Technically you should get a condensation risk analysis done.

The bottom of the joists will be nearer the cold side (50mm) than the warm side (125mm) so if the insulation was uniform you might expect the temperature down there to be approx 50/125 ths of the way to the cold side temperature (eg if cold side is zero and warm side is 20C then about 20 x 50/125 =8C). Add a vapour barrier above the joists somewhere?

I would fit a vapour barrier on top of the joists, either above or below the floor boards. Most likely you wont have a problem but no the foil and sheet insulation are in the wrong place to be the vapour barrier. I would also fill the void formed by the joists fully.

Well hang on a second.

You already have a vapour barrier in the from of the rigid insulation!

You only ever want one vapour barrier, and you already have it, so don't go sticking another one in ontop of the joists, as that will cause moisture to be tapped in the void between the joists.

If you already have the rigid insulation installed, go round and make sure the floor void is air tight at the perimeter with some expanding foam and mastic, then leave it at that with NO additional barrier.



In theory the vapour barrier must always go on the WARM side of the structure. So in the case of a floor you would idealy have have floor boards, then vcl, then mineral wool insulation.

If you go for a rigid insulation solution, it must either go under the joists with nothing between them and no additional barrier, or between the joists with nothing below. By putting rigid below the joists then filling with mineral wool between you end up with a 'hybrid' system that is not a cold or warm design.

With a warm design, all the structure is on the warm side of the rigid insulation, and as such no condensation will form in the structure because there is no temperature differential.

With a cold design the insulation is within the structure, so moist air will condense as it passes though, but with a cold design, ventillation is provided which allows that moist air to be taken away without causing issue. With a hybrid, you get moist air passing though the structure and cooling, but there is not ventillation to take it away, and that is where problems begin. Although with the rigid insulation below the joists, the tempurature on the warm face of that insulation is unliekly to be cold enough to cause condensation to form, it can still cause issues.

If you put a vcl over the top of the joists it will stop that moist air getting into the floor void in the first place, but the timber joists will not be able to breathe and so could be subject to elevated moisture contents, with mould growth and even decay a possibility (quite likely!).

Do one thing, or the other, NOT both!

Timber

Posted By: tonyTry and understand the original post and make up -- with foil layer not on warm side.

Say what now?

Is the cellar heated?

Well, for my money, I would go with the 50 mm cut between the joists just below the floor boards and sealed tight with foam, with the rest of the void packed with mineral wool.

that way you get to use your stuff you already have, you have a vapour barrier on the warm side, and breathability to the bottom of the joists.

Timber

Indeed!

Could I just ask???

Hypothetically speaking.

Suppose you did get the vapour barrier wrong - or didn't bother with one.

How bad would it be?

So we have moisture in the house, from living. We also have moisture outside the house.

If we assume the moisture from either of these areas get trapped, how long would it take before the house rots/we get overcome by fumes/mould/mildew ruins out emulsion/ the joists/beams rot away and the house falls down??

Are we talking 6 months, 6 years or 60 years?

I too have a cold suspended timber floor which needs sorting out. I've been following the multifoil thread so have a head full of radiation etc. I've concluded that there is little benefit to be gained for the price/hassle in having a radiant barrier in the loft as the main means of transfer here is conduction and convection. But conversely, under the floor boards i suggest most heat loss downwards is from radiation, with very little from convection, some from conduction out through the joists and not much from conduction through the air. Thus the resistance to conduction provided by the rockwool and kingspan are probably overkill in this application. Using the heat loss to price(or hassle) ratio, i reckon you may as well line the underside of the floorboards with newspaper to stop dust and wafts but allow breathability, then pin a load of foil mylar to hang from the joists (maybe put a strip of polystyrene between the foil and wood to stop any condensation touching wood, and reduce conductive losses). This is not advice, merely a thought. Mad? I 'm sure it wouldnt be as effective as putting rockwool between the joists and kingspan beneath, but would it be significantly less effective, or that much more dangerous for condensation?

"It would be nice to get rid of thermal bridge through joists but not essential. "

To reduce any thermal bridging could I fit 50mm kingspan between the joists to act as a vapour barrier then fit 75mm glasswool between and also extra under the joists held up with netting of some sort? or does the bottom of the joists need to be open to the air for ventilation?

Hi Radiohead

I would forget about Kingspan in your floor. Better to use a Vapour open breathable insulation below the joists like Wood fibre board and pump the space between the joists with Cellulose to a high density, this will give you a good level of Decrement Delay, Windtightness and Breathability. 9mm OSB directly under the floorboards will act as a vapour control layer when the joints are taped

Another option would be to fill the void under the floor with Light Weight Aggregate and block up the vents, this option gives you a warm floor and improved Airtightness in one go.

Agree

May work in Scandanavia, but in the UK we have lots of penetrating moisture into sub-floor walls - Its more than condensation thats the risk here

Hi Mike and Tony

Any moisture in the walls will be given off to the LWA due to its strong hygroscopic power and the timber-joists will be wrapped in LWA which has an extremly low moisture content so the timbers will give off any moisture to the LWA.
The close proximity of the timbers and the LWA keeps the timbers in an extremly dry envoirnment similar to the timbers in German Fokwork houses (clay/timber) walls where the timbers are still healthy in 500 year old houses.
The complete floor can then breathe like a Limecrete floor and the Anti capillary action of the LWA prevents rising damp.
Due to the insulation effect of the LWA the sub-floor walls will now be at least 2 degrees warmer at the top than at the bottom eliminating any capillary effect.

I've tried this in a few renovations, even in a house for my sister with dampish sub-floor walls, I often return to check the joists with my moisture meter but they are as dry as a bone!

Ooh, does this mean you're happy Aerogel doesn't disintegrate under foot traffic - or are you taking a v expensive punt? Will be very interesting to see this at the 5 year point...