Where Does The 150mm Requirement Come From?

[Onoff]

Chippy mate mentioned he's helping out on a new floor job in an extension. Original builder has been given the push.

 

Build up is:

 

100mm Type 1

50mm sand blind

DPM

125mm concrete with A142 mesh

75mm pir

Membrane

70mm screed with UFH

 

I said I thought the insulation should be a minimum of 150mm pir. He reckons it's being done to proper drawings and all the bco was worried about was the original builder using broken roofing tiles instead of Type 1. 

 

All I can find ref floor insulation is this and it says to use 75mm "foam" or 150mm wool:

 

https://energysavingtrust.org.uk/advice/floor-insulation/#:~:text=The U-value is a,floor type%2C shape and size.

Edited February 20, 2022 by Onoff

[JohnMo]

I think it is the min insulation required to meet passivhaus levels of insulation with u value of 0.15.  Could be well wrong in the assumption.

[hb1982]

U-Value for floor, in solid slab on grade, is subject to the perimeter to area ratio.

 

150mm in general is an overkill.

 

Regards

[ProDave]

Yes if they want it to perform well, 75mm is woefully poor for a floor with UFH.  Many of us find it sad that people are still building like this, and the customer is probably not aware of the poor job they are getting and not been advised that more insulation = lower running costs and a warmer house.

[SteamyTea]

19 minutes ago, Onoff said:

I said I thought the insulation should be a minimum of 150mm pir.

It is an extension to an existing building, so a lower U-Value of 0.7 W.m^-2.K^-1, so the drawings are probably correct.

Don't mean it is a wise decision. 

[SteamyTea]

5 minutes ago, hb1982 said:

150mm in general is an overkill.

why do you think that when the UFH makes the temperature gradient to the ground a lot higher?

[SuperJohnG]

Scottish regs attached..

 

5 minutes ago, hb1982 said:

150mm in general is an overkil

Overkill why? 

[nod]

1 hour ago, ProDave said:

Yes if they want it to perform well, 75mm is woefully poor for a floor with UFH.  Many of us find it sad that people are still building like this, and the customer is probably not aware of the poor job they are getting and not been advised that more insulation = lower running costs and a warmer house.

Plus 1

I would never go Les than 150 

and it ties in perfectly with a block off the floor 

[Temp]

I have 80mm under our UFH and would go for more next time.

[Onoff]

1 hour ago, Temp said:

I have 80mm under our UFH and would go for more next time.

 

I have 150mm pir + 25mm eps under my bathroom floor and the room is still bloody freezing...

 

?

[hb1982]

The floor buildup suggested is a solid concrete slab on ground (not suspended) in a house extension.

If you check out the Kingspan Insulation Calculator

75mm Insulation (PIR) - 0.40 P/A ratio (e.g. 6m extension for a 8m wide room), the resultant U-Value is 0.17W/m2K.

For 150mm insulation, the result is 0.10 W/m2K, that what I consider as an overkill (at least from the builders perspective) 

 

https://www.uvalue-calculator.co.uk/calculator/floors/ground floor/solid concrete - insulation below screed/0.4/75/

 

 

[SteamyTea]

1 minute ago, hb1982 said:

The floor buildup suggested is a solid concrete slab on ground (not suspended) in a house extension.

If you check out the Kingspan Insulation Calculator

75mm Insulation (PIR) - 0.40 P/A ratio (e.g. 6m extension for a 8m wide room), the resultant U-Value is 0.17W/m2K.

For 150mm insulation, the result is 0.10 W/m2K, that what I consider as an overkill (at least from the builders perspective)

That is missing the point. This extension is getting UFH, so the slab will be at 30⁰C plus, not at 17 or 18⁰C.

You would not expect a plumber to bury a radiator in the garden.

[hb1982]

11 minutes ago, SteamyTea said:

That is missing the point. This extension is getting UFH, so the slab will be at 30⁰C plus, not at 17 or 18⁰C.

You would not expect a plumber to bury a radiator in the garden.

 

For a 50sqm area, the 0.07W/m2K difference is going to give you around 40W extra heat losses theoretically. For a ground slab, it could be less or more subject to the ground temperature fluctuation. We may all have different perspectives on this.

From a builder perspective, 75mm PIR in a 0.4 P/A ratio scenario, it meets the building regulation and he saves £800 on insulation material and more on avoiding extra digging / grab lorries / time. 

From a homeowner perspective, it will take a lot of heating hours to payback that £800 + £ xxxx costs.

 

It is a similar conversation like if electric cars are more eco-green considered that huge amount of lithium is required to be dug from ground. 

[SteamyTea]

2 minutes ago, hb1982 said:

0.07W/m2K difference is going to give you around 40W extra heat losses theoretically.

What delta T have you calculated that on?  Room temperature to Ground temperature (about 10°C) or slab temperature to ground temperature (about 22°C). 

2 minutes ago, hb1982 said:

It is a similar conversation like if electric cars are more eco-green considered that huge amount of lithium is required to be dug from ground. 

