Choosing a structural cement board for spreader style UFH with carpet

[Mmark]

We are installing a Wunda spreader plate with 50mm of celotex below. I need some guidance on switching from 22mm chipboard to structural cement boards.

Wunda have cautioned me about the carpet flooring that is now at 2.42 tog when combined with underlay. I'm using Tradaire King, at 0.8 tog, which seemed to be the lower tog I could find for carpets - lower ones were for wood/laminate floorings.

I cannot change the carpet, and we don't want to switch systems and lose head height, so I need to try and ensure this spreader system works with 2.42 tog of carpet. There are two new loft rooms (L shaped loft) (6mx4m and 4mx3m). Each with 100mm celotex in walls and roof (cold roof).

To give this the best chance of working, does it seem prudent to invest in structural cement boards, i'm trying to understand if the Knauf Gigafloor is actually twice as good as the other two. I've copied in pastes from their data sheets. The knauf is 25mm vs the others 22mm. Also I'm seeing a different use of units, STS has R for m2k/w while other have R for m2/kw, which makes it hard for me to compare.

I see the 22mm P5 chipboard has k value of 0.14, which doesn't seem that far behind the STS 22mm. So I'm also trying to understand the relative performance between the different systems and chipboard.

Knauf GifaFloor:

STS:

HardieFloor:

I've seen the gifafloor specced over at nu-heat too http://files.nu-heat.co.uk/core/media/media.nl?id=300642&c=472052&h=672c89d1105850def2d3&_xt=.pdf

GifaFloor is more expensive, so I want to be sure it's worth it, compared to the other two.

 

[Mmark]

Wunda got back and provided me details to show that Knauf Gifafloor FHB25 is the best of the three. I'll share this, so it's helpful to others in the future.

They normalised the tog values for all 4 structural decks, based on my carpet + underly tog of 2.42:
Ply (22mm) + Carpet = 4.12
Hardie + Carpet = 3.18
STS 22 + Carpet = 3.52
GifaFloor + Carpet = 3.02

It's really expensive, but I'm going with the Gifafloor fhb25. I just don't want to be in a situation where I end up having to pay more to redo it, if it's not effective enough.

 

[muggles]

What guarantees to Wunda give that their proposed system will actually perform as expected, IE it'll heat the room satisfactorily?

It seems to be rather a lot of hassle and expense to save half an inch of height build on an overlay system...

 

[Mmark]

What guarantees to Wunda give that their proposed system will actually perform as expected, IE it'll heat the room satisfactorily?

It seems to be rather a lot of hassle and expense to save half an inch of height build on an overlay system...

I have to be able to determine heat loss of the rooms, for them to be able to do that. All they can do is provide me calculations for heat output, for the various configurations. I hope to calculate the heat losses soon.

Changing from 22mm chipboard to 25mm Gifafloor raised the heat output by about 20% - in the calculations they showed me. It does cost more money, but happy to do so, if the heating is better. I actually went for 18mm GifaFloor FHB in the end, as it works for my 300mm centre joists and has even lower thermal resistence.

 

[dilalio]

IMO you'd be better off with air con in the loft. It gets hotter in the summer than it does colder in the winter.

I put pipes in for 2 rads in mine... Never fitted the second rad and the other is rarely on... Never on full. It's always warm enough up there through rising heat.

 

[Mmark]

IMO you'd be better off with air con in the loft. It gets hotter in the summer than it does colder in the winter.

I put pipes in for 2 rads in mine... Never fitted the second rad and the other is rarely on... Never on full. It's always warm enough up there through rising heat.

aircon is going up too. don’t ever plan to move, so getting all the home comforts in