An idea for insulation under an old uninsulated block and beam floor

[Carol W]

I think that block and beam floors need up to a 225mm void beneath them (we're on clay so I'm being cautious). 

We have about double that. How about (by taking out a few blocks and wriggling under) I use plinth/pedestal patio stands to hold Celotex up against the underside of the block and beam? 

It would take a lot of wriggling and effort to make sure each board was butted up to the next but would I end up with a warmer floor?

[Mr Punter]

It sounds OK in theory.  The beams are normally deeper than the blocks, so you need to stop air flowing between the insulation sheets and the blocks.  Is there concrete blinding to work off?

[Carol W]

Just clay soil unfortunately

1 hour ago, Mr Punter said:

It sounds OK in theory.  The beams are normally deeper than the blocks, so you need to stop air flowing between the insulation sheets and the blocks.  Is there concrete blinding to work off?

Just clay soil unfortunately so maybe we could use a block under each pedestal to spread the push force ?

 

[Redbeard]

My concern would be what residual space you'd leave yourself to crawl - and indeed *work* in. If you currently have no insulation (you don't say - how old is the house?) you'd probably want at least 150mm of PIR. That will leave you 300mm for you to crawl in. Mainly it will be wriggling as you won't be able to bend your limbs much. Are you very very not claustrophobic? I have done loads of tight underfloor insulation jobs, but few where I end up with as little as 300mm. Nevertheless I now get claustrophobic in small spaces. Think seriously before you proceed! Getting half-way through and realising you cannot complete the job is very frustrating!

[Mr Punter]

I wonder if you could get some EPS beads installed?  If you don't get any water under the floor it could be that the insulation will give lots more benefit than the risk of lack of sub floor ventilation.

[SimonD]

+1 - have a look at full fill with eps beads. Otherwise I fear there will be a lot of swearing and cursing and a poorly finished insulation layer. I also believe that once it's filled with eps it no longer needs to be ventilated as it ceased to be a ventilated void.

[Carol W]

3 hours ago, Mr Punter said:

I wonder if you could get some EPS beads installed?  If you don't get any water under the floor it could be that the insulation will give lots more benefit than the risk of lack of sub floor ventilation.

I've seen others mention doing that but I'm not sure how much of the house the void extends to ....I need to take a good look. And I'm guessing the quantities involved would make it really expensive

[Ferdinand]

It sounds very difficult.

 

At least the floor area of the house would give you an estimate of the maximum multiplied by your 0.5m height. 

 

Here is a price suggesting ~£50 per cubic m for polybeads - which feels about right. May save for buying more, but will have fitting costs:

https://ecclestons.com/product/10-cu-ft-expanded-polystyrene-bead-bags/

 

At £25 per sqm, that is the same material cost as a posh carpet, or decent tiles or laminate.

 

The way full fill works is it can come in a tanker and is pumped in, usually after you have lined the space with polythene to keep moisture out (!) If you are not able to line, you need to pay a lot of attention to moisture and ground conditions.

 

Also I think to keeping Roland out when he considers visiting; polybeads are probably toasty for rodents in winter.

 

It has also been done with a material called LECA, which is Light Expanded Clay Aggregate - a clay version of an Aero Chocolate Bar, in granules. It is more resilient to moisture. Here are some "LECA insulation fill" prices in bags - looks pricey!

https://www.specialistaggregates.com/advanced_search_result.php?search_in_description=1&keywords=Leca Insulation Fill Price&delivery=pallet

 

That LECA is around £130 per sqm at 500mm thick, at which price you might be better off using Aerogel Spacetherm on top in a subfloor.

 

I think you need to map your void - do you have access? If you can see it can you map it with a torch and an estate agent laser measure, a protractor and graph paper?

 

Plan B: Can you do it on top as a raised insulated floor, instead?

 

Plan C might be perimeter 'skirt' insulation in a trench round the house, and letting the ground heat up underneath. I do not know how an underfloor void affects that.

Either way, it will need some careful staff work first.

 

HTH

Ferdinand

[nod]

It sounds a mammoth task 

Back in the day you could pushed a small child under there and pay him thrupence But the PC brigade have put a stop to that now Or maybe a small Polish builder 

If you manage to pull it off Take plenty of photos As this is something that will keep coming up on here 

  Best of luck 

[George]

Suitable fixings to stop them dropping off in future would be my concern. I'd rather go for a jetfloor type system, or lower the floor and increase the insulation on topside. 

[MaxEmery]

On 08/12/2023 at 14:14, Carol W said:

think that block and beam floors need up to a 225mm void beneath them (we're on clay so I'm being cautious). 

We have about double that. How about (by taking out a few blocks and wriggling under) I use plinth/pedestal patio stands to hold Celotex up against the underside of the block and beam? 

It would take a lot of wriggling and effort to make sure each board was butted up to the next but would I end up with a warmer floor?

What did you end up doing here in the end? I'm looking at doing the same thing but with sticky foam and screws through the blocks into insulation wallplugs.