Floating floor or concrete in extension

[bajgal]

A rather specific and technical building question here for those of you who've extended your terraced house! OH and I are about to have a rear and side extension built on our terraced house. The floor of the main house is a floating wooden floor which is warm enough, even in the current freezing temperatures. However, our kitchen's at the rear of the house and has an extension added on to it by a previous owner. These both have a tiled floor which is freezing cold. OH suspects this extension may have a concrete foundation and that concrete's been poured into the original kitchen foundation where it's been tiled. My question is, can we dig up all this concrete and put in a floating floor like we have at the front of the house, so our feet don't freeze and will this be better value than putting underfloor heating in? Could we then have a floating wooden floor in the extension we're about to build? Thanks, hopefully there's some engineering minded folk out there who can help!

A modern concrete floor can (should) have a damp-proof membrane to keep it dry, and an insulating slab to make it less cold. It will never be warm, though, unless you add UFH. It is only worth the cost of adding UFH if you are going to dig up and relay the floor anyway. Wet UFH (supplied by a gas boiler) is not very expensive to run.

Electric UFH IS very expensive to run.

If you are digging out and laying a new floor, you can have wooden joists and a wooden or ply floor if you want (not chipboard, please) and you can insulate between the joists. If you insulate and carpet it, it will not be cold. You can add UFH if you want to.

Breaking up and digging out a concrete floor is very noisy and dusty. In most cases they are not very thick and can be broken up within a day or two. Think of road menders digging up a hole or watermain. With the right heavy gear they can do it in an hour or so.

It will depend on the age of the extension whether it is (of should be) insulated: floor insulation is a relatively new thing.

A suspended timber floor as suggested by @PigletJohn may not work well due to ventilation and DPC requirements; it will depend on teh specific circumstances. They're not easy to insulate safely, either: there's a reason why no new houses are being built with them.

Thanks PigletJohn and Sensus. I think my question should have been this - is it going to be better value (cheaper!) for DH and I to have a concrete foundation and subfloor in the new and old extension, rather than having wooden joists inserted for a floating wooden floor, bearing in mind that we'll have to pay to have wet UFH heating in the former? Thanks!

Scatterbrained, I should have mentioned too that we'd hoped to avoid UFH and to use refurbished floorboards for the extension, with rads in the house to heat it. These floorboards being cold in the extension would be a no, no though.

Sensus, why might you not be able to insulate between joists safely?

I think it is a building regs requirement for floor insulation in an extension, no matter what the floor type?

Regarding removing the existing concrete, we have just had this done to relay with insulated concrete and ufh. Hoping it will be worth it as not cheap!

In a new extension I mean.

Sorry, I should have also asked you all whether laying a timber floor on the insulated slab for the new extension will be cold underfoot or not, with the insulation and without UFH? Thanks!

if you lay, say, engineered wood flooring on a concrete slab, it is usually laid "floating" on rigid insulating foam board, so is not cold. Some people think it has to be nailed onto battens, but it doesn't.

If you have a ground slab, then an airspace, then a timber floor, you would use joists and the airspace would have to be ventilated, so you would insulate between the joists. This is easy while the floor is being built, very much harder afterwards, but it would all be included in the design and construction plans.

@PigletJohn:

"Sensus, why might you not be able to insulate between joists safely?"

Interstitial condensation, in a word (well, two words).

It's very difficult to maintain adequate vapour control, on a suspended timber floor, and as the insulation requirements for new-builds have steadily increased, so has the condensation risk.

With new-builds there's also the requirement for air tightness, which is similarly difficult to achieve with a traditional suspended timber floor.

Where would this interstital condensation occur?

Not when cool air ventilating the subfloor void is warmed if it enters the house.

Thanks OctoberOctober, my fear is that it won't be cheap and we'd be happy with radiators if the floor isn't ice like!

Thanks PigletJohn, I understand (I think). Have you any idea which foundation and build up which would be most economical, the floating floor or solid slab?

no, not for a one-off extension.

Pouring slabs is cheap and easy when you're throwing up lots of new houses.

But extensions are one-offs, and driving a readimix truck into your kitchen is difficult.

@PigletJohn: "Where would this interstital condensation occur?"

Not when cool air ventilating the subfloor void is warmed if it enters the house.

When the warm air within the house cools as it filters through the floor insulation (warm, moisture-laden air exerts a 'vapour pressure' so the usual rule of 'heat rises' doesn't necessarily apply... the vapour is continually trying to force its way out), or when air trapped within the insulation cools as it goes through the usual day/night cycles.

but water vapour is lighter than air.

if there is air movement due to imperfect sealing between the cool air in the ventilated void, and the warm air in the house, which way will it go?

Why is the floor at risk of damp, and the walls and ceilings not, when the subfloor void is constantly ventilated?

@PigletJohn: "but water vapour is lighter than air."

It is, but that doesn't matter. As I said, it exerts a vapour pressure, which means that it tries to find its way out in all directions. If you stick a pin in the bottom of a helium-filled party balloon, does the helium in the balloon stay where it is?

The walls and ceiling are at risk. In fact, cold roof construction is by far the biggest risk risk of all and loft condensation is becoming a really serious problem in new-build housing.

Walls are less of a risk because:

a) If they are timber frame, they have a vapour barrier toward the inside (warm) face, which theoretically prevents the water vapour reaching a dew point within the construction (but if the vapour barrier is not perfect, they most certainly are at risk, too).

b) If they are masonry, then they are designed so that the dew point profile is such that the interstitial condensation occurs where it can do no damage - unlike timber, brickwork or concrete blockwork can stand elevated moisture levels with no harm.

If we could install a vapour barrier in an insulated timber floor, in the same way that we do in a timber frame wall, and rely on it remaining intact, then it wouldn't be so much of an issue, but in practice this is very difficult to achieve.