General: What to do with this problem subfloor void

31 to 42 of 42

- CommentAuthorKenny_M
- CommentTimeSep 5th 2024 edited
Submitted the Building warrant application for this floor yesterday. I've been specific about the Foam Glass but a little more vague about the timber subfloor, as I think the Foam Glass is the thing they will focus on as the structural component and I am still trying to think through the sub floor. The council here won't talk to you about a proposal until you have submitted an application, so I thought it best to get it submitted and then I can hammer out any concerns with them once the officer engages.

I have started to sketch out a design for this below. I have not built a subfloor like this before so I would welcome any feedback.

A few things I am still not sure of and would be grateful for any comments

- Not sure whether I should have a vapour barrier between the Foam Glass and the timber subfloor or should just leave it as a fully breathable structure?
- Not sure what to use as perimeter insulation, maybe corkboard (maybe PIR if I have a vapour barrier)?
- If I am still a bit short on height once the Foam Glass is in, could I put XPS in between the Foam Glass and the battens? This is presuming that its level enough to take them or I have the depth to add a dry levelling compound + XPS. I've used XPS under a floating floor before but not sure if it would take the slightly more concentrated loads of the battens

Or if there is a better way to do all of this, with the proviso that I need to finish the floor with the solid wood flooring I took up, I would be happy to hear it.
- CommentAuthorGreenPaddy
- CommentTimeSep 6th 2024
A few thoughts...

- the warrant will check against what's in the regs. If you match what's in the regs then you pass. Check versus Section 3 points 2,3,4,10,15, and Secion 6 point 2. (April 2024 Regs)
- if you are doing something different, you will need evidence from manuf doc's that your approach has been validated. Since you have sketched the build-up, and not included a manuf image from a data sheet, I'm guessing you don't have evidence the timber battens can sit loosely on the compacted glass, or the load on the batten areas can be transmitted to the insulation with neglibile compression?
- If you do great, include it and put a written design spec that explains and relates it. Don't assume they have the time to work through and fully understand/interpret/correlate sketches with seprate data sheets.
- have you confirmed you are not in a Radon area, ie if you are then you have to install a radon membrane, or some other method of preventing radon entering the subfloor and room
- you're choosing an insulation that is not a particularly good insulator compared to what else is on the market. You'll likely need to confirm that alternative better insulative barriers are not suitable, if you don't meet the required Uvalue of 0.15 W/m2K

I'm not trying to be clever, or knock your design, but this is the sort of response you may get from the BSO - it can be very frustrating, but their job is to ensure your documents inform the "installer" of all the things they must do to meet the regs. They have no remit to encourage env friendly or novel approaches.
- CommentAuthorKenny_M
- CommentTimeSep 6th 2024
Thanks for this

Posted By: GreenPaddy- the warrant will check against what's in the regs. If you match what's in the regs then you pass. Check versus Section 3 points 2,3,4,10,15, and Secion 6 point 2. (April 2024 Regs)

I was hoping (maybe naively), that the fact that I am in a 200 year old house would lead to a recognition from building standards that its not always as simple as following regulations designed for modern houses. There does seem to be an explicit recognition of this in the regs. I've tried to make the argument in my submission that I can repair the rotten joists and keep the original design, but that this will lead to a repeat of the problems of the past.

Posted By: GreenPaddy- if you are doing something different, you will need evidence from manuf doc's that your approach has been validated. Since you have sketched the build-up, and not included a manuf image from a data sheet, I'm guessing you don't have evidence the timber battens can sit loosely on the compacted glass, or the load on the batten areas can be transmitted to the insulation with neglibile compression?

The only thing I really have is that Mike Wye suggested this approach and sent me a Geocell document with a picture of a timber subfloor sitting on the Foam Glass, but they didn't have any specs to go with it. I agree this is probably the weakest component of my submission as there are not a lot of examples of it. I have seen a video on youtube with someone doing similar, and the pic from Mike Wye, but that's it.

Posted By: GreenPaddy- have you confirmed you are not in a Radon area, ie if you are then you have to install a radon membrane, or some other method of preventing radon entering the subfloor and room

I checked this out before and according to the UK government site I am in the lowest probability area for Radon, the <1% category, so think I am fine here.

Posted By: GreenPaddy- you're choosing an insulation that is not a particularly good insulator compared to what else is on the market. You'll likely need to confirm that alternative better insulative barriers are not suitable, if you don't meet the required Uvalue of 0.15 W/m2K

I think I have a strong argument for this one. Its a fairly shallow subfloor space, no DPM in the walls, structure is below ground level etc. Options are limited, solution needs to allow for free draining of any water ingress. The proposal dramatically improves the insulation value of the existing floor while at the same time rectifying an unsafe structural element.
- CommentAuthorsgt_woulds
- CommentTimeSep 6th 2024
In England & Wales Part L of the Building regs defines a â€˜threshold valueâ€™ of 0.70 W/m2K for floors where it is not practical to achieve the current requirements. Some additional flexibility is given for certain defined scenarios, such as where fitting floor insulation might create an access problem

I believe there are similar allowances in the Scottish regs, but best to check and certainly worth discussing with your local authority / BCO; for an older building, most BCOs are now sympathetic to the argument that breathability is more important than the insulation value.

