Insulating cavity of existing beam and block sub-floor

[embra]

Hi everyone,

 

Looking for some advice on insulating the cavity of an existing beam and block sub floor. Our property was originally built ~1990 and is located in Edinburgh, Scotland. I’ve included a sketch of the current and proposed arrangements.

 

There is currently no access to the void space. I have managed a limited inspection of the void by fitting a GoPro through a penetration the last owners made in one of the blocks. There is currently a ventilated void of 300 to 380 mm (depending on interface with beams) beneath the subfloor. The ground is covered in what looks like a transparent damp proof membrane. I can’t see for definite how the membrane is terminated, so I’m not sure if it is simply loose around the edge of the block work, or whether it continues through the course of inner block work. There is no vegetation etc present, and everything in the void looks dry.

 

Upon inspecting the outer leaf of brickwork, I have found evidence of a damp proof course immediately above the telescopic underfloor vents.

 

I’ve identified the Super Bead system by Energy Store that we may be able to install to effectively insulate this void. The installer would pump EPS beads into the cavity in the same way as is done to retrofit cavity wall insulation. We’d manage the installation as part of other building works planned later this year, so are comfortable with installation process and level of disruption.

 

The installation guidelines state that the Damp Proof Membrane (DPM) should continue up the inner leaf and be secured beneath the beam & block floor system.

 

The installer estimates we can achieve a u value of ~0.10 W/m2K, compared to estimate of ~0.49 W/m2K for the current set up.

 

Energy Store Super Bead Beam and Block System Manual: https://source.thenbs.com/literature/energystore-beam-block-system-manual/ureVc5R4tbVEzchxsCNEWs/8hwTa8EYcje93NK8ytqXD1

 

I’ve read some discussion on this topic before, and would like some opinions on our specific situation:

·        Given the current arrangement, where is there risk of rising damp? What checks should I be doing to give confidence that this won’t happen?

·        If there is a risk of rising damp, are there any easy ways to mitigate?

·        Assuming we can mitigate risk of rising damp, should the telescopic vents be blocked?

·        I don’t know the detail between the concrete beam and block, and the inner leaf of blockwork. Is it fair to assume any thermal bridging around this perimeter would be insignificant in comparison to the benefit of insulation when compared to current / un-insulated arrangement?

 

Thanks as ever for your input

[nod]

Correct me if I’m wrong Are you talking about filling the void under the BB

[embra]

11 minutes ago, nod said:

Correct me if I’m wrong Are you talking about filling the void under the BB

That's right

[nod]

In the 30 plus years since your BB was installed It hasn’t change All allow for airflow to prevent a buildup of stale air Blocking this isn’t a good idea 

[Iceverge]

54 minutes ago, embra said:

Given the current arrangement, where is there risk of rising damp? What checks should I be doing to give confidence that this won’t happen

 

I would say this is fine. The beads don't wick water and if it's dry now it won't get wet later. 

 

54 minutes ago, embra said:

Assuming we can mitigate risk of rising damp, should the telescopic vents be blocked?

 

No. They will provide an escape for any gasses that may find their way I to the beads. 

 

54 minutes ago, embra said:

I don’t know the detail between the concrete beam and block, and the inner leaf of blockwork. Is it fair to assume any thermal bridging around this perimeter would be insignificant in comparison to the benefit of insulation when compared to current / un-insulated arrangement?

 

A U value of 0.11 is excellent for the main part of the floor. 

 

Unfortunately this means the thermal bridge at the perimeter will be really significant. Can you fill the cavity to below the level of the block and beam with EPS beads too?

 

[embra]

1 hour ago, Iceverge said:

 

I would say this is fine. The beads don't wick water and if it's dry now it won't get wet later. 

 

 

No. They will provide an escape for any gasses that may find their way I to the beads. 

 

 

A U value of 0.11 is excellent for the main part of the floor. 

 

Unfortunately this means the thermal bridge at the perimeter will be really significant. Can you fill the cavity to below the level of the block and beam with EPS beads too?

 

 

Those are useful observations, thanks for sharing.

 

We'd be targeting to fill the entire ventilated cavity below the beam and block floor with the beads. Is that what you mean?

