insulating under suspended floors

[Mr Sparkle]

Having my suspended floors insulated

The chaps that are doing the insulation are removing a few boards and then crawling underneath to place the insulation between the joists

However the installers have not mentioned a word about adding a vapour barrier and I'd like to know whether it is absolutely essential for a vapour barrier to be laid on top of the insulation/joists before the floorboards are screwed down

If it helps the installers are using the following products

Knauf Earthwool loft roll 44

Insulation is held by chicken wire

 

[freddiemercurystwin]

No

 

[veedee]

I would have thought that something to prevent condensation and potential for joist rot like a vapour barrier would be installed.

 

[JohnD]

ventilation under the floor is correct.

Why do you think there will be warm humid air in the space under the floor?

 

[endecotp]

I believe the important thing is that you have air bricks so that the space below is vented to the outside.

 

[Mottie]

ventilation under the floor is correct.

Why do you think there will be warm humid air in the space under the floor?

Many years ago, I laboured for a friend who was doing a full rewire on a big house that a car customer of mine had just moved into. When we got the downstairs floorboards up we were surprised to see what looked like insulation had been sprayed everywhere. On closer inspection, the previous owner had simply vented his unfiltered tumble drier into the floor void and it was years of clothing fluff that had built up - that must have been warm humid air and it’s a wonder the joists weren’t rotten or a fire didn’t break out!

 

[Bullet-Proof_Biscuit]

ventilation under the floor is correct.

Why do you think there will be warm humid air in the space under the floor?

There will be warm humid air in the habitable space ABOVE the floor, caused by human occupation. Without a vapour barrier this moist air will migrate down through the insulation and condense when it reaches the dew point, mid way through the insulation, as the sub floor void will be COLD.

You absolutely DO require a vapour barrier, best way is a big sheet of DPM btw. Also I believe this is a BCO requirement.

Further more, not only will the condensation mid way through your new rockwool start to go mouldy, it will rot your joists..

Sounds like you've got some cowboys on your hands.. If you aren't happy with what they're doing and they refuse to accept the above, call in your local BCO to inspect (Which the builder should do anyway, or ensure that you do as part of their works).

I joined because I thought this forum was full of good advice as well!

HH

 

[JohnD]

Tell us more about how WARM air doesn't rise, it falls.

 

[Bullet-Proof_Biscuit]

Tell us more about how WARM air doesn't rise, it falls.

It expands Now unless your ceiling has holes in(?), when warm air expands in a space it will exit through any penetration correct? Namely this guys floorboards & rockwool, the rest is history. Caveating with door being closed.

Having said that I believe the correct term is 'vapour pressure'. And it causes interstitial condensation

 

[JohnD]

Now unless your ceiling has holes in(?)

unless you live in a submarine, no part of your house is airtight. Expansion does not cause a rise in pressure within the house.

warm air naturally rises.

water vapour naturally rises.

warm, humid air does not pass through a ground floor into the ventilated void below. The purpose of the ventilated void is to evaporate and blow away any water vapour within.

 

[endecotp]

You absolutely DO require a vapour barrier, best way is a big sheet of DPM btw. Also I believe this is a BCO requirement.

Find some evidence of that “BCO requirement” please.

 

[Bullet-Proof_Biscuit]

Assuming you can read, just spend a few minutes googling interstitial condensation

 

[JohnD]

Oh biscuit, you are silly.

 

[Bullet-Proof_Biscuit]

You install no vapour barrier in any building fabric you install?

 

[JohnD]

now you're being silly again.