Ground floor insulation

[Apasc]

For thermal and noise benefit, considering insulting under floors in single storey 1930s property, brick cavity wall (filled).
Important factors:
- not sufficient crawl space to work underneath, so need to do from above.
- timber T&G floorboards.
- soil solum.
Original plan was simple:
1. Loft floorboards.
2. Fit netting up and over floor joists to create wells for insulation.
3. Friction fit hygroscopic insulation to wells.
4. Refit floorboards.
5. Fit floor coverings.

However, got complicated when I “googled” and various other ideas thrown in the mix, with most modern suggestions now being as below:
Modern suggestion:
1. Loft floorboards.
2. Fit breathable VCL up and over floor joists to create wells for insulation - to prevent wind wash.
3. Friction fit hygroscopic insulation to wells.
4. Fit air tight vapour barrier - to stop any moisture from above getting through to insulation.
5. Refit floorboards.
6. Fit floor coverings.

My concern is if the modern suggestion will create possible timber issues, or issues for the brick walls? Is this modern suggestion suitable for a 1930s house? Is it just a way for insulation sellers to sell more products? Having experience of “damp proof” companies selling “solutions” that actually make things worse, I’m a bit sceptical of enhanced solutions.

Any informed experience by forum users? Thank you.

 

[paulc]

Slightly older property here, also with T&G floorboards. Lifting them would be a right royal pain in the backside, and not something I would willingly do. Yes, it would be nice to add a bit of insulation under the boards, but being 3x2" joists, there is a real risk of ventilation being compromised where the dwarf walls are supporting the joists. For those reasons, I've decided insulating under the floor is more trouble than it is worth. Instead, I've gone round the perimeter of the rooms sealing the gap between floor & wall, and then putting down a layer of woodfibre laminate underlay. This has killed the cold draughts and added a small amount of insulation to the floor. Rooms are much warmer as a result.

 

[fernicarry]

The second approach is closer to what I've done. I'm able to work from below although in some areas its not so much crawling as excavating... Depends on your tolerance for confined spaces.

As your source says the airtight (but NOT vapour tight, so NOT a VCL) membrane below the insulation at step 2. improves the performance of the insulation by allowing it to hold on to more air. Breather membrane isn't terribly expensive so no reason not to use it rather than netting. You can fix the membrane to the sides of the joists with battens to give you a more uniform rectangular cross section into which to fit the insulation than you would get with netting. Its quite important that the insulation is tight fitting with no gaps. I recommend Indinature products as having a nice balance of sitffness to friction fit in place but some squashiness to take up small variations in dimension (no connection with them).

The modern advice is a VCL above the insulation to stop vapour from the habitable space leaking into the fabric of the building and potentially causing condensation. I'm not really convinced on the use of VCL's in traditional properties. We usually try to allow vapour to pass in and out of the fabric without hinderance even when adding insulation. In modern construction the entire internal space is basically made vapour proof and the vapour is removed by other means (mechanical, trickle vents) which isn't generally achieveable when you are retrofitting so there will still be gaps where warm air can leak. I fell that using a VCL in some parts but not others concentrates that warm air leakage into smaller areas which could more quickly become problematic.

Instead I've used another layer of breather membrane as the top layer (but fitted from below so not a continuous layer) on my ground floor. The purpose of this layer is mainly to stop debris from above contaminating the insulation. Whatever you do you should try to ensure that the membrane is detailed around the perimeter of the floor so that you get an airtight seal onto the external wall. I don't always achieve this, but if I've taken skrting boards etc. off for any reason I lap the membrane up onto the plaster and fix it with airtight tape before refitting the joinery. IMO airtighness under a ventilated floor is the biggest gain from this exercise rather than the addition of insulation but if you've gone to the trouble of ripping up the floor then adding the insulation is a small extra step.

As paulc says lifting T&G floorboards is likely to create many casualties but if you are going for a floor covering that may not be a concern. Note that internal walls often float over the floorboards so you'll need to figure out how you are going to cut the boards that run under the walls.

 

[Apasc]

Thanks fernicarry.
When you say “I lap the membrane up onto the plaster and fix it with airtight tape before refitting the joinery.” Are you referring to the hammock membrane or your top membrane? Thanks.

 

[fernicarry]

The top layer.

 

[Apasc]

The top layer.

Thanks so much. Have looked at Indinature and looks good, just need to work out which product best for floor insulation - which did you use, and which membrane did you use?

 

[fernicarry]

Yeah Indinature seem to have expanded their range of products and for the life of me I can't work out the difference between them. I think it might be to do with the stiffness of the batts making them more suitable for standing up in walls versus lying down in lofts. Maybe drop them a line and ask.

I have noticed that the density seems to vary from pack to pack. Whether that's because I'm getting different products each time I buy or its them fine tuning their production process I'm not sure (they are quite a young company I think). Having a range of densities is quite useful when using it on the uneven wall but for under the floor the denser stuff is defniitely easier when I'm fitting from below so that it doesn't all sag around my ears before I can get it pinned in place.

I buy from a place in Stirling which is close enough to pick up a car load and avoids having to pay for pallet delivery fees. Same place sells breather membranes and a whole range of natural/breathable accoutrement. I just tend to get whatever is most reasonably priced, last one was thermafleece branded.