[Iceverge]

Not quite. 

 

The residual wall cavity below the block and beam level. 

 

 

 

Like this to make the heat have the longest most difficult path to leave the house. 

 

[ProDave]

Usually the cavity is filled below DPC so there is no "cavity" there.

 

As there will be a cold bridge from the beams out through the wall, I would perhaps mitigate this a bit by not taking the UFH all the way to the edge of the floor, as another way of giving the heat further to travel to get out of the house.

[saveasteading]

43 minutes ago, ProDave said:

giving the heat further to travel to get out of the house

Simple and pragmatic.

Nobody sits or stands at the perimeter. Some insulation can be contrived too.

[Iceverge]

4 hours ago, ProDave said:

there will be a cold bridge from the beams out through the wall, I would perhaps mitigate this a bit by not taking the UFH all the way to the edge of the floor, as another way of giving the heat further to travel to get out of the house.

 

I was thinking about this. 

 

Maybe it would be pragmatic to include some insulation above the block and beam just at the perimeter.  Say 300-600mm from the walls. 

 

 

[Iceverge]

3 hours ago, saveasteading said:

Nobody sits or stands at the perimeter. Some insulation can be contrived too.

 

In terms of energy use it may be significant. 

 

I'm away from my laptop for a few days. I might try to do a therm model if I get a chance at the end of the week. 

[saveasteading]

1 minute ago, Iceverge said:

Say 300-600mm from the walls. 

Yes that's what I meant. 

 

Looking forward to your model. The journey of unit of energy through a longish linkage of materials, to earth.

Variables: indoor and outdoor temperatures, general temperature of the earth by season.

Time lag day and night? Does that heat woozle end up not quite getting outdoors and start moving back in on a warm day?

Do you need to know the composition of the ground and the location of the property?

 

I did once calculate heat loss through cladding screws into steel purlins, thinking it might be quite a lot (10 screws/m2 extending from outside hex, to an exposed  point into and through purlins.  It was tiny though as the actual c/s of the screw is inside the thread. 

[Iceverge]

5 minutes ago, saveasteading said:

Yes that's what I meant. 

 

Looking forward to your model. The journey of unit of energy through a longish linkage of materials, to earth.

Variables: indoor and outdoor temperatures, general temperature of the earth by season.

Time lag day and night? Does that heat woozle end up not quite getting outdoors and start moving back in on a warm day?

Do you need to know the composition of the ground and the location of the property?

 

I did once calculate heat loss through cladding screws into steel purlins, thinking it might be quite a lot (10 screws/m2 extending from outside hex, to an exposed  point into and through purlins.  It was tiny though as the actual c/s of the screw is inside the thread. 

 

Therm isn't that sophisticated. At least not in my hands. It's a static model. 

 

Yes point losses are really tiny. Especially if spread over any distance above a few cm. 

 

I did the calcs on our 250mm cavity wall for SS wall ties Vs Teplo and there was almost no benefit. 

 

[Dave Jones]

On 09/04/2023 at 21:31, embra said:

That's right

 

DO NOT do this. 

 

There has to be a minimum 150mm air gap from the underside of beam. 

 

Insulation goes above the floor not below it.

[embra]

7 hours ago, Dave Jones said:

 

DO NOT do this. 

 

There has to be a minimum 150mm air gap from the underside of beam. 

 

Insulation goes above the floor not below it.

Unfortunately I cannot find a practical way to add more insulation above the sub-floor, without knocking the house down and starting again.

 

The supplier has obtained an NHBC acceptance certificate for this arrangement in a new build setting, which is why I wanted to understand if there are any other potential issues in a retrofit scenario.

 

Can you help me understand the specific concerns you have?

[saveasteading]

I can't see anything wrong with it. My concern would have been getting approval from BCO for a hybrid design but there is NHBC confirmation so should be ok.

 

so do not have air bricks.

Make sure that the dpm is utterly seales so that water cant get in (there is no way out again)

presumably the superbead is placed slightly prod so that there are no gaps.

 

How do you keep the superbead in place against the wind, and dry until the vcl goes in? or is this within a constructed house?

