Insulating under block and beam floor with perlite

[Patrick]

I was wondering what you think about insulating under a block and beam floor. Building control specifies approx 200mm ventilated gap under a suspended block and beam floor. I am quite unsure of why this is. Apart from expanding of clay floor underneath, I can't quite see a reason (with suspended timber, it would be air circulation, but for block and beam?)

Would it be possible to fill the space underneath with insulation. For example perlite, or some other no moisture/rot sensitive materials, maybe eps in granulated form. What would be the problems that could arise from it? Anybody got some experience in doing this?

[PeterW]

Polystyrene beads..?? It’s been discussed before !

[bassanclan]

The problem is the manufacturer also states a minimum amount of air clearance/circulation so any warranty would be invalidated.

[nod]

It would also render your telescopic vents pointless 

Though it would save you a few quid if you didn’t have to put them in 

[Patrick]

49 minutes ago, PeterW said:

Polystyrene beads..?? It’s been discussed before !

Where? Couldn't find it.

[Patrick]

You are both right @bassanclan
 And @nod

Leaves the question, where these rules are coming from. I really can't see the point. What should happen to a concrete floor if not ventilated?

[PeterW]

9 hours ago, Patrick said:

Where? Couldn't find it.

 

 

[scottishjohn]

I think the point is  that IF it did get wet underneath --flood -etc

then you need an air flow to dry it out or you would get mold--so it needs to be hollow + vented or SOLID like concrete 

[Jeremy Harris]

I'm aware of two methods that have been tried, blown-in EPS beads and poured in Leca (fired clay beads).  I believe that Leca may be approved for use under a suspended floor, at least a beam and block one where there isn't really any risk of mould growth.

[Patrick]

3 hours ago, JSHarris said:

where there isn't really any risk of mould growth.

Exactly my thinking. Will check it out.

[Declan52]

Is it not also to stop the build up of certain gases like radon gathering up underneath your floor. Obviously this will depend on the geological makeup of the area your building in.

[j_s]

I thought it was to do with radon/gas most of the time. It is the reason building control stated for our extension where we were blocking up 2 vents for our block/beam floor on the main house and had to route a deeper duct under our solid extension floor to join the original vents. Luckily we could go down quite deep so it didn't cause issues with the extension slab.

[Radian]

This thread went quiet a long time ago but the issue has cropped up again recently. There must be an awful lot of houses with suspended concrete B & B floors that have just experienced -10^oC rushing around beneath. To meet building regs in the late 90's our house had just 50mm of XPS sheet topped with chipboard flooring:

 

 

A U-value of 0.5W/(m²K) (0.6 without the carpet 🙄) is not very clever at all. But it exceeded the minimum requirement of 1.2 at the time:

 

Those numbers look criminal now. So what to do?

[Radian]

The obvious answer of lifting all the carpets, pulling up the chipboard and replacing the EPS with 50mm PUR brings us down to 0.372 W/(m²K) Simply not worth the trouble and the 0.144 W/(m²K) saving will never, ever, cover the cost of the materials let alone the whole job. There's also no prospect of raising the floor level at all.

 

Now, pumping in 300mm of EPS beads (the average depth of the void below floors here) would achieve something like 0.09 W/(m²K) which is well into passive house territory. Apart from the addition of virtually unlimited amounts of insulation in the loft, there's no other single improvement that I can think of that would make such a big difference. I also suspect that there is no XPS in the floor makeup for the kitchen/utility rooms as these were screeded for tiling, and there's not enough depth - maybe 25mm XPS and 43mm screed but I very much doubt it. Insulating under these rooms alone could make a huge difference (the kitchen and utility room have always baffled me as to why they're so cold).

[MikeGrahamT21]

One reason for the ventilation gap is to remove radon gas build up if you are in an area which suffers from this. And the other which has been mentioned is to protect from flooding, again depends on where you are located

[S2D2]

 

2 hours ago, Radian said:

There must be an awful lot of houses with suspended concrete B & B floors that have just experienced -10^oC rushing around beneath

I have the same setup, B&B floors with 50mm polystyrene, apart from one room with 50mm PIR instead. Probably 600+mm void underneath the floor though. The only sensible thing I've seen is the little spray foam robot you can send under which retains a ventilation gap, but that turned out to be prohibitively expensive. Watching with interest.

