insulating under suspended floors

[Bullet-Proof_Biscuit]

Good luck with that

 

[JP_]

Surely, if the air bricks are all clear then vapour barrier is not essential? Yes, it would be better, but if the subfloor is ventilated then the moist air is not going to hang around for long...

my house, all air bricks were sealed / covered. No insulation, but the electrics were done with junction boxes on the floor. Apparently the previous owner was forever having powercuts, probably due to junction boxes getting wet from condensation.

 

[endecotp]

Assuming you can read

Don’t be rude.

 

[Bullet-Proof_Biscuit]

Surely, if the air bricks are all clear then vapour barrier is not essential? Yes, it would be better, but if the subfloor is ventilated then the moist air is not going to hang around for long...

my house, all air bricks were sealed / covered. No insulation, but the electrics were done with junction boxes on the floor. Apparently the previous owner was forever having powercuts, probably due to junction boxes getting wet from condensation.

The problem is the moist air condenses half way through the insulation when it reaches the dew point (top half of insulation is exposed to internal heat is warm, so the moisture condenses when it reaches the bottom half of insulation exposed to atmosphere which is cold). This is the problem... Hence a vapour barrier is installed to the all warm side of all insulation. This is why PIR is foil faced and you are required to seal the joints with foil tape.

 

[JP_]

That makes sense.
But, why isn't it a problem in lofts?

 

[endecotp]

This is why PIR is foil faced and you are required to seal the joints with foil tape.

The bubbles in PIR boards contain whatever gas was used to blow the foam (pentane?) and that is important for the thermal performance. At exposed faces, air will slowly diffuse in and replace the blowing gas over the years, reducing the thermal performance to eventually be similar to less-exotic materials like polystyrene. Preventing this is one reason for the foil facing. Another reason is that the low-emissivity ("shiny") surface helps with the thermal performance when it faces into a void.

It is true that the foil can also provide a vapour barrier, but I still don't believe that this is required or often done under suspended floors. I refer to this Kingspan document:

https://ks-kentico-prod-cdn-endpoin...Original/22173_ProductBrochure_TF70_01_UK.pdf

See page 10, "Installation in a suspended floor". It gives detailed instructions but does NOT specify that joints should be taped.

I had hoped to find a similar document for mineral wool, as that is what is being asked about in this case, but have failed to find one. My understanding is that the air flow in the under-floor void is supposed to be sufficient to remove any moisture that is present, without the need for a vapour barrier.

 

[JP_]

That makes sense too. It really shouldn't be any different for lofts. If there is air flow any interstitial condensation in the installation will be born away when ir reaches the surface, and not be able to cause damp problems in wood.

 

[Bullet-Proof_Biscuit]

Lofts are different because ceilings are not vapour permeable..

 

[Bullet-Proof_Biscuit]

Find some evidence of that “BCO requirement” please.

The Building Regulations 2010
Site preparations and resistance to contaminants and moisture
APPROVED DOCUMENT C
Refers to:
BS 5250:2002 Code of practice for control of condensation in buildings
Which states:
3.15.5 Interstitial condensation
A floor, wall, roof or other building element should minimise the risk of interstitial condensation in any part of a building that it could damage. Walls, roofs and floors should be assessed and/or constructed in accordance with Section 8 and Annex D of BS 5250:2002.

BS 5250 is explained in layman's terms from about page 15 here:
https://www.property-care.org/wp-content/uploads/2014/12/Technical_appendix.pdf

Basically moisture in a warm space will condense when it contacts anything cold... Whether that's window glass, a cold bridge, or the cold side of insulation, regardless of it being up down or sideways.. Ergo, it is good practice to use a vcl to prevent moisture entering the fabric of any building wall/floor/ceiling. Apologies, I thought this was a fairly simple concept.

 

[frutbunn]

Find some evidence of that “BCO requirement” please.

There is none and I've inspected hundreds, we had some pretty severe flooding when a river burst it banks a few years ago affecting several hundred properties mainly in my part of the city, most of which required new floors. Didn't see or request a single vapour barrier.

 

[JohnD]

Apologies, I thought this was a fairly simple concept.

No need to apologise, you're not the only one who can read, even though you think you are.

Tell us about some of the occasions you've personally found interstitial condensation in a ventilated wooden ground floor.

I'm especially interested in what led you to think that the air in the ventilated void will be warmer than the floor above. I find your belief most curious.

 

[frutbunn]

Tell us about some of the occasions you've personally found interstitial condensation in a ventilated wooden ground floor.

I've not seen a vapour barrier installed once in this situation since the introduction of insulation in floors in the B Regs, I've yet to hear of a problem, though with a vented sub floor I can't see why you would.
Just to clarify its not a B Reg requirement in spite of what bullet proof biscuit believes.

 

[JP_]

Lofts are different because ceilings are not vapour permeable..

I thought vapour could pass through normal plasterboard and paint - if ceilings are not vapour impermeable, how are walls different? #confused

 

[frutbunn]

I thought vapour could pass through normal plasterboard and paint - if ceilings are not vapour impermeable, how are walls different? #confused

It does that's why you ventilate over the insulation or use a warm roof.

 

[JP_]

So in that case, ceilings and floors can be treated the same - no vapour barrier so long as you have ventilation?