Retro Therma

[Onoff]

Looks like a neat, near dust free install method for fitting UFH in existing concrete floors but guessing the losses downward (& to the side?) must be pretty large without decent insulation in the floor make up?

 

 

[SteamyTea]

12 hours ago, Onoff said:

but guessing the losses downward (& to the side?) must be pretty large without decent insulation in the floor make up?

Yep, it relies on people thinking that low temperature automatically means low energy.

I was recently talking to someone about an UFH system that was being fitted to a new house.  I asked how much insulation was being fitted under it, the reply was "80mm, it's the building regs standard".

[Jeremy Harris]

A quick and dirty estimate of losses, assuming the floor is set to deliver around 30 W/m² (which is on the low side for a house built to just meet building regs) gives the percentage heat lost down through a 100mm concrete slab set on 80mm PIR underfloor insulation of around 14.35%, for a room temperature of 21°C.

 

With no insulation under the floor the heat loss increases so that around 4 times more heat would flow down to the ground beneath than would flow into the room (assuming a 150mm thick slab bearing on the ground, it would be worse for a 100mm slab).

 

 

[SteamyTea]

What would be the losses if the insulation was increased to 120mm?

[Jeremy Harris]

1 minute ago, SteamyTea said:

What would be the losses if the insulation was increased to 120mm?

 

With the same 100mm slab, 30 W/m² output from the floor, and 21°C room temperature, with the same 8°C ground temperature, the losses would be around 9.64% for 120mm of PIR, with a λ of 0.022 W/m.K.

[SteamyTea]

So worth it for an extra 40mm.

Especially as builders can't work to that accuracy.

 

Edited May 7, 2019 by SteamyTea

[Jeremy Harris]

13 minutes ago, SteamyTea said:

So worth it for an extra 40mm.

Especially as builders can't work to that accuracy.

 

 

 

Yes, especially as 30 W/m² is probably a bit higher than would be needed with extra insulation under the floor, due to the reduction in whole house heat loss.

[epsilonGreedy]

32 minutes ago, JSHarris said:

With the same 100mm slab, 30 W/m² output from the floor, and 21°C room temperature, with the same 8°C ground temperature, the losses would be around 9.64% for 120mm of PIR, with a λ of 0.022 W/m.K.

 

 

I thought my original heat loss of 5.8% for 150mm of insulation was low, having adjusted the insulation number from the PIR default to EPS I now get 9% at a floor temp of 25 c.

 

Would I be correct in thinking EPS is the default choice under a screed in a traditional build?

[Onoff]

Anyone care to calc the loss of 150mm pir with 25mm eps underneath?  Cheers.

[jack]

1 minute ago, epsilonGreedy said:

Would I be correct in thinking EPS is the default choice under a screed in a traditional build?

 

Tends to be PIR I believe, largely due to better performance for a given (often height-limited) thickness.

EPS tends to be used under slabs in raft constructions where a separate screed isn't used, largely because of its durability in that application.

[epsilonGreedy]

51 minutes ago, JSHarris said:

the losses would be around 9.64% for 120mm of PIR, with a λ of 0.022 W/m.K.

 

 

One thing I noticed while adjusting the whole house heat loss to 50% or 30% of worst case is that the heat loss to ground rockets up to > 20%.

[Jeremy Harris]

1 minute ago, Onoff said:

Anyone care to calc the loss of 150mm pir with 25mm eps underneath?  Cheers.

 

For a 100mm slab, delivering 30 W/m² to the room, with a ground temperature of 8°C and a room temperature of 21°C, the losses through the insulation would be about 6.98%

[SteamyTea]

2 hours ago, epsilonGreedy said:

heat loss to ground rockets up to > 20%.

That is to be expected because it is a percentage of the whole.  This is not really the way to look at it.  Really should look at the kWh/year and what that is costing in terms of emissions and cash.

[epsilonGreedy]

3 minutes ago, SteamyTea said:

That is to be expected because it is a percentage of the whole.  This is not really the way to look at it.  Really should look at the kWh/year and what that is costing in terms of emissions and cash.

 

 

 

I guess so but previously I had scaled down my under screed insulation thickness to 150mm on the basis that anything below 10% was acceptable, now I realize the majority of the time the ufh will be coasting along well below the max output in the zone where the 10% to 20% factor applies. 

 

[Jeremy Harris]

Always best to look at the actual heat loss numbers as well as the percentage to see if it is worth worrying about, IMHO. 

[SteamyTea]

1 hour ago, epsilonGreedy said:

where the 10% to 20% factor applies.

Generally, in science, it is always worth looking at the actual units rather than in percentage terms.

To give you an example, dairy farming uses about 1 GWh of electrical energy a day, Sound a lot, but is about 3% of our mean energy usage.

Well worth it for something to put in tea.

That is about the same as electric cars currently use.

 

 

Edited May 7, 2019 by SteamyTea