100mm cavity U value regs fix

[Surftuzzer]

Our builder used fullfill wool insulation in our 100mm cavity (a simple 1m x 2m porch).

Building control have told us “this construction doesn't comply with Approved Document L. This construction achieves 0.35W/m2K when it needs to meet 0.18W/m2K. Please provide details of how this porch will meet Part L which may include compensating elsewhere for the under provision in the cavity (e.g. roof or perhaps additional insulation over the inner face of the blockwork).

What is the likely way forward? What will get this over the hump (0.17U to find).
Anyone else had to address a shortfall like this with a 100mm cavity?

Interesting that they might be happy if we compensate in the roof area?

Roof is on and wool roof insulation is in place. Plaster board isn’t yet in place.
Assume there is no expectation of sliding anything more into the cavity or taking away blocks to do so, so it’ll be some sort of internal layer.


Incidentally, our 100yr old house has no insulation in the cavity’s elsewhere at all (just in the loft).
So this little porch of ours is ironically the only bit of our home with wall insulation!

Is there any hope Building control take a view so to speak? Adding internally will rob us of 10% of our internal floor area

 

[noseall]

Our builder used fullfill wool insulation in our 100mm cavity (a simple 1m x 2m porch).

Building control have told us “this construction doesn't comply with Approved Document L. This construction achieves 0.35W/m2K when it needs to meet 0.18W/m2K. Please provide details of how this porch will meet Part L which may include compensating elsewhere for the under provision in the cavity (e.g. roof or perhaps additional insulation over the inner face of the blockwork).

What is the likely way forward? What will get this over the hump (0.17U to find).
Anyone else had to address a shortfall like this with a 100mm cavity?

Interesting that they might be happy if we compensate in the roof area?

Roof is on and wool roof insulation is in place. Plaster board isn’t yet in place.
Assume there is no expectation of sliding anything more into the cavity or taking away blocks to do so, so it’ll be some sort of internal layer.


Incidentally, our 100yr old house has no insulation in the cavity’s elsewhere at all (just in the loft).
So this little porch of ours is ironically the only bit of our home with wall insulation!

Is there any hope Building control take a view so to speak? Adding internally will rob us of 10% of our internal floor area

Is this a true 'porch' i.e. a thermally accountable (external spec') door, separating the porch from habitable? If so, tell BC to do one.

Do you intend heating the space, using the house heating system?

 

[Surftuzzer]

Yes true porch new front door/thermal boundary.
The space will be heated switch plumbed in radiator installed within the new porch

 

[noseall]

Yes true porch new front door/thermal boundary.
The space will be heated switch plumbed in radiator installed within the new porch

Either keep the heating separate or add 25mm of PIR to the inside walls, prior to plasterboarding.

 

[freddiemercurystwin]

But check BC will accept 25mm beforehand.

 

[^woody^]

Ask a manager for relaxation of the requirement based on your comments re the rest of the house.

Unless you instructed him otherwise or any plans provided to him were incorrect, the builder would be responsible for your costs and his failure to meet the regulation requirements.

 

[Deluks]

0.35W/m2K doesnt sound right.
What is the actual construction? Brick/insulation/block? what type of block?

 

[wessex101]

Three quick options-
1. Ask the building inspector if he will accept additional insulation in the roof to compensate for the lack of insulation in the walls.
2. SAP calculation: in a nutshell compensate for the energy loss due to the lack of insulation by installing energy saving items elsewhere such as LED lights in the house etc. Calculation costs around £200- £300.
3. Area Weighted Calculation: install extra insulation somewhere else such as the roof to compensate for the lack of insulation in the walls. There are free calculators online or maybe £100 to pay someone such as your designer to do it. To be honest I have only ever used this for excess areas of glazing but in theory it might work in this case.

 

[stevie888]

If it's a true porch then you need no insulation. I have however come across BCOs in the past who insisted on full code requirement if you put which is illogical in the extreme. anything in.

I doubt very much if you'd cram enough insulation into the porch roof to compensate: my go to option in this case woud be sto look at the whole structure and bung insulation in the existing roof: very rare to find the full 270 mm up there and it's an easy option.

 

[Surftuzzer]

Thought I should report back what came out the wash… (after over a week of chasing BC for a call back).
BC gave us ok to fit 37.5mm insulated plasterboard (25mm PIR, VCL and 12.5mm plasterboard) dabbed to the inner blockwork face “increase the overall insulation and is accepted as meeting part L”.

I’m very happy with this compromise achieved by a phone call between my contractor and the BCO.
I think it’s possibly due to the small floor space ( doc L has a note about lesser provision if 5% of a floor is lost through applying layers to the inner face).
It won’t have brought us to 0.18 but probably circa 0.22 ish.
Had they insisted on 50mm PIR on the inner face (which would have got us to 0.18) I would have likely not done that and instead obtain a SAP calculation (as some suggested) for whole house and compensