Suspended timber floor insulation

[Dylan123]

I’ve had a lot of help on this so far on another thread, I’ve bought my kit and I am soon to be going south and pot holing!

suspended wood floor, 4 inch joists insulating with fibreglass from below.

I was going to use netting (bought some) but at the 11th hour I’ve seen 2 “how to” guides say to use breather membrane if going from below.

Not come up before and 2 other sources say it’s not necessary.

anyone have a view on this? Would like to avoid cost as it’s dear but if it’s going to be draughty without it I can pay the extra £150. I don’t want to go to all this fuss to find it’s not made a big difference.

so, netting ok or VPM?
Thanks

 

[Mr Chibs]

Netting is fine, seen string used, chicken wire and barrier mesh to hold the rock wool in place.

I put a building paper on top of the insulation/joist before putting the floor down.

If your skirts don’t fit well, go around with the silicon sealant gun.

 

[Dylan123]

Netting is fine, seen string used, chicken wire and barrier mesh to hold the rock wool in place.

I put a building paper on top of the insulation/joist before putting the floor down.

If your skirts don’t fit well, go around with the silicon sealant gun.

damn i have already bought VPM. Swings and roundabouts I guess, slightly better airtight seal.


One other thing which is bugging me and which I appreciate it may be getting very into the detail.


My joists are 4 inch. The Government best practice says when insulating between joists, if the joists are less that 125mm or 5 inches their R value means this will potentially cause cold bridging which lowers the overall value of the floor (not too fussed as it will still be tons better than before) but this may also cause damp where the warm insulation meets the cold bridge. It then recommends to insulate the joists as well.

I’ve checked a couple of other sources which say 4 inch joists main issue from the cold bridge is mould and provided well ventilated crawl space (it is very well ventilated, can feel the breeze!) then no worries.

I suppose I can install then check in a year with a moisture tester? Obv would like to get it right.

I also note the best practice guide goes to ridiculous lengths on safety so not sure if this is a serious issue or just typical nanny state. I have read of plenty of people doing 4 inch joists without insulating the joists themselves no issues.

if you or anyone else can answer this much appreciated.

 

[Mr Chibs]

Not sure I'd worry about this.

Well it might lower the value, of insulated floor, but it will be higher that no insulation.

You could run some sort of battens to the underside if you wanted to make them deeper... but I think it will be a waste of time.

As said, if the ventilation is good, there won't be a mould problem.

My old joists were 4" and they were rotten at the ends, but this was due to vent being blocked and now damp proof under where the wood was on sleeper walls etc.
Still not bad for over 100 years of use.

Crack on.

 

[Dylan123]

Not sure I'd worry about this.

Well it might lower the value, of insulated floor, but it will be higher that no insulation.

You could run some sort of battens to the underside if you wanted to make them deeper... but I think it will be a waste of time.

As said, if the ventilation is good, there won't be a mould problem.

My old joists were 4" and they were rotten at the ends, but this was due to vent being blocked and now damp proof under where the wood was on sleeper walls etc.
Still not bad for over 100 years of use.

Crack on.

thanks. Last question. I am using butyl tape under the joists and using staples.

it’s quite a tight fit and thought battens tricky.

do you think butyl tape and a lot of staples will hold it long term?

I think so.

 

[Mr Chibs]

Are you Intending using the tape and staples to attach the membrane to the underside of the joists?

I reckon it will take the weight of the insulation... but if it comes unstuck, you won’t know unless you take up a floor board.

why not just staple on chicken wire.

Whats the plan for placing the membrane... under the joists or on top?

 

[Dylan123]

Are you Intending using the tape and staples to attach the membrane to the underside of the joists?

I reckon it will take the weight of the insulation... but if it comes unstuck, you won’t know unless you take up a floor board.

why not just staple on chicken wire.

Whats the plan for placing the membrane... under the joists or on top?

