SiloSi
Member
- Messages
- 15
- Location
- Garrigill, Alston Moor, Cumbria
Hi all,
Wondering if anyone has any experience with foam glass balls for sub-floor insulation over shallow bedrock. I’ve started the process of digging up massive flagstones in the living room floor that had damp issues, no doubt caused by using a patchwork of discontinuous damp proof membrane and bedding/pointing with standard cement mortar.
The (slate) bed rock is very shallow, which means any notion of underfloor heating or any standard thickness of insulation and screed is out of the question. We initially reckoned there was on average 6cm space under the flags. After speaking to three lime suppliers (Ty-Maur, Unity Lime and Mike Wye), the consensus was the best solution would be to lay a 4cm thick bed of foam glass 2-4mm aggregate mixed with lime and water, then a 2cm layer of lime bedding mortar.
However, although there is more than enough space under some flags, there are lots of areas where there’s only about 3cm space (or less!). Although the guy at Mike Wye (who supplied the materials) said in theory it would be possible to bed straight onto the foam glass/lime mix, he suggested the best thing to do would ideally be to hack out slate to enable enough depth where necessary, to enable the full 6cm depth needed for uniformity of insulation and performance.
Besides all the extra work of digging out a lot of slate, the other issue I have is how to deal with the different thickness of all the flags, that range between c4 and 8cm thick. The lime glass mix needs to go off before laying the flags on bedding mortar, but that means the lime glass mix would need to set at different levels under each flag, which I can see would be very difficult to achieve.
I’ve come up with another solution, but would welcome any comments. I plan to remove as much bedrock to enable a 4cm layer of foam glass where necessary, but where there is more space, I’m proposing to lay enough sharp sand and lime mix directly onto the bedrock (and probably bigger aggregate/slate pieces where significantly more than 6cm depth available), to enable 4cm of foam glass/lime mix on top, but I wouldn’t add any water. There’s a certain amount of moisture in the sharp sand, and also the bedrock itself seems damp in places (hard to say if this is just the DPM trapping moisture or if it’s actually rising damp, though cleared areas seem to have dried after a week or so…).
This way, the sharp sand/lime mix will slowly absorb any moisture and go off over time, but there’ll be a uniform layer of foam-glass/lime mix under directly under the flags. It’s this layer that’s providing the thermal insulation (such as it is)… I don’t think the different thicknesses of sand/aggregate/lime base layer will significantly alter that performance. It also means that I can lay the subfloor under each flag in one go rather than waiting for the foam glass layer to go off before bedding, but also means that I don’t have to lay the whole floor in one go, but do it gradually and level up to any un-lifted flags.
I’m working by myself (and some help from my wife for lifting flags) with no time restraints, so this feels a lot more doable than digging up lots of slate, laying the foam glass mix at multiple levels to compensate for different thicknesses of flag, and waiting for it to go off before laying the flags.
Here’s a couple of pics to show the bedrock that’s clearly uneven, and also a flag resting on the bedrock to show the limited space under it relative to the unlisted flags.
Although I’ve put this to Mike Wye, unfortunately the guy who was advising me was out the office til some time next week, so I thought I’d put a posting here to see what anyone thinks. Apologies for the long post!
Thanks
Si
Wondering if anyone has any experience with foam glass balls for sub-floor insulation over shallow bedrock. I’ve started the process of digging up massive flagstones in the living room floor that had damp issues, no doubt caused by using a patchwork of discontinuous damp proof membrane and bedding/pointing with standard cement mortar.
The (slate) bed rock is very shallow, which means any notion of underfloor heating or any standard thickness of insulation and screed is out of the question. We initially reckoned there was on average 6cm space under the flags. After speaking to three lime suppliers (Ty-Maur, Unity Lime and Mike Wye), the consensus was the best solution would be to lay a 4cm thick bed of foam glass 2-4mm aggregate mixed with lime and water, then a 2cm layer of lime bedding mortar.
However, although there is more than enough space under some flags, there are lots of areas where there’s only about 3cm space (or less!). Although the guy at Mike Wye (who supplied the materials) said in theory it would be possible to bed straight onto the foam glass/lime mix, he suggested the best thing to do would ideally be to hack out slate to enable enough depth where necessary, to enable the full 6cm depth needed for uniformity of insulation and performance.
Besides all the extra work of digging out a lot of slate, the other issue I have is how to deal with the different thickness of all the flags, that range between c4 and 8cm thick. The lime glass mix needs to go off before laying the flags on bedding mortar, but that means the lime glass mix would need to set at different levels under each flag, which I can see would be very difficult to achieve.
I’ve come up with another solution, but would welcome any comments. I plan to remove as much bedrock to enable a 4cm layer of foam glass where necessary, but where there is more space, I’m proposing to lay enough sharp sand and lime mix directly onto the bedrock (and probably bigger aggregate/slate pieces where significantly more than 6cm depth available), to enable 4cm of foam glass/lime mix on top, but I wouldn’t add any water. There’s a certain amount of moisture in the sharp sand, and also the bedrock itself seems damp in places (hard to say if this is just the DPM trapping moisture or if it’s actually rising damp, though cleared areas seem to have dried after a week or so…).
This way, the sharp sand/lime mix will slowly absorb any moisture and go off over time, but there’ll be a uniform layer of foam-glass/lime mix under directly under the flags. It’s this layer that’s providing the thermal insulation (such as it is)… I don’t think the different thicknesses of sand/aggregate/lime base layer will significantly alter that performance. It also means that I can lay the subfloor under each flag in one go rather than waiting for the foam glass layer to go off before bedding, but also means that I don’t have to lay the whole floor in one go, but do it gradually and level up to any un-lifted flags.
I’m working by myself (and some help from my wife for lifting flags) with no time restraints, so this feels a lot more doable than digging up lots of slate, laying the foam glass mix at multiple levels to compensate for different thicknesses of flag, and waiting for it to go off before laying the flags.
Here’s a couple of pics to show the bedrock that’s clearly uneven, and also a flag resting on the bedrock to show the limited space under it relative to the unlisted flags.
Although I’ve put this to Mike Wye, unfortunately the guy who was advising me was out the office til some time next week, so I thought I’d put a posting here to see what anyone thinks. Apologies for the long post!
Thanks
Si
! 