 

[embra]

Yeah this for retrofit into an existing building. Actually, even in the new build scenario the beam and block floor is installed as normal, and then the beads are injected through holes drilled between the blocks (similar to retrofitting cavity wall insulation). This keeps everything in place. For reference the system manual including installation process can be found here.

 

Since I already have telescopic vents installed, I'm trying to work out if I should do something to specifically block these up? In theory they would be blocked by the beads (coated with PVA) upon injection anyway...

 

I haven't seen many examples of this overall approach, so I'm really interested to hear all opinions.

Edited April 11, 2023 by embra

[saveasteading]

14 minutes ago, embra said:

if I should do something to specifically block these up?

At least insect mesh. I think I would fix it inside for security and outside too, to keep the vents empty of creatures.

 

When the beads are glued together is it a mass, or are there gaps?

[Dave Jones]

how are you going to add the DPC and make it continuous into the blockwork without taking the floor up ?

 

Why wasn't the floor insulated when it was built, building control would have required it ?

 

Ventilation is a requirement and a warranty fail if the missed or vents not installed. Do you have a new build warranty already ?

 

 

 

 

[saveasteading]

28 minutes ago, Dave Jones said:

Ventilation is a requirement

Not according to that NHBC sketch.

 

To me it is ending up as an overdesigned floor over an expensive substitute for slab eps, (which would not require a vented space), but entirely due to circumstances.

[ADLIan]

Adding insulation under a beam & block floor changes it from a suspended floor to a ‘solid’ floor. This is OK provided there is a correctly detailed DPM under the insulation - possible in a new build but impossible in a refurb or upgrade. The Energystore system is approved for new build only, not upgrade work.

 

As above, an expensive solution looking for a problem.

[Iceverge]

BEFORE

 

 

Without insulation. Assuming 100mm EPS in the cavity and full fill below ground level. 

 

 

 

 

 

AFTER

 

 

 

 

With 300mm of EPS under the floor. 

 

The U value drops significantly but given the effect of the thermal bridge not as far as you'd think. 

 

 

 

 

 

 

[Iceverge]

 

Here's another option. 

Lay 75mm PIR boards over the block and beam. Carefully foam all joints, trim and tape. Then lay 2 x layers of 11mm OSB floating over the top, joints staggered, glued and screwed. If you wanted to bump to Phenolic insulation you could lower the U value to 0.24 from 0.28 w/m2K but I wouldn't imagine it's worth the cost. You can still see the effect of the thermal bridge at the wall floor junction. 

 

Then use large radiators for the ASHP. 

 

 

Personally I think that you could fill the floor void as is with EPS beads nothing bad would happen. I can't under stand how moisture would magic it's way vertically through 300mm EPS beads. 

 

 

[S2D2]

1 hour ago, Iceverge said:

Personally I think that you could fill the floor void as is with EPS beads nothing bad would happen. I can't under stand how moisture would magic it's way vertically through 300mm EPS beads. 

Isn't the main concern with this approach ground gasses with the missing airtight layer on retrofits?

[embra]

On 11/04/2023 at 18:24, ADLIan said:

Adding insulation under a beam & block floor changes it from a suspended floor to a ‘solid’ floor. This is OK provided there is a correctly detailed DPM under the insulation - possible in a new build but impossible in a refurb or upgrade. The Energystore system is approved for new build only, not upgrade work.

 

As above, an expensive solution looking for a problem.

 

On 11/04/2023 at 17:40, Dave Jones said:

how are you going to add the DPC and make it continuous into the blockwork without taking the floor up ?

 

Why wasn't the floor insulated when it was built, building control would have required it ?

 

Ventilation is a requirement and a warranty fail if the missed or vents not installed. Do you have a new build warranty already ?

 

These are great points, and the reason I wanted to get opinions from others. I need to follow up with the supplier to see if there is any other approved arrangement for retrofit installation that are compatible with non-continuous DPM. I have been able to confirm the presence of a DPM in good condition on top of the ground in the void, but I can't confirm how it interfaces with DPC in blockwork etc and whether there is continuity. 

 

The floor was not insulated when built in ~1990, I believe this was allowable back then (source). This is why I'm looking for creative solutions to reduce energy loss through my existing floor.