The Back to Earth system uses a layer of woodfibre over the Geocell/levelling compound before the floor finish is added to provide better U-values (I believe 250mm of Geocell is needed to provide U 0.18).

This might also be possible for the floor/batten approach if the battens are wide enough to spread the load on the woodfibre, (probably at least 80mm wide battens) You could also fill between the battens with flexible wood fibre) This would maintain a fully breathable finish.

Woodfibre depths from 22mm upwards depending upon manufacturer. If needed whisper me and I'll ask our technical team for advice.
- CommentAuthorKenny_M
- CommentTimeSep 6th 2024
Thanks for this

Posted By: sgt_wouldsIn England & Wales Part L of the Building regs defines a â€˜threshold valueâ€™ of 0.70 W/m2K for floors where it is not practical to achieve the current requirements. Some additional flexibility is given for certain defined scenarios, such as where fitting floor insulation might create an access problem

I believe there are similar allowances in the Scottish regs, but best to check and certainly worth discussing with your local authority / BCO; for an older building, most BCOs are now sympathetic to the argument that breathability is more important than the insulation value.

I think this is the case here too. There is a mention of 0.7 for "individual elements". There also this - "Existing buildings - it may be appropriate to vary the guidance contained in this Handbook when assessing the guidance against the constraints in existing buildings especially those buildings which are listed in terms of their architectural or historic interest."

To be honest it would seem a bit daft to be obsessing over 0.15 for a floor, in a room with stone walls and lath/plaster, especially given the improvement over a damp uninsulated suspended timber floor, so hopefully the officer will show a bit of flexibility.

Posted By: sgt_woulds
The Back to Earth system uses a layer of woodfibre over the Geocell/levelling compound before the floor finish is added to provide better U-values (I believe 250mm of Geocell is needed to provide U 0.18).

I do like this system, but it seemed like it was designed for a tiled finish. Think I will give them a shout and see what they say about how a solid wood floor might fit into this.
- CommentAuthorGreenPaddy
- CommentTimeSep 6th 2024
of couse you won't be expected to necessarily reach the reg's Uvalues, but my point was that you are chosing a material to insulate with a relatively poor lambda value (worse than 0.05 W/mK I think I read somewhere, though I'm not sure if that was the solid material, or with the void fraction). Just be aware that you may be asked to justify it.

The 0.7 for individual elements is not really relevant, as what you're proposing is a continuous layer.

Warrants usually get a response in a couple of weeks, so you'll know soon enough.
- CommentAuthordjh
- CommentTimeSep 7th 2024
Posted By: Kenny_MI do like this system, but it seemed like it was designed for a tiled finish.
It's shown with wooden floorboards as the final finish?

Posted By: GreenPaddyJust be aware that you may be asked to justify it.
The main reason for choosing geocell is that it acts as a capillary break, I believe.
- CommentAuthorKenny_M
- CommentTimeSep 8th 2024
Posted By: djh
Posted By: Kenny_MI do like this system, but it seemed like it was designed for a tiled finish.
It's shown with wooden floorboards as the final finish?

I saw that pic after I posted. I think its a floated, engineered wood floor though and its on top of ufh tiles. I've emailed back to earth to ask them whether it can be built up to take a solid wood floor. Of course I could float the solid wood floor, but manufacturers seem to advise against floating solid wood. I am presuming that the wood fibre boards in that system won't be suitable to screw into, but maybe I could float plywood on top and screw into that. Will see what back to earth say if they get back to me.

Posted By: djh
Posted By: GreenPaddyJust be aware that you may be asked to justify it.
The main reason for choosing geocell is that it acts as a capillary break, I believe.

Yes, this is part of my justification. Its free draining, breathable, impervious to water, capillarity breaking etc. As part of the outside perimeter is above finished floor level, the walls don't have damp proof course, adjoining solid floors etc, this feels like a good fit. My argument is that the first priority is that the floor is dry and safe, that's not possible with the current design, and the improved u value from the Foam Glass is a bonus. With the limited depth I don't think that modern materials, minus the depth required for a slab, would provide a much better u-value anyway.
- CommentAuthorKenny_M
- CommentTimeSep 20th 2024
Just a quick update to this. Had some back and forth with building standards and it seems tricky but not impossible to get the foam glass approved. Would probably need to be the system on the Back to Earth/Glapor sites with cemwood-fibreboard-ply/osb as although the 'glass deck' that Mike Wye suggested is in all the foam glass manufacturer brochures, no one seems to have any more information beyond the pictures.

