Insulation below or above concrete? To screed or not to screed?

[karenw]

Hi,

Remodelling/extending a 1970's house, as much DIY as possible.  Putting in water UFH throughout. to replace a really old radiator system. On the ground floor we currently have a suspended timber floor in the existing house and were going to do a suspended concrete beam and block in extensions with lewis deck/screed in existing house.  However, joists on ground floor are not strong enough for this and when preparing the site we have discovered a huge concrete slab externally under patio slabs that is 150mm deep and covers most of the extensions areas.  The depth from this to dpc is approx 340mm.  In addition, we have a void of 240mm from dpc to a concrete oversite in existing house. So we have changed our minds and think insulation, concrete slab and screed makes more sense.  The questions we have are:

 

1. As we have a lot of solar gain, we were considering putting insulation under concrete to take advantage of the thermal mass with anyhdrite screed over ufh pipes for self levelling and efficiency. We can't seem to figure out how to fix pipes to concrete if we do this and ufh companies we have asked advise against doing it because of the lag in heating effectiveness.  We know this but think gains outweigh disadvantages for us. Has anybody put insulation under concrete and how has it worked out and any advice on fixings to use ?

 

2.  Putting ufh in concrete and not having screed?  Concerned about levelling floors and efficiency.  Are there any other advantages apart from thermal mass, saving cost of screed? Finished floors likely to be tiles.  Was considering polished concrete a long time ago but costs scared us off unless it could be done diy when ufh is in the concrete. Is this done by fixing to mesh or any other ways?

 

3. First floor of existing house: was going to use lewis deck and screed as want to use wet ufh.  Now considering dry screed boards such as screedboard20 or knauf brio to save drying time etc. Has anybody used these?  Do they live up to their claims that they are more efficient than a wet screed?  Any other cost effective ideas for first floor water underfloor heating?  The space is open plan and a vaulted ceiling so no concern about height build up, although we need to tie in a flush floor with the roof terrace of the single storey extension at the rear.

 

I know you pay your money and take your choice to a certain extent but when there are so many options out there it is helpful to hear other people's experiences and hindsight.  If this has already been discussed, happy to be pointed to the post but I tried to look and there are a lot of ufh posts.

 

Thanks,

 

Karen  

[ToughButterCup]

You can't waste time on this board Karen. It's not easy to drill down to the core message sometimes but mostly a bit of effort repays handsomely. 

 

Or you could just PM @Russell griffiths, he knows everything. 

[joth]

56 minutes ago, karenw said:

We can't seem to figure out how to fix pipes to concrete if we do this and ufh companies we have asked advise against doing it because of the lag in heating effectiveness.

Key reason for this is most houses (even those built to current regs) are drafty and have too little insulation. Result is people leave the heating off as long as possible, then when a cold snap hits they want instant heat everywhere. This is compounded by having too little insulation under the slab meaning it's just as easy (or easier) for the UFH to escape down into the ground than up through the slab.

 

The decision to have a large heated slab (be it concrete or a decent thickness of screed) on top of UFH is often coupled to having a very well insulated house everywhere, and especially good levels of insulation (150mm min, up to 300mm often advised) under the UFH. This means heating can be left on year round, just driven by a room-stat to keep the whole house at a constant temperature. This way you're never trying to heat the house back up from cold, and so the time it takes for heat to come up through the slab is largely irrelevant. (And as you say, provides additional benefit)

 

What insulation and airtightness standard will you be aiming for?

 

1 hour ago, karenw said:

we were considering putting insulation under concrete to take advantage of the thermal mass with anyhdrite screed over ufh pipes for self levelling and efficiency

 

This is what we wanted to do but our architect and SE weren't open to considering it. Our local building control messed us about with the slab enough as it was (we're renovating and extending a 60s build; originally it was signed off with a raft foundation for the extension but after it was dugout the BCO had a change of heart and wanted strip founds dug) so in retrospect I'm glad we kept away from doing anything more ambitious on that front.

 

We have 100mm concrete, 160mm Celotex, 75mm Screed (embedded UFH), which for us is enough to meet EnerPHit+ standard and hopefully give us some reasonable thermal mass(TM)  but on the low side compared to most on here.