Extension UFH combined with existing floor

MarkUFH · Oct 14, 2017

Hi.

I'm currently building a single floor rear extension that will have UFH, I'm pretty confident about the extension UFH. 100mm insulation, 70mm screed, etc. My question is about the existing kitchen and dining room that will become one open plan space as part of the extension.

The make up of the existing floor is 60mm of screed on a block and beam floor. I would like to install UFH in this existing area also. Is it worth/possible to take up the existing screed and install a insulated solution option B, or just go for an overlay solution option A?

I would prefer option C if possible due to it being insulated and introduces less of a height difference with the rest of the house. Any special considerations needed for option C?

Thanks in advance.

Chalker · Oct 14, 2017

Option b, every time .
Rip up the old floor and screed right through.
It's not perfect with 25mm insulation, but it's the best that you can get with the limitations of floor build up.
Also for tiling, it makes a continuous floor. With no differing rates of expansion.

Dumbo · Oct 14, 2017

Option b but at that thickness of screed you would have to use a poured screed but I'm not sure if it ok at 40mm @Ajax123 is your man to answer that .

MarkUFH · Oct 14, 2017

Thanks both.

I was planning on using and poured/liquid screed anyway. Is there an optimum insulation/screed ratio as I am height limited?

Also am I able to use the same pipe throughout or should the loop in the existing maybe user smaller pipe?

Cheers.

Ajax123 · Dec 6, 2017

If you use Gypsol HTC anhydrite screed over a 16mm pipe you can lay it at a depth of 41mm from the top of the insulation to the top of the screed. This screed is specifically designed to offer very thin depth and very high thermal performance to make the underfloor heating system as efficient as possible. Typically it increases the coefficient of performance for the heating system by some 25%.

If you intend to use a cement based screed then the minimum depth would be 65mm. This means that the minimum depth is over the top of the pip so with a 16mm pipe you would need 81mm. Most people compromise at 75mm...it may still crack.

MarkUFH · Dec 6, 2017

That's great thanks. Is there a maximum depth? The new extension screed will be about 70/80mm to make it level with the existing floor.

Ajax123 · Dec 6, 2017

MarkUFH said:
That's great thanks. Is there a maximum depth? The new extension screed will be about 70/80mm to make it level with the existing floor.

If you wnted a liquid screed you could just put deeper insulation into fill the void. This might allow you to use polystyrene instead of foil backed PIC thus saving quite a bit of money per m2

MarkUFH · Dec 11, 2017

Great idea. So just bring all levels to 40mm. Any recommendations for company's that do this screed in the Leicester area?

Ajax123 · Dec 11, 2017

MarkUFH said:
Great idea. So just bring all levels to 40mm. Any recommendations for company's that do this screed in the Leicester area?

will message you

MarkUFH · Aug 21, 2018

Hi all.

I eventually went for a cementitious screed . It was pumped in February. I'm only now in a position to commission the UFH and lay tiles.

Unfortunately some cracks have developed in the screed. Some only hairline, others maybe 2-3mm wide. They developed slowly in the first few months after installation and haven't really changed since then. These cracks are both in the existing section of the house, block and beam floor. And also in the extension, solid slab.

1. Should I be concerned with these cracks?
2. How do I stabilise them?
3. Am I OK to tile directly on the screed with porcelain tiles? 600mm x 300mm

Many thanks all.

Flintstone · Aug 22, 2018

Mark, you could repair the cracks with a resin crack repair system, there’s a few out there is too google. I would deffinately fit an uncoupling mat such as ditra over the floor to protect the tiles from any expansion / contraction and also to help take the risk away from the cracks transferring through the tiles. Also, the screed looks like it needs sanding, was it a pumped in fast setting type of cement screed ?

Ajax123 · Aug 22, 2018

theyre some impressive cracks. check for vertical movement and if there is any you would need to resin stitch them. See the following link for the repair kit I tend to recommend. If there is no vertical movement then you could just resin fill but either way uncoupling membrane is a requirement as per localtilers comment above

UZIN KR 516 (Subfloor Preparation | Substrate Repairs) - http://www.uzin.co.uk/products/product-search/details/uzin-kr-516-10788/

MarkUFH · Aug 22, 2018

Thanks both.

It was a pumped cement based screed as per the link Flowing Cementitous Screed | MaxiUltimate - https://maxi.co.uk/screed-applications/maxiscreed/ that shouldn't need sanding. There is some surface "dust" it I think it's just from general building/plastering.

There is no vertical movement that I can see. I was hoping to avoid the membrane. More expenses on an already expensive screed! :-(

Arrrggghhh! Thanks again.

Flintstone · Aug 22, 2018

That will need sanding mark, despite what the manufactuers claim. Here’s one I recently did. Not the same exactly but the same system, sprayed on curing agent etc

The first two are pre sanded, with a skin on, and the second are after sanding.

MarkUFH · Aug 22, 2018

Bigger, best get my sandpaper out! Matting over the whole area or just the cracks?

It's about 60m2

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