The breather membrane will go under the joists, as you describe. Each joist will have butyl tape along its bottom edge. This will stick the membrane in place while I staple through into the joist to join membrane, through the tape and into the joist.

I was going to use wire/mesh but the issue is this is more prone to draughts. The membrane can tape on to the wall, it’s recommended over mesh/wire in the gov guide so I’ve gone and bought it .

I’m just considering using a baton above the tape instead of staples. I’m sure enough galvanised staples will hold it, I am probably being too cautious. Tape + staples should be enough.

main headache now is how to attach it to the wall. It’s permeable so I don’t think I need to worry about lining up with the damp course (not sure if I have one, 1939 building)

some say butyl tape will attach to brick provided its clean and dry. Others say put Christmas tree fixings or a baton as tape won’t hold it.

A lot of conflicting info out there!

 

[Notch7]

Ideally you wouldve used celetex.

Half the u value of fibreglass

 

[Dylan123]

Ideally you wouldve used celetex.

Half the u value of fibreglass

Too late bought it all now

 

[Mr Chibs]

It will stick to the wall if you prime the wall with pva/primer. Although it’s a bit of a faff.

As you’ve got your membrane and tape. Why not try fitting a section near the wall. To see if you can get away with your method? Personally I’d prefer to have a physical fixing holding it.

As mentioned celotex would be easier. but more expensive.

What about your method plus a few bits of batten at 90° To the joists, stopping 10/15mm from wall. This will make sure nothing will slip down once you added the insulation.

 

[Dylan123]

It will stick to the wall if you prime the wall with pva/primer. Although it’s a bit of a faff.

As you’ve got your membrane and tape. Why not try fitting a section near the wall. To see if you can get away with your method? Personally I’d prefer to have a physical fixing holding it.

As mentioned celotex would be easier. but more expensive.

What about your method plus a few bits of batten at 90° To the joists, stopping 10/15mm from wall. This will make sure nothing will slip down once you added the insulation.

right, so going 90 degree to the joists, forming a cross kind of thing?

so one at each end of the joists and perhaps at 1m intervals? Or just at the ends? The spans I have are small. I have an odd construction. The joists are 4b2 but there are lots of low walls under the house, so the spans are only about 5-6 foot anyway.

that leads another question, the bit where the joists sit in on the wall support, I guess just loose fill fibreglass. I can’t physically lift the joists to put the membrane underneath. Just tape it up on side of wall and loose fill I guess

 

[Mr Chibs]

yes you’ve got it, like crosses. I’ve done this before, but access was tight.

Those low walls as you call them could be sleeper walls, that the joists rest on, this is to support and take bounce out of the floor.
Quite often these don’t have and damp proof on them, so the joist is just plonked on and over time the damp rises and transfers into the wood. If you have this arrangement try to lift slightly whilst person 2 slides in some dpc, plastic sheet.

Yes, some areas will have to be loose fill.

I have also used cable sheathing (cheap) zig zagged from joist to joist and stapled to make a cradle to sit the insulation in.

Try not to compress the rock wool, As you lower its efficiency when compressed... fluff it up!

Is that really your last question

 

[Dylan123]

yes you’ve got it, like crosses. I’ve done this before, but access was tight.

Those low walls as you call them could be sleeper walls, that the joists rest on, this is to support and take bounce out of the floor.
Quite often these don’t have and damp proof on them, so the joist is just plonked on and over time the damp rises and transfers into the wood. If you have this arrangement try to lift slightly whilst person 2 slides in some dpc, plastic sheet.

Yes, some areas will have to be loose fill.

I have also used cable sheathing (cheap) zig zagged from joist to joist and stapled to make a cradle to sit the insulation in.

Try not to compress the rock wool, As you lower its efficiency when compressed... fluff it up!

Is that really your last question

one more. How much wood would a wood joist Chuck if a wood joist could Chuck wood?

 

[Mr Chibs]

Edwood wood wood might know.

 

[Dylan123]

Edwood wood wood might know.

wood he?