Insulating suspended floor - where to put the DPM?

[RichD1]

I’ve read all the previous threads about this topic but it doesn’t resolve my query.

I am going to use 100mm fibre insulation between the joists supported by mesh/netting underneath but do I need a vapour proof membrane over the top before relaying the floorboards?

The floor is well vented underneath and the insulation will be well clear of the air vents.

Richard

 

[frutbunn]

Simple answer no. However the amount of insulation probably won't comply with B Regs.

 

[RichD1]

I'm assuming on a renovation building regs don't apply.

I'm thinking now that I will replace the wire mesh with permeable roofing paper as I understand this reduces heat loss due to the air movement in the vented void under the floor.

Why do you say one is not required?

Richard

 

[frutbunn]

I'm assuming on a renovation building regs don't apply.

I'm thinking now that I will replace the wire mesh with permeable roofing paper as I understand this reduces heat loss due to the air movement in the vented void under the floor.

Why do you say one is not required?

Richard

B Regs can apply to renovation work. it was changed a few years ago.

 

[Notch7]

I'm assuming on a renovation building regs don't apply

It is to do with building regs part L:

"When refurbishing a property it is necessary to be aware that if more than 25% of any existing thermal element is affected by works then the whole element should be improved to the required target U-value for that element"

I can't remember the target U value for a floor, I'd be guessing you would need more like 200mm, without looking it up.

There are differing opinions on use of VCLs on a suspended timber floor. If you are laying carpet, vinyl, hardwood flooring over your floorboards you could buy underlay with a VCL incorporated.

I know Tyvek recommend the use of a breather membrane below the joists -supro I believe, or in between and up and over, but I've never done that -probably because I've always used celetex PIR rigid boards rather than fibreglass.

 

[frutbunn]

"When refurbishing a property it is necessary to be aware that if more than 25% of any existing thermal element is affected by works then the whole element should be improved to the required target U-value for that element"

Where does that quote originate from as its incorrect, its 25% of the entire building envelope or 50% of an individual element.

I can't remember the target U value for a floor, I'd be guessing you would need more like 200mm, without looking it up.

Its 0.25, in reality fibreglass is probably impracticable due to the required thickness. I've personally never seen it used for years, it was never particularly popular even when the standards were much lower.

 

[RichD1]

I've spent many hours over the last week reading every article I can find on Google on insulating the underfloor and nowhere did any of those state that Building Regs would need to be followed. I just can't see the rationale for invoking Building Regs on old properties apart from when a new extension is built. Surely fitting some insulation is better than leaving it as it is?

Richard

 

[frutbunn]

I've spent many hours over the last week reading every article I can find on Google on insulating the underfloor and nowhere did any of those state that Building Regs would need to be followed. I just can't see the rationale for invoking Building Regs on old properties apart from when a new extension is built. Surely fitting some insulation is better than leaving it as it is?

Richard

I'm actually a Building Control Surveyor, I can assure its correct.

 

[JohnD]

Surely fitting some insulation is better than leaving it as it is?

quite.

luckily the secret police will not raid your house to see if you have insulated under the floorboards.

 

[dontbelieveawordofit]

quite.

luckily the secret police will not raid your house to see if you have insulated under the floorboards.

but you will know he’s done it.
and that’s worse .

 

[frutbunn]

quite.

luckily the secret police will not raid your house to see if you have insulated under the floorboards.

Such things have a habit of being reported by neighbours whilst in progress.

 

[RichD1]

If I’m just replacing a few rotted joists and about 5 floors boards surely this is well less than the 50% regulation.

About my original question; can I put the dpm layer over the floorboards?

 

[frutbunn]

If I’m just replacing a few rotted joists and about 5 floors boards surely this is well less than the 50% regulation.

About my original question; can I put the dpm layer over the floorboards?

It wasn't clear it was only part of the floor, less than 50% won't require B Regs.
No one ever bothers with a vapour check in this situation, nor does B Regs require it, although there's no reason not to install it if you wish. I never use them on my own work.

 

[RichD1]

We suffer with high humidity which causes condensation. Due to underground water courses in the area our underfloor is always damp. I’ve installed more air bricks and cleared out the couple of inches of mud on the oversite, so the use of a dpm either over the joists or preferably over the floorboards would further reduce any moisture rising into the house.

I’d prefer fitting over as it doesn’t entail removing skirting and lifting all the boards. Dpm can be tucked in under the skirting And then sealed with mastic.

Richard.