Wet UFH on ground floor suspended floor. Marriage with an extension

[DCG]

Hi folks

 

I am new here. First post

 

To introduce myself, I have planning for a double storey side extension, full width rear extension and partial first floor rear extension in NW greater London. 

 

I am looking for some advice on how to get wet UFH on the ground floor. 

 

It's straightforward enough for the new bits of the ground floor but the existing bits of the house are on a suspended floor above a void. I think the void is probably around 300-500mm but probably closer to 300mm. I say this because I have seen some of the void depth at the back of the house when running some new power cables 5y ago. The house is on a flat road so I assume the same depth at the front of the house. 

 

My issue is how best to arrange the UFH on the suspended floor. What to do with the void? How to avoid a creating thermal bridge? How to keep air circulating assuming the back of the house will not have any void? Ideally, I would want the whole ground floor to warm up and cool down uniformly. Is this possible if you are marrying a suspended floor to an extension floor? The UFH in the extension will be laid in screed, is this the case for the UFH on the suspended floor too? I want the screed to act as a uniform thermal store and release heat evenly across the ground floor. 

 

My friend has UFH laid on a suspended floor. He says his builder filled the void with rubble (!) and then celotex type insulation, then UFH pipes, screed and tiles. His project only finished earlier this year so not really enough time for any issues to present themselves. 

 

Is this an acceptable way of doing UFH on a suspended floor? I have heard that filling the void with rubble is a very good way of cutting air circulation and introducing rising damp through thermal bridges. 

 

Obviously, I am very keen for the UFH solution not to be heating the void under the floor and instead sending heat upwards. 

 

I'd be grateful for anyone who has seen this issue to give some advice on how to correctly go about getting UFH. If it means I have to keep radiators at the front of the house to avoid issues, I'd rather do that than do UFH incorrectly and store up a problem for later down the line. 

 

This is a project to create a forever home. I want to get it right first time and happy to spend money on doing it correctly (although there is obviously an upper end to my budget!)

 

thank you folks

[DCG]

The grey bits are all new. 

 

The bit where it says DATUM will be demolished and absorbed into the open living area so will not be suspended floor. 

 

The entrance hall, living room and part where dining table and chairs are shown are all on suspended floor. 

 

Would be grateful for advice on:

 

1. Air bricks at the back of the house. Do I need to create a tunnel to the end of the patio (patio extends 5m beyond sliding doors) so that air can enter the void? 

 

2. How to create the insulation for the suspended floor and lay UFH on top.

 

3. Is is reasonable to wish for uniform heating and cooling of the ground floor given we are marrying two floor types. 

 

4. Optimum number of zones for my ground floor. 

 

5. Should I have hive or similar controlling the upstairs radiators and downstairs towel rails. And UFH controls for the UFH zones? 

 

Thank you!

 

 

 

 

 

 

 

 

 

 

 

 

[JohnMo]

Unless the insulation is really good under the floor in the new and old parts I would only expect high flow temps, not the best performance and large heating bills, when compared to radiators.

 

Are you planning for carpets or hard floors?

 

In the kitchen living dining you will have about 3 loops, the lounge another one or two loops and toilet another, I would treat these as a single zone.  Locate the manifold in the cupboard under the stairs, borrow heat from these loops to heat the hall. Locate a single thermostat in the hall to control the lot.  Balance the loops to get the room temps where you want them and so they all heat up at a similar rate.

 

Your spare bedroom and en suite I would install radiator and towel radiator.

 

[DCG]

Thanks

 

Why would you expect performance to be sub optimal in the new bits? I thought UFH was really good for extensions? 

 

What would constitute really good insulation for the new and old bits? 

 

And what to do with the void?

 

Any insights very gratefully received

 

Thank you

[JohnMo]

Really you need a u value better than 0.15, under the house (ground) will never be warmer than about 6 deg.  Ventilated area will be at outside temp.  So heat will travel down rather than up given half a chance. 

 

The worse the U value the more heat from the UFH is lost downwards, so the higher the flow temp needs to be, so you run the flow temps hotter to compensate...

 

Will leave others to advise on the voids.

[DCG]

I have come across this solution from continal. But I cannot understand what is going to hold up tons of 8:1 mix? They suggest laying the 8:1 on the celotex or similar insulation that goes between the joists of a suspended floor

 

Anyone  used this or seen this used and working well?

