Wooden subfloor with 4 inch joists and rockwool

[Rocket Ron]

Hello everyone we have recently purchased a bit of a monster Property and taken on a bit more than I can chew however insulation is a priority. The ground floor is a wooden subfloor with about a 70 cm crawlspace. All the joists are 4 inches tall not 6 inches and I wanted to insulate it with Rockwall and netting from below without lifting any floorboards because the floorboards have all been sanded and varnished and I do not want to damage .

 

Kingspan is just not an option. The joists do not run true and the distances between the joists or vary by an inch two besides which getting the material under the floor is virtually impossible Rockwool on the other hand would be a lot more practical to install. I’m just wondering whether the joists being 4 inches are too shallow?

 

Thanks in advance

[ProDave]

4" will be a lot better than nothing.

 

It will be a miserable job from below you will need full suit and respirator imho.

 

Use something like Frametherm 35, a lot less nasty than most flavours of rockwool style insulation and quite stiff so should push into place and stay there while you get the netting in place.

[Oz07]

Do you mean structurally or thermally? Structurally if they've lasted this long then should be fine. Must have quite a few intermediate walls? Thermally as @ProDave says be a hell of a lot better than nowt. Some modern houses still have 4" in the cavity. 

[Rocket Ron]

Thanks all. This is an Edwardian house that is huge. I was surprised that it only has 4 inch deep joists.

 

I’m talking about thermal insulation installed from the crawl space just to be clear.

 

it’s it’s going to be a horrible job, but I think it will pay dividends well I hope it will.

 

No, it’s a case of finding the right material to use as already mentioned I will check out frametherm

 

 

[torre]

I'd echo what @ProDave said about PPE. Definitely a full face respirator mask.

 

We did a similar job but from above when replacing floorboards and used Thermafleece (wool) which is way nicer to handle but pricey and hard to cut. Of the rockwools Knauf products like Frametherm do seem the nicest to handle and shed less fibres.

 

Instead of netting, use a breather membrane (and tape it to the wall edges all round, you may need to PVA walls before taping) as that will cut down hugely on draughts. The whole job will be difficult and time consuming but the results are transformative for comfort.

 

Are you sure you have access to all the crawlspace? Our joists were supported at half span by a low wall.

[Rocket Ron]

Yes I’ve got access to the entire under floor of the house. There is only one section that is very very tight but that has plywood Floor on top so I might access through a hatch that an electrician has made prior to today otherwise I have full access yes there are partition walls down there but there are holes in them for crawling through. 

[Rocket Ron]

Because it’s such a large floor space, I think I’m gonna have to go with rockwool  loft insulation as the surface area of the floor space is close to 100 m²

[ProDave]

7 minutes ago, Rocket Ron said:

Because it’s such a large floor space, I think I’m gonna have to go with rockwool  loft insulation as the surface area of the floor space is close to 100 m²

Before you make the choice, price up all the options.

 

Frametherm is so much less nasty than most wool insulation and it is quite stiff so when pushed in place it should stay there while you get the netting etc in place, many will just fall out.

 

https://knauf.com/en-GB/p/product/frametherm-r-roll-35-ready-cut-26329_4206

 

Search for best price, I got mine from SIS as they were the cheapest I could find it at the time.

[Rocket Ron]

18 minutes ago, ProDave said:

Before you make the choice, price up all the options.

 

Frametherm is so much less nasty than most wool insulation and it is quite stiff so when pushed in place it should stay there while you get the netting etc in place, many will just fall out.

 

https://knauf.com/en-GB/p/product/frametherm-r-roll-35-ready-cut-26329_4206

 

Search for best price, I got mine from SIS as they were the cheapest I could find it at the time.

Thanks for the info if my choices are only 4 inches do I go for the 140 mm or 90 mm thickness?

[Russell griffiths]

I’m going to say this is such a horrible job to do that you will end up doing it badly 

if done badly with air gaps then you might as well not do it. 
 

I would do room by room

take the skirting off, lift the floor, re fit floor re fit skirting and then re decorate each room. 

[Rocket Ron]

4 minutes ago, Russell griffiths said:

I’m going to say this is such a horrible job to do that you will end up doing it badly 

if done badly with air gaps then you might as well not do it. 
 

I would do room by room

take the skirting off, lift the floor, re fit floor re fit skirting and then re decorate each room. 

This is a Period property. There is no way the skirting boards are coming off after having re-plastered all the rooms it has to be done from below and I won’t leave any gaps.