Two things, BEVs, even if powered purely from coal fired powerstation have a lower life cycle analysis than ICE cars.

Lithium is not, generally, 'dug from the earth'.  It is dissolved in water and then naturally evaporated, leaving a salt that then gets processed.  Unlike iron and aluminium ore which is mined, generally open cast mining.

[joth]

1 hour ago, hb1982 said:

For a 50sqm area, the 0.07W/m2K difference is going to give you around 40W extra heat losses theoretically.

 

The variable missing here is what floor finishes are put onto of the slab. If the homeowner likes wood floors, or especially thick carpets, then the slab is insulated on both sides, so they will have to run the UFH flow temperature that bit hotter and longer to get the room to stay at temperature. This means the 40W of losses may well be more like 80W (for a 35ºC flow temperature) and experienced for more hours of the day.

Aside from the financial impact, some users may simply not want this design due to:

a) potential damage to wood floors from the higher flow temp

b) the lower level of comfort achieved in having hot/cold cycling on the flooring (vs well insulated floor and a stable constant temperature)

c) the CO2 cost of the energy wasted

 

In the grand scheme of putting in an extension and UFH, £800 is not a great deal really

 

 

[hb1982]

 

3 hours ago, SteamyTea said:

What delta T have you calculated that on?  Room temperature to Ground temperature (about 10°C) or slab temperature to ground temperature (about 22°C). 

Two things, BEVs, even if powered purely from coal fired powerstation have a lower life cycle analysis than ICE cars.

Lithium is not, generally, 'dug from the earth'.  It is dissolved in water and then naturally evaporated, leaving a salt that then gets processed.  Unlike iron and aluminium ore which is mined, generally open cast mining.

 

The delta T was based on 12C (30degC in UFH vs 18degC in non UFH) as you suggested in previous post.

 

The BEV/ICE may not be a good example but that is not really the conversation here. 

 

As a home owner, there is no harm to go with better insulation as long as the decision is informed. 

 

1 hour ago, joth said:

 

The variable missing here is what floor finishes are put onto of the slab. If the homeowner likes wood floors, or especially thick carpets, then the slab is insulated on both sides, so they will have to run the UFH flow temperature that bit hotter and longer to get the room to stay at temperature. This means the 40W of losses may well be more like 80W (for a 35ºC flow temperature) and experienced for more hours of the day.

Aside from the financial impact, some users may simply not want this design due to:

a) potential damage to wood floors from the higher flow temp

b) the lower level of comfort achieved in having hot/cold cycling on the flooring (vs well insulated floor and a stable constant temperature)

c) the CO2 cost of the energy wasted

 

In the grand scheme of putting in an extension and UFH, £800 is not a great deal really

 

 

 

£800 is cost difference on the insulation only. There will be extra due to extra digging and removal of earth subject to your site condition. 

Subject to your location, the carbon payback could be questionable as well.

 

My original reply, to be honest, was trying to answer the OP question, i.e. if 75mm floor insulation is sufficient. 

My answer was/is yes from the building regulation perspective. Solid slab U-Value calculation is different from suspended floor calculation. If the P/A ratio is right, 75mm PIR in the said sbuil-up could meet the building regulation requirements.   

[bassanclan]

8 hours ago, Onoff said:

 

I have 150mm pir + 25mm eps under my bathroom floor and the room is still bloody freezing...

 

?

This suggests that the extra money you spent on 150mm pir over say 75mm could have been diverted to improving the other elements e.g. wall roof etc to get more bang for your buck

[SteamyTea]

11 minutes ago, hb1982 said:

The delta T was based on 12C (30degC in UFH vs 18degC in non UFH) as you suggested in previous post.

Be more like 20⁰C difference, and I assumed UFH with thicker insulation and radiators with thinner insulation, where the dT may only be 10⁰C (assumes winter ground temperature is 8⁰C).

But think the original question was with regards UFH and where does the thickness come from.

It comes from Part L building regs. Probably why they are going to be updated to take low temperature UFH into account.

[Onoff]

22 minutes ago, bassanclan said:

This suggests that the extra money you spent on 150mm pir over say 75mm could have been diverted to improving the other elements e.g. wall roof etc to get more bang for your buck

 

I've 6" of pir above the ceiling.  On the one external 3m,  cavity wall, I've 50mm of pir one end tapering to about 150mm the other. 

[hb1982]

2 hours ago, Onoff said:

 

I've 6" of pir above the ceiling.  On the one external 3m,  cavity wall, I've 50mm of pir one end tapering to about 150mm the other. 

 

that is like an highly insulated box. sounds like the cold could be from air infiltration or generally the room is not heated?

[Onoff]

3 hours ago, hb1982 said:

 

that is like an highly insulated box. sounds like the cold could be from air infiltration or generally the room is not heated?

 

The room is not heated...yet.

 

There WAS a continuous VCL on the inside of the pir but I've cut lots of holes in it for spots, speakers and the body dryer. Not to mention a number of conduits open to the cold loft space.