[Radian]

So yes, gas build-up. And it's not just radon that's a concern! Part of the requirement for voids to be ventilated is to prevent lethal explosions from gas leaks coming from defective pipework and harder to spot sources of methane coming from rotting material in the ground. But to what extent would loose EPS granules restrict the airflow and how would their presence affect the explosive potential of the gas air mix?

 

These are questions for which I've seen no research. My instincts tell me the volume of potential explosive gasses would be reduced in proportion to the reduction in ventilation making the effect of loose fill neutral in this regard. But I have no feel for what EPS does for airflow. It's a windy day - I could go up in the loft and find out.

[MikeGrahamT21]

EPS is open cell, I suppose you could go as far as saying some gas may enter the cells, can imagine that would cause a fair explosion.

 

 

https://www.houseplanninghelp.com/bens-self-build-story-laying-a-beam-and-block-floor/
 

found this which may offer some solutions?

[j_s]

Would love to fix mine, poorly laid polystyrene insulation somewhere in the floor with various draughts where the plastic sheet/ radon barrier wasn't folded and fitted properly around the perimeter 🤬. In the rooms I've renovated it has taken me hours and hours to fix those junctions with doubt at the back of my mind that it wont be a long term fix. I hope it will be as airtight foam, tescon tape and blowerproof used excessively. 

[Radian]

7 hours ago, MikeGrahamT21 said:

EPS is open cell,

 

Is it though? Progressive Foam don't seem to agree:

 

What is EPS?

Expanded Polystyrene Insulation, more commonly referred to as EPS, is a closed cell insulation that has been around since the 1950’s. EPS is made of 98% trapped air and only 2% plastic, making it an efficient insulator with a small amount of raw material.

[MikeGrahamT21]

19 minutes ago, Radian said:

 

Is it though? Progressive Foam don't seem to agree:

 

What is EPS?

Expanded Polystyrene Insulation, more commonly referred to as EPS, is a closed cell insulation that has been around since the 1950’s. EPS is made of 98% trapped air and only 2% plastic, making it an efficient insulator with a small amount of raw material.

yes you are right, I’ve always thought of it as open cell, which in board form it is, but the beads themselves are closed cell…

 

“EPS is, in other words, composed of 2% polystyrene and 98% air. The manufacturing process results in a closed-cell structure, but not a closed-cell insulation board (due to voids that can occur between the beads).”

 

[Radian]

So here's where I come unstuck. The Kepler conjecture states that the maximum volume occupied by identical spherical units is 74% of the containing volume. This is an optimised arrangement of spheres while a random arrangement results in a typical packing density of 65%. Non-identical sized spheres would tend to increase this percentage but I instinctively feel 75% could be the limit.

 

But in any mass flow situation, the pressure increases when the flow is restricted tending towards conservation of volumetric flow. This is why achieving airtightness is so damn difficult - as holes are plugged, a bigger draught comes through the ones you've missed. So does this mean that a space filled with beads would actually be safer than a totally empty void? The volume of potentially harmful gasses might be reduced by 65% to 75% while the airflow purging them would remain largely the same? Seems too good to be true.

[Iceverge]

Have you a properly sealed DPM/radon barrier or is this an old house? 

[MikeSharp01]

On 19/12/2022 at 22:38, Radian said:

The volume of potentially harmful gasses might be reduced by 65% to 75% while the airflow purging them would remain largely the same? Seems too good to be true.

If you have some airflow through the beads then although you have insulation you don't have air tightness is this unimportant?

[Radian]

49 minutes ago, MikeSharp01 said:

If you have some airflow through the beads then although you have insulation you don't have air tightness is this unimportant?

 

This question often keeps me awake in the night. If all the insulation value of EPS beads was lost due to airflow then why would anyone bother with injecting it into cavity walls? Obviously the volume of polystyrene displaces the volume of air by around 70% so the potential for heat transport is reduced as would be the mean velocity. But this is only considering the displacement effect of the beads. The expanded polystyrene beads are trapping air in their internal cellular structure so maintaining a lower thermal conductivity than, for example, solid polystyrene spheres when acting as a conducting bridge. But I think it may be an open question as to whether 0.033 W/(mK) quoted for EPS accounts for airflow or not.