below is 5 degrees) the power required will be (35-5) x 25 x 0.19 = 143W 

 

If I had 150mm PIR insulation the U-value would drop to 0.11 W/m^2K and the power required would be 83W

 

So the additional burden on the heat pump is 60W or 1% of it's 6kW capability, in the worst case winter. So I can see why the BCO is cool about it.

 

60W is not nothing, but do I really dig the floor back out for a 1% improvement in the GSHP? 

 

I could scrape 25mm off, add a layer of polystyrene and reduce the excess loss to only 30W.