UFH Screeds

[gc100]

There is no reason to avoid anhydrite based screeds. You just need to use them correctly like any other building material. You just re-move the lattice with a grinder (it took me 1 day for 180m2). With any UFH best to always use a separation matting (DITRA etc) if tiling on top. Not rocket science. We went with the anhydrite screed as the whole house was poured in a couple of hours and we had no more than 2mm height difference from one end of the house to the other making for lovely flat tiling . It was also cheaper than concrete for us.

Edited February 8, 2021 by gc100

[Thorfun]

5 minutes ago, gc100 said:

There is no reason to avoid anhydrite based screeds. You just need to use them correctly like any other building material. You just re-move the lattice with a grinder (it took me 1 day for 180m2). With any UFH best to always use a separation matting (DITRA etc) if tiling on top. Not rocket science. We went with the anhydrite screed as the whole house was poured in a couple of hours and we had no more than 2mm height difference from one end of the house to the other making for lovely flat tiling . It was also cheaper than concrete for us.

thanks for this reassuring post. we have our FFL designed around an anhydrite screed so would've been a real ball ache to change it! we will proceed as planned with the anhydrite. cheers.

[Temp]

Yes sure you can still use them...if you grind off any laitance. Its just a job I'd avoid if possible.

[Adsibob]

On 02/01/2021 at 22:01, johnbarleycorn01 said:

liquid, quick drying Gypsum based screed- not cheap £1600 approx. If you can, put as much PIR into the ground as you can afford, i could only only afford 100mm, most pewople recommend 150mm as a minimum.

If I only have space for 100mm of insulation in between my slab and my screed, am I better off combining two types of insulation. Eg. 70mm of PIR and 33mm of aerogel such as Thermablok Aerogel https://www.thermablok.co.uk/our-products/thermablok-aerogel-magnesium-floor-board/

This aerogel product has is 0.015W/m2K so about 30% better than PIR such as Thermafloor TF70 which has 0.022 W/mK.

 

An alternative to those two products, which would save labour, would be to just use 100mm of Kooltherm K103 Floorboard which is 0.018 W/mk.

 

[Declan52]

Get a price for aerogel and then rule it out. As you have been told in the other thread look at a method to remove the concrete sub floor which will give you enough depth to boost the insulation. 150mm of pir is the minimum depth you should be looking at. 

[Adsibob]

16 hours ago, Declan52 said:

Get a price for aerogel and then rule it out. As you have been told in the other thread look at a method to remove the concrete sub floor which will give you enough depth to boost the insulation. 150mm of pir is the minimum depth you should be looking at. 

My project is incredibly structurally complicated - we have about 12 RSJs and 7 or 8 columns, and we're dealing with a property that is almost 100 years old and has a fairly random pattern of differing corbel heights as its foundations. You'll therefore understand why I'm confused by the suggestion that I (a layman with no engineering expertise) can second guess my structural engineer and tell him that he has to do away with the concrete subfloor. He has specified it for a reason. 

[Declan52]

The sub floor is just usually there to give you a surface with which to work of which it it takes a bit of damage it won't matter. It doesn't do anything else normally. 

Ask your structural engineer can you do away with it and use one single layer of concrete that contains all your ufh pipes plus steel mesh. 

[Adsibob]

15 hours ago, Declan52 said:

The sub floor is just usually there to give you a surface with which to work of which it it takes a bit of damage it won't matter. It doesn't do anything else normally. 

Ask your structural engineer can you do away with it and use one single layer of concrete that contains all your ufh pipes plus steel mesh. 

Spoke to the Structural Engineer. Any suggestion of losing the slab or combining it with the screed was a no. He has already specified that the slab be reinforced with steel mesh so can't really make it any stronger. It may be that your experience is not of London clay, but unfortunately we have metres of it beneath our house, squelching with water. We are already having to do some underpinning because the council planners insisted we sink our extension into the ground to reduce the height of it. Engineer said that if I want to lower the slab to make more room for more insulation, I will have to do more underpinning - at £1k plus VAT per linear metre, it's just not an option. It will also delay our works increasing our rental costs, which are significant. So I'm back to my original quest which is to find the best insulation I reasonably can that will fit in the 100mm of space I have for it.

[Moonshine]

46 minutes ago, Adsibob said:

Spoke to the Structural Engineer. Any suggestion of losing the slab or combining it with the screed was a no. He has already specified that the slab be reinforced with steel mesh so can't really make it any stronger. It may be that your experience is not of London clay, but unfortunately we have metres of it beneath our house, squelching with water. We are already having to do some underpinning because the council planners insisted we sink our extension into the ground to reduce the height of it. Engineer said that if I want to lower the slab to make more room for more insulation, I will have to do more underpinning - at £1k plus VAT per linear metre, it's just not an option. It will also delay our works increasing our rental costs, which are significant. So I'm back to my original quest which is to find the best insulation I reasonably can that will fit in the 100mm of space I have for it.

 

Aerogel - 0.015 W/mK

Kingspan Kooltherm K103 - 0.018 W/mK

PIR insulation (many brands) - 0.022 W/mK

 

Get quotes for each of those at 100mm and see how much your wallet can handle

[Declan52]

2 hours ago, Adsibob said:

Spoke to the Structural Engineer. Any suggestion of losing the slab or combining it with the screed was a no. He has already specified that the slab be reinforced with steel mesh so can't really make it any stronger. It may be that your experience is not of London clay, but unfortunately we have metres of it beneath our house, squelching with water. We are already having to do some underpinning because the council planners insisted we sink our extension into the ground to reduce the height of it. Engineer said that if I want to lower the slab to make more room for more insulation, I will have to do more underpinning - at £1k plus VAT per linear metre, it's just not an option. It will also delay our works increasing our rental costs, which are significant. So I'm back to my original quest which is to find the best insulation I reasonably can that will fit in the 100mm of space I have for it.

What about lifting door head heights to allow you to add extra insulation to the floor. 

How big of a renovation job is this going to be??? 

Or just accept the fact the ufh will lose more heat to the ground than you would like and live with it.

[Adsibob]

7 hours ago, Declan52 said:

What about lifting door head heights to allow you to add extra insulation to the floor. 

How big of a renovation job is this going to be??? 

Or just accept the fact the ufh will lose more heat to the ground than you would like and live with it.

It's a huge renovation project (it would have been easier to knock the house down, but being semi detached that wasn't really an option). 
I had considered increasing ceiling height, but it complicates things upstairs as we have a first floor extension that council planners have limited in height quite severely, so that would need to be on a different level if we raised the ground floor ceiling height.

I think we will end up going with Kingspan Kooltherm K103 in most of the ground floor and cheaper stuff in the utility room and hallway, but I need to check what the price difference will be.

[Nickfromwales]

How much focus have you given to air-tightness? You can get away with the lower level of insulation IF your infiltration rates are hugely improved. And I mean hugely. Are you going for AT ( airtightness ) and MVHR? The cooler you can run the UFH the lower the losses will be. The real fear here is that if the dwelling needs very high w/m2 output to get ( and keep ) the rooms at temp, then the losses to ground increase significantly. Can you squeeze in 120mm of KIngspan? What thickness are you allowing for screed btw?

[Adsibob]

8 hours ago, Nickfromwales said:

How much focus have you given to air-tightness? You can get away with the lower level of insulation IF your infiltration rates are hugely improved. And I mean hugely. Are you going for AT ( airtightness ) and MVHR? The cooler you can run the UFH the lower the losses will be. The real fear here is that if the dwelling needs very high w/m2 output to get ( and keep ) the rooms at temp, then the losses to ground increase significantly. Can you squeeze in 120mm of KIngspan? What thickness are you allowing for screed btw?

We are installing MVHR, yes. All glazing in the house will be trickle vent free, and I will try and tape any other gaps, but I still struggle with this concept of taping, as with a poured screed on the ground floor and new insulated plaster board and plaster going on all the inside of the walls and ceilings, 20mm of external insulation plus 5mm of external render outside and new flooring on first and second floor, and all voids filled with minderal wool, I struggle to see where air leaks will come from. I'm not building to passive haus standards, so whilst I do want the MVHR to run well, I still struggle conceptually with this taping requirement.

Screed on ground floor will be 60mm deep.

No space for 120mm kingspan unfortunately.

[ProDave]

9 minutes ago, Adsibob said:

We are installing MVHR, yes. All glazing in the house will be trickle vent free, and I will try and tape any other gaps, but I still struggle with this concept of taping, as with a poured screed on the ground floor and new insulated plaster board and plaster going on all the inside of the walls and ceilings, 20mm of external insulation plus 5mm of external render outside and new flooring on first and second floor, and all voids filled with minderal wool, I struggle to see where air leaks will come from.

 

Usual issue is if you are relying on the plasterboard and skim to be your air tight layer is you then leave the possibility of it being a "plasterboard tent"  What usually happens through simple lack of attention, is this gap behind the plasterboard ends up being open to the cold loft space.

 

It is almost normal up here in the winter when i unscrew a socket or a light switch in a house, to be greeted by a blast of icy cold air coming out from behind the plasterboard tent.

 

You really want to make the fabric of the building air tight so the plasterboard remains within that.

 

[DonkeyTable]

56 minutes ago, ProDave said:

Usual issue is if you are relying on the plasterboard and skim to be your air tight layer is you then leave the possibility of it being a "plasterboard tent"  What usually happens through simple lack of attention, is this gap behind the plasterboard ends up being open to the cold loft space.

 

It is almost normal up here in the winter when i unscrew a socket or a light switch in a house, to be greeted by a blast of icy cold air coming out from behind the plasterboard tent.

 

You really want to make the fabric of the building air tight so the plasterboard remains within that.

 

That should have been fixed by the dryliner adding a layer of gunge along the top and bottom of the dot and dab. The difficulty is when the house is using plasterboard on Gypliner or batten. Fixing all the holes is a right pain. Then throw in ceilings on resilent bars.

[DonkeyTable]

On 01/01/2021 at 22:22, sheepie said:

Hi Folks,

 

I have had a look at some plans for a house + land that has PP and founds in place.

 

The Construction of the sub floor is 100m concrete then goes the DPM then Insulation 100mm poly but the cut away shows a further 100mm screed

 

But say I wanted to put UFH in does this add further to the 100mm screed or do I skip this and put in a lower 60+mm screed. 

 

The second wee question is there a difference between Liquid Screed (That seems to be laid over UFH) vs Sand and Cement mix?

 

Cheers

If its a new build you will have to get an energy survey carried out. This will tell you what thickness of floor insualtion you can get to meet current building regs. I often hear you need 150mm of floor insulation, however this is not the case. The survey is a whole house calculation, should cost about £250. In my case 130mm of floor insulation met current building regs.

[ProDave]

1 hour ago, DonkeyTable said:

That should have been fixed by the dryliner adding a layer of gunge along the top and bottom of the dot and dab. The difficulty is when the house is using plasterboard on Gypliner or batten. Fixing all the holes is a right pain. Then throw in ceilings on resilent bars.

But it is usually NOT sorted.  To get an air tight house it is all down to detail being done properly.  Sadly a lot of people do not know or do not care about doing it properly.

 

It is far better to make the whole fabric of the building air tight, then all the wiring and plumbing can be contained in a service void without having to penetrate the air tight layer.