Have to say that I am questing the time and expense of all of this though. Considering just going back to the suspended timber floor with shallower joists, monitoring the humidity and adding a subfloor fan if its a problem. The cost of the foam glass with all the fibreboard/cemwood added is about 5 times the cost of a new timber subfloor and PIR to go between the joists, and even 90mm PIR between the joists gets a marginally better u value than 200mm of foam glass + fibreboard/cemwood. Even if I had to shell out Â£300 for a fancy subfloor fan I would still be well under the cost of the foam glass.

Learning a lot though! :)
- CommentAuthorKenny_M
- CommentTimeOct 11th 2024
In the end I decided to abandon the building warrant application and foam glass and switch back to a timber subfloor. When the floor and joists were removed I had found an additional vent, which I originally thought was a disused drain pipe but turns out runs under the solid floor of the adjacent room to an external vent which had been rendered over. With the vent now cleared I should have through ventilation, and with the joists now off the ground there is a good chance that there will be enough air circulation to avoid a repeat of the dampness of the past.

The trigger that made me switch back to timber was the insistence from Building Standards that a DPC must be fitted over the Foam Glass, and Back to Earth saying they normally do that to pacify them, even though the foam glass already does that. It just made me think that the need for ventilation/void in that particular room is only partly about keeping the timbers dry, but also about carrying away moisture that may seep through from both under the solid floor of the adjacent room which is at a higher ground level, and the outside ground. It struck me that if I made that floor solid, or block the void with beads and with no DPC in the walls, I am asking for that moisture to accumulate in the base of the walls.

I went back and forward on natural versus PIR insulation, but with only 90mm available I opted for the better u value achievable with PIR. I plan to fit this tight with foam to fill any gaps, but was also thinking of the belt and braces of a breather membrane like in the pic below. My thinking is that joists will naturally move over time and gaps in the PIR might develop and this would at least ensure they can't fall down or leak air past. I have a full roll of cromar vent in the shed that will likely never be used for anything else anyway. From what I can see on the internet this method only ever seems to be used with rolls and batts of natural insulation or maybe glass wool. Is there any reason I am missing why doing this with PIR would be an issue?
- CommentAuthorsgt_woulds
- CommentTimeOct 11th 2024
Ill informed BCO's are the bane of green buildings.

I didn't use a DPC for my slab, just fleece. Very low radon risk in my area luckily. Luckily my BCO for that part of the project was well versed in MMC.

If I'd had the chap I'd had for the earlier parts of the build it would probably have held things up for weeks. I had to point out changes in building regs that he wasn't aware of which was quite worrying. Electricians are tested regularly on the amendments, why not BCO's, architects, etc?

I am due to discuss the Geocell foundations and 'glass deck' with my structural engineer tonight - will add any info here if relevant.

For floor refurbishments using our flexible woodfibre batts we always recommend using a breather membrane as shown in your picture. Carefully sealed this can be used as an airtightness layer and will considerably improve the performance and comfort levels.

If using PIR I'd use compriband in addition to expanding foam to seal the gaps. The PIR will contract as it ages in addition to the timber expanding and contracting over time. Flexible batts, (glassfibre/woodfibre/mineral) work better long term - even if they are not as effective insulators they will always prevent thermal bypass.

Can you add any insulation above the floor? We have breathable systems available with integrated battens for fixing floorboards in 40-60mm depths... Whisper if required.
- CommentAuthorKenny_M
- CommentTimeOct 11th 2024 edited
Posted By: sgt_wouldsIll informed BCO's are the bane of green buildings.

To be fair to my guy he was willing to discuss and I think might have been brought around, but he was looking for a lot more information, more drawings etc but it was only when I paused to think about it again, and discovered the hidden vent, that I had a rethink myself and figured that the suspended floor might be the better option.

Posted By: sgt_woulds
If using PIR I'd use compriband in addition to expanding foam to seal the gaps. The PIR will contract as it ages in addition to the timber expanding and contracting over time. Flexible batts, (glassfibre/woodfibre/mineral) work better long term - even if they are not as effective insulators they will always prevent thermal bypass.

To be honest I had no idea that PIR contracts over time but I have just done a search and can see that this problem has been reported before on reopened installations. That's horrendous when you think of all of the buildings this is in and it may now have gaps all over the place. This has given me second thoughts as once this is closed up and finished floor is screwed back down I can't just check and top up every few years!

However, I wonder if this is much different from other insulations. I'm pretty sure I have seen old mineral wool shrink, and maybe there will be some reported problems with wood fibre in 20 years.

Is comproband really going to expand 5 or 10 years down the line to take up the gap from shrinking PIR? Don't know how convinced of that I would be, it seems from looking it up that its more about sealing an existing gap.

Posted By: sgt_wouldsCan you add any insulation above the floor? We have breathable systems available with integrated battens for fixing floorboards in 40-60mm depths...

Not now, as the joists are now finished, but I had very little depth to play with to begin with. Had to switch to 95mm joists just to get a 100 to 150mm gap underneath for airflow so not a lot to play with. This particular room is a bit of a nightmare due to the changes made lowering the front of the house in the 1880's, so its all about compromises.

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