09_PUG_Dry_Mix_System_Datasheet_GB-EN.pdf

[Conor]

A drastic, but best performing option is to rip up all the existing floors, level the ground with some hardcore, sand binding, DPM, 300mm EPS, 100mm floated concrete floor with UFH. Marry that to the extension new floor; say 150mm PIR, 100mm screed, so you have consistent floor type, U values, heating loop lengths, levels etc.

Edited December 29, 2022 by Conor

[DCG]

Thanks. That is basically what my builder was suggesting but on another forum some people said that solution might cause rising damp if no air is circulating in the void. 

 

I guess your suggestion of DPM addresses that potential problem. 

 

Might you have some links where I can read about your suggestion more?

 

 

 

 

[HughF]

There isn’t a void, you rip the whole lot up and turn it into a slab floor…. The damp issue is caused by damp in the floor being pushed towards the walls (which are still in contact with the ground)…

 

It might be a non-issue, or it might be an issue. It all depends on what the water table is like where you are.

 

If you end up with two separate systems, a standard screed system in the extension and an overlay system in the old part of the house then you’ll have two separate response times and probably require different flow temperatures.

[DCG]

Garden does flood a bit in heavy rain. But that's about 30m from the house. 

 

I've never seen any water collecting near the house. 

 

I'd be grateful for recommendations on exactly what to ask my builder to do. I'm willing to take a punt on the water table 

 

 

[DCG]

1 hour ago, HughF said:

There isn’t a void, you rip the whole lot up and turn it into a slab floor…. The damp issue is caused by damp in the floor being pushed towards the walls (which are still in contact with the ground)…

 

It might be a non-issue, or it might be an issue. It all depends on what the water table is like where you are.

 

If you end up with two separate systems, a standard screed system in the extension and an overlay system in the old part of the house then you’ll have two separate response times and probably require different flow temperatures.

Thanks for this explanation as well btw. It's helped me understand the issue a lot more

 

Now trying to ascertain water table level from neighbours who may have taken their floors up 

[HughF]

12 hours ago, DCG said:

Garden does flood a bit in heavy rain. But that's about 30m from the house. 

 

I've never seen any water collecting near the house. 

 

I'd be grateful for recommendations on exactly what to ask my builder to do. I'm willing to take a punt on the water table 

 

 

150mm whacked hardcore, 200mm eps/xps, 100mm concrete with ufh pipes in. That’s 450mm to find, from FFL down…

 

If you do decide to keep the suspended timber floor then you will need to carry the ventilation through the slab of the extension and use telescoping vents and air bricks on the footings of the extension to provide the airflow.

 

Or just do what my neighbours did and concrete straight over the existing air bricks 🤣

Edited December 31, 2022 by HughF

[DCG]

Thank you! 

 

One final question

 

Is what your neighbours did problematic? 

 

I'm planning on doing as you suggest after ripping out the suspended floor. Was also planning on laying a new DPM. What layer would you suggest the DPM needs to be laid at? 

 

Thank you and happy new year!

 

 

[HughF]

9 hours ago, DCG said:

Thank you! 

 

One final question

 

Is what your neighbours did problematic? 

 

I'm planning on doing as you suggest after ripping out the suspended floor. Was also planning on laying a new DPM. What layer would you suggest the DPM needs to be laid at? 

 

Thank you and happy new year!

 

 

If you keep the suspended timber floor, it needs to be ventilated. BC should have picked this up on my neighbours extension, but didn’t.

 

I would put the dpm on top of the insulation.

[DCG]

I am effectively intending to do exactly as your neighbours did. 

 

I'm in a semi and planning on filling my floor out which will cut the ventilation that exists between my void under my floor and next door's void under their floor. 

 

Is this going to be a problem from a BC perspective? I should have mentioned I was in a semi earlier! 

[HughF]

No, because the air bricks for all of you will run front to back. They can’t run sideways because it’s a semi.

 

BC might not be happy if you have to excavate downwards to get enough depth for the floor buildup, they can get twitchy about exposing the footings. But that’s only an issue if you need to dig down a long way and your footings are shallow.

[DCG]

Thank you. 

 

Very informative and clear for a layman like me! 

 

Happy new year