Insulating suspended floor from below - VCL?

[andyscotland]

I don't have enough on my list 🤣 so I'm thinking about sorting out insulating our suspended timber floor (traditional 1960s floorboards on joists).

 

In my old flat I did that from above - I took up & sold the floorboards, draped breather between the joists, then insulation, then a polythene VCL across the whole floor, then laid a new chipboard deck and carpet. Lots of common references to that approach online.

 

But that's not an option here due to disruption, so I will need to do it from the crawl space below.

 

My main concern is what (if anything) to do about a VCL:

• The bottom of the joists will need to breathe, so I don't think I can take it between & under.
• I considered cutting strips of DPM to roughly the joist spacing, double-sided tape to the bottom of the floorboards then silicone seal along the floorboard / joist junctions. But given there will still be a moisture gradient I'm wondering if sealing between the joists will actually force more into the timber, making things worse.
• No matter what, I think I'll need something between insulation and floorboards just to reduce airflow and keep dirt, dog sick 🤢 etc out of the insulation (there are quite a few small gaps in floorboard joints) . But that could obviously be a breather membrane.

 

I was also considering fixing a layer of Steico/etc board across the bottom of the joists for a few reasons - to support the insulation, to get a better u-value than achievable just with wool between joists, to improve airtightness from below, and for a reasonable surface to fix cables / pipes etc too (most of which I've inherited just draped loose across the void).

 

What do you all reckon?

[Linto]

I'm doing something similar. I've used a roofing breathable membrane to hold the rockwool in place. Stops air/wind from the air vents from washing the trapped heat out of the insulation and holds it in place. You just need to think through the joins to the walls where I've used orcon f as the adhesive with some wooden bats screwed into the brick to clamp the membrane in place. 

You just need to be methodical making sure the overlaps are taped etc. 

I didn't come up with approach myself, I saw it on a website somewhere but for the life of me i can't find it now. 

You could use sealant around the skirting boards to stop anything going down into the cavity. Unless you mean by going between the floor boards which the answer to is probably a floor covering of some description. 

[Linto]

Here's the official government guidance on how to do what you're thinking of. May be some help.

[Linto]

Or here ,3rd option which is similar to what I am doing.

You could also, if feeling flush, paint the floor with something like blower proof to stop air movement. 

[TonyT]

I just squished in friction fit ROCKWOOL in 2 staggered layers, didn’t add any membrane due to pipes,cables  and having to lie on my back to fit insulation.

 

better than what it was!

[Gus Potter]

9 hours ago, andyscotland said:

I don't have enough on my list

I too have a 1960's house, want to insulate the living room floor. Good news for me is that it's not a kitchen or bathroom so moisture levels will be not too severe.

 

Problem I have is that the joists are built into the inner leaf of masonry on a timber wall plate. Yes.. the inner skin is carrying all the roof load plus two stories of masonry above!

 

I'm still trying to get my head around the best way of doing this. Big worry is that if I cause damp / condensation in the wall plate the whole house will drop!

 

Will keep mulling it over. There must be a way! That is as far as I have got to date, like you that list is a journey.

 

 

 

 

 

 

 

[TonyT]

Mine is similar except it’s 100 year older and 600mm stone.

 

no issues in 10 plus years, have been down a few times to alter pipes, cabling and it’s dry as a bone

[andyscotland]

9 hours ago, Linto said:

I'm doing something similar. I've used a roofing breathable membrane to hold the rockwool in place. Stops air/wind from the air vents from washing the trapped heat out of the insulation and holds it in place. You just need to think through the joins to the walls where I've used orcon f as the adhesive with some wooden bats screwed into the brick to clamp the membrane in place. 

You just need to be methodical making sure the overlaps are taped etc. 

I didn't come up with approach myself, I saw it on a website somewhere but for the life of me i can't find it now.  

 

Thanks, that and the subsequent links are broadly what I did last time when I was working from above.

 

The only thing being that doing it supported on breather membrane was easy/felt very secure from above - just draped it over the joists and then I'd made myself a little jig to push it down to form trays the right depth to hold the insulation flush to the top. So then it just took a few staples into the joist tops to stop the breather moving.

 

And also that the manufacturer's instructions specified an airtight/vapour layer between the insulation and the room which again was easy with the boards up.

 

I'll read that government doc carefully, maybe it will shed some light. Most of the details/guides I've read all assume either working from above, working with a continuous/solid floor covering e.g. chipboard/new floorboards etc, and don't feel like they're quite so easy to replicate from below without disruption.

[andyscotland]

10 hours ago, Linto said:

You could use sealant around the skirting boards to stop anything going down into the cavity. Unless you mean by going between the floor boards which the answer to is probably a floor covering of some description. 

 

Yes, going between floorboards is the issue. Floor covering is not an option. At some future point we will sand floors and restore joints but that is further down the list.

 

Generally I think it's good to keep dust etc out of insulation as it compromises performance over time. And ideally to prevent spilt liquids getting down there.

[andyscotland]

1 hour ago, Gus Potter said:

Problem I have is that the joists are built into the inner leaf of masonry on a timber wall plate. Yes.. the inner skin is carrying all the roof load plus two stories of masonry above!

 

Yikes! Fortunately mine are on sleepers so that makes life easier.

 

1 hour ago, Gus Potter said:

Will keep mulling it over. There must be a way! That is as far as I have got to date, like you that list is a journey.

 

I'm finding one of the biggest issues bringing a house up in stages while living in it is the constant journey of "well, this would be viable at the moment but in X years time when we do Y then [something else will be easier/that will cause us a problem]"

 

Identifying the critical path and the logical way to approach things is a journey & a job in its own right!

 

Not to mention there is a budget for how many things can be in progress/look a bit of a mess for a while because finishing them would be undone in a future phase, without triggering divorce 🤣

 

Many evenings turning over ideas in the mind and trying to work out how it all fits together, then energy goes through the roof and you start to think "JFDI"!

Edited October 14, 2022 by andyscotland

[andyscotland]

22 minutes ago, TonyT said:

Mine is similar except it’s 100 year older and 600mm stone.

 

no issues in 10 plus years, have been down a few times to alter pipes, cabling and it’s dry as a bone

Thanks, that's useful to know.