[Redbeard]

If the house is Edwardian there is a fair likelihood that the skirting boards were hollow-fixed first, then the plaster  carried down to the skirting.

 

If this is the  case one of the few ways you'd get the floor airtight at the perimeters while leaving the skirtings on would be to pump air-tight foam up behind the boards from the bottom. This has the disadvantage that you are very unlikely to be able to use a foam gun safely (i.e. with the can upside down). In my relatively limited experience if you attach plastic pipe to the nozzle you do not get enough pressure for the foam to 'stick'. The alternative is to find something with the consistency of 'sticky chewing-gum' and ram it up the back of the skirting from below. (You can see why foam was my first thought!).

@Russell griffiths's suggestion to take off the skirting gives you a chance to get really good perimeter air-tightness.

Edited 6 hours ago by Redbeard

[Rocket Ron]

Do we really want the floor airtight ? I was under the impression that some airflow would still be required to help with damp and Mold issues and allow the house to breath ? 
 

what I’m trying to do is minimise heat loss through the floor

[Redbeard]

Heat is lost by conduction (in this case through the floorboards, which you aim to limit by insulating - though note there will still be heat lost via the joists) and convection - air-borne heat loss. Yes, you want the floor structure air-tight, but water-vapour permeable. Water-vapour-permeability is what energy geeks call 'breathability'. You need to sort both sets of losses.

[Iceverge]

None of the above! 

 

Screw timbers ( thickness space dependant, more is better) under the existing joists. Staple a good breather membrane like VP400 underneath. Tape all joints and seal to the walls. 

 

Screw 75*25mm battens or similar underneath to keep the membrane in place. 

 

Then blow the cavities full of cellulose insulation. 

 

[Rocket Ron]

15 minutes ago, Redbeard said:

Heat is lost by conduction (in this case through the floorboards, which you aim to limit by insulating - though note there will still be heat lost via the joists) and convection - air-borne heat loss. Yes, you want the floor structure air-tight, but water-vapour permeable. Water-vapour-permeability is what energy geeks call 'breathability'. You need to sort both sets of losses.

Thanks for the clarification. Looks like I’m gonna try the foam gun with a plastic hose first in that case.
 

I take it even if I didn’t manage to get the skirting is 100% airtight. There is still a great benefit to insulate between the joists? At least a certain degree of heat loss will be mitigated.

[Redbeard]

31 minutes ago, Iceverge said:

Screw timbers ( thickness space dependant, more is better) under the existing joists. Staple a good breather membrane like VP400 underneath. Tape all joints and seal to the walls. 

 

Screw 75*25mm battens or similar underneath to keep the membrane in place. 

 

Then blow the cavities full of cellulose insulation. 

@Iceverge, I definitely agree with you in principle, but the part of the world you are in (and the size of the job) can have a big bearing on whether you'll get a cellulose blower out of bed... I've known colleagues struggle for large extensions, let alone one floor. I don't know if it still is, but Wales used to be a good place to be for cellulose. Is Ireland similar? I don't know where @Rocket Ronlives, but it could be pivotal w.r.t cellulose.

 

I've talked about blocking 'unintentional ventilation', and you @Iceverge, have suggested cellulose and membranes, but I don't think any of us yet has (in this thread) explicitly stated the need for excellent cross-ventilation below the insulated 'sandwich'. Yes, getting rid of warm moist air from the house entering the void is good, but if *any* vapour-laden air gets into the void, which is now colder because of the insulation, there's a potential chance of condensation occurring on the bottom of the membrane. Good, unimpeded cross-ventilation (straight lines and diagonals) is a life-saver for suspended floors. Vents in one elevation only mean you have 'ins', but no 'outs', and the air 'short-circuits'. Edit: Come to think about it @Iceverge's  idea of adding to the bottom of the joists could be done in home-made OSB/plastic insulation/OSB 'fillets' to cloak the thermal bridges of the joists. For example, perhaps instead of 50 x 50 PSE, 2 x lengths of 12mm x 50mm wide separated by 25mm PIR or EPS/XPS and glued up with waterproof glue. (I used EVA nearly 40 years ago and the 2 elements are still stuck together!)

Edited 3 hours ago by Redbeard

[Rocket Ron]

Thanks all but it’s getting super complex by the minute. 
 

I think I’d like to stick to the mineral wool type insulation at this stage because I don’t have vast amounts of money and time to a screw strips of wood to the bottom of the joists aside from which there are so many of them it could be a very costly experience. If the mineral will route is effective, then I’m going to stick with that.

 

For reference, I’m based in Liverpool

Edited 3 hours ago by Rocket Ron