useful link:
http://www.superhomes.org.uk/resources/insulate-a-floor/

I'd suggest having a look under the floor before you get too far down the planning route. Old houses have a way of messing up the best laid plans.

Personal preference but I like the idea of glass or rock wool insulation under a suspended floor in case of vermin, fire, leaks etc.).

We attempted to future proof it and put 50mm drytherm 32 batts on chicken wire under 200mm joists with 200mm reused glass wool between (because we had bags of it for other parts of the renovation).

You may also want to insulate down the walls a bit under the floor but this depends somewhat on where your wall insulation is.

Hi Tom, I'm wondering about the following points...I'm deliberately not giving an answer, as every case needs to be considered and inspected rather than applying a "one size fits all".

- you plan to use sheeps wool insulation, which can allow vapour passage, uptake, release. So why are you then planning to use a VCL, to control the amount of vapour passage, allowing for the plan to improve the air circulation with more air bricks?.

- the vapour/moisture from inside the house has passed through the floor and away (presumably if the void is ventilated) prior to your purchase. How is that vapour now going to be released in the future once you seal the floor to a much greater extent? Condensation increase at other now relatively colder spots around the house (wall corners, windows)?

- if the VCL is only controlling the rate (checking) the vapour flow, ie. allowing some vapour through, are a few nail holes really going to alter the desired effect? You've said you're not applying it as a vapour barrier.

- if you do have more than the minimum 150mm depth below the joists (for air passage) I would recommend you let the breather membrane barrier sag below the joists, and fill with say 2 layers of 150mm insulation, so the insul goes right under the joist and meets it's saggy neighbour, keeping the joist bottom edge warm (much more important than vapour control in this case), and giving much better thermal performance. Watch where the air bricks are positioned, so they aren't covered, in which case add the plastic periscopes to bring the air below the sagging insulation.

- you could lay a solid timber floor onto the joists, and forego the chipboard. That one depends upon who's installing the floor (saving on labour costs, but you'd need a secret nail applicator) and of course the cost/m2 of solid versus engineered. Buy it untreated, and brush on the Osmo oil yourself?

Always more to think about than meets the eye..

Thanks for the replies.

Posted By: jms452
I'd suggest having a look under the floor before you get too far down the planning route. Old houses have a way of messing up the best laid plans.

That’s what I intend to do. Will be getting an electrician, plumber and damp surveyor in to inspect the condition under the floor before any materials are purchased or plans set in concrete.

Posted By: djh
I also agree that inorganic wool insulation would be one less concern, although obviously they don't buffer humidity either. Depending on what the conditions near the walls are like it might be possible to combine inorganic wool in the centre area with sheeps wool near the walls.

The intention to use organic insulation is based on its hygroscopic properties. A number of the companies chemically treat the insulation to detract moths etc, is there still a rodent problem? Mineral wool is breathable, but doesn’t have the collecting and releasing properties sheep’s wool has. Would that be the next best option? Would it also be sensible to put a chicken wire or galvanised steel mesh under the joists to protect from the rodents?

Posted By: djhYou may also need to rewire to avoid the cables being surrounded by insulation.

This is something I have read about, I haven’t read up on how the electrics should pass through the insulation, I imagine this is something the electrician should know?

Posted By: djh How much space is there underneath the joists? (another good reason to have a look before going much further). I take it there's no cellar. The space will make a difference to the possibilities and the ease of doing the job.

I’m not sure I don’t have access yet. But hopefully in the coming weeks I will, when I can take up the floor boards. Then I will be in a better position on understanding the current situation and what work needs to be carried out.

Posted By: djh Having said that, some people don't like membranes at the top of floor insulation. It might be possible to use the membrane underneath your insulation as the airtightness barrier, or the T&G plywood subfloor.

Does the VCL not need to go on top of the joists to limit the warm air (which holds more water vapour than cold air) coming into contact with a material at a colder temperature, where the warm air meets a cold porous structural element the air may be cooled to such a point (especially as the floor cavity will be a lot colder than present) that the air becomes saturated (technically speaking it reaches its 'dewpoint', further cooling of the warm air then results in condensation forming? What’s the thought process/ reasoning for putting the VCL below?

Posted By: GreenPaddy
- you plan to use sheeps wool insulation, which can allow vapour passage, uptake, release. So why are you then planning to use a VCL, to control the amount of vapour passage, allowing for the plan to improve the air circulation with more air bricks?.

The materials have the ability to ‘breathe’, allowing them to pass both air and moisture vapour controlled and slowly through, thus minimising and diffusing the danger of condensation. The surveyor has noted that there is currently inadequate air circulation in some parts of the floor and more air bricks would enable better circulation and removal of moisture as its released from building.

Posted By: GreenPaddy
- the vapour/moisture from inside the house has passed through the floor and away (presumably if the void is ventilated) prior to your purchase. How is that vapour now going to be released in the future once you seal the floor to a much greater extent? Condensation increase at other now relatively colder spots around the house (wall corners, windows)?

The materials I’m proposing to use are all breathable. I don’t want to significantly reduce the permeability of the floor, but it will be reduced a bit. That said, the walls are going to be re-plastered with lime plaster, this should assist with limiting the build up of condensation and help control moisture levels inside.

Posted By: GreenPaddy
- you could lay a solid timber floor onto the joists, and forego the chipboard. That one depends upon who's installing the floor (saving on labour costs, but you'd need a secret nail applicator) and of course the cost/m2 of solid versus engineered. Buy it untreated, and brush on the Osmo oil yourself?.

I’m planning on insulating the floor and laying the plywood floor myself ; I will get registered professionals in to carry out the electrics and plumbing, and will get a professional in to lay the floorboards. I will oil and seal the floor myself. Currently my budget is looking like it can stretch to a solid oak floor but engineered floor is a little too expensive. We are planning to get tongue and grooved solid oak boards to lay. We are thinking to install a plywood layer under the boards to enhance the thermal qualities (the ply is breathable though) and structural quality of the entire floor. Laying a ply layer would also allow for the option to not nail the floor if that’s a more suitable method…


Thanks so much for everyone’s input so far

Posted By: thebeaconThanks for the replies.

Posted By: djh
I also agree that inorganic wool insulation would be one less concern, although obviously they don't buffer humidity either. Depending on what the conditions near the walls are like it might be possible to combine inorganic wool in the centre area with sheeps wool near the walls.

The intention to use organic insulation is based on its hygroscopic properties. A number of the companies chemically treat the insulation to detract moths etc, is there still a rodent problem? Mineral wool is breathable, but doesn’t have the collecting and releasing properties sheep’s wool has. Would that be the next best option? Would it also be sensible to put a chicken wire or galvanised steel mesh under the joists to protect from the rodents?

You certainly need to keep moths out of wool, or kill any that do get in. And you certainly need to keep rodents or any other animals out - insulation makes attractive nesting material and/or sites. So if there's any way for small creatures to get into the under-floor space then yes I would put steel mesh of some kind. I'd think chicken wire has slightly too big a mesh size to keep small mice etc out.

One question to answer is whether you actually need hygroscopic buffering there? What is its purpose?

Posted By: djhYou may also need to rewire to avoid the cables being surrounded by insulation.

This is something I have read about, I haven’t read up on how the electrics should pass through the insulation, I imagine this is something the electrician should know?

Well, it's something they should know, but I wouldn't count on it. The current-carrying capacity of a cable has to be derated (i.e. reduced) where it is surrounded by insulation. So you must either run the cables outside the insulation or use a suitably larger cable size, which an electrician should be able to calculate. There's also the separate issue of polystyrene (EPS) causing the insulation of cables to self-destruct - put cables in conduits if required.

Posted By: djhHaving said that, some people don't like membranes at the top of floor insulation. It might be possible to use the membrane underneath your insulation as the airtightness barrier, or the T&G plywood subfloor.

Does the VCL not need to go on top of the joists to limit the warm air (which holds more water vapour than cold air) coming into contact with a material at a colder temperature, where the warm air meets a cold porous structural element the air may be cooled to such a point (especially as the floor cavity will be a lot colder than present) that the air becomes saturated (technically speaking it reaches its 'dewpoint', further cooling of the warm air then results in condensation forming? What’s the thought process/ reasoning for putting the VCL below?

You need to distinguish between a VCL (VAPOUR control layer) and an AIRTIGHTNESS barrier. Inorganic insulation such as mineral wool doesn't much care about some condensation near the bottom, and it's more likely to form right at the bottom, on the surface or membrane.

‘....wool doesn't much care about some condensation near the bottom, and it's more likely to form right at the bottom, on the surface or membrane.”

I worry about organic insulation getting wet repeatedly and it will, having a breathable membrane underneath will allow vapour through but there will be water on top of it, this will soak into the wool and I think cause decay, smells and potentially fungal growth.

I have removed mouldy mineral wool from a poorly ventilated loft with polythene sarking had it been wool I dread to think what it would have been like hygroscopic, breathable etc wouldn’t have helped.

Sorry trying to put the op off real wool

thebeacon asked: "Is there anywhere i can be point to for further reading on best practice for cables running through insulation?"

https://www.voltimum.co.uk/articles/thermal-insulation-and-flat-twin-and

I use strawberry netting under and combined vb/air tight layer on top (poly)

Hi all,

i finally got into the house over the weekend and lifted some floor boards. I had an electrician come in and he will be redoing the the entire circuit.

The floor joists are 90mm deep, at the front of the property there are two 75x230mm air bricks that have been blocked up. We plan to get these reopened and add an additional 1 so we will have 3. and at the rear we will add 2 more.

The good news is we have 500mm of space between the joists and the ground.

Currently the sleeper walls are solid. the only gap between the floor boards and the wall is between the 90mm joists.

The current plan is to lay a floor like this:

Solid or engineered oak T+G boards
Pro Clima Intello VLC
Insulation between the joists with a breathable insulation. Not Sheep wool due to animals and rotting, strongly considering rock wool now.
layer hanging between the joists to sit the insulation in. (not sure what)
Additional layer required under joists?

To reach the recommended U value i will probably need 200mm of insulation. Is it a case of adding 130mm of baton to the joists or could i let the layer that is hanging over the joists simply hang lower?

There will need to be air flow through the sleeper wall. I will take bricks out lower down the wall, below the level of insulation. Is there are suggestion on how often you should be removing a brick?

The walls have been patched together over the years with various types of plaster. Installing the Pro Clima Intello VLC layer between the floor boards and insulation, i need to stick it to the walls to ensure its airtight. Should i remove the various plasters on the brickwork so the VCL attaches directly to the bricks?

At the front of the property there has been rot, it didn’t feel that damp in there when i opened it up, but there has certainly been rot, to the point where the ends of the floor joists were rotting away. They will either need to be braced or replaced. In the past, someone had clearer removed what the joists were resting on and placed in a new piece of wood, then added small bits of wood to make the floor level. See the photos. this will need to be corrected.

If you're using rockwool between and under the joists, an option could be to fit wood wool boards bonded to rockwool and screw them into the bottom of the joist. We considered these for our ceilings, but just went with 25mm woodwool boards with rockwool between the joists.

https://www.savolit.co.uk/products/savotherm

That looks good. How did you keep the netting taut? What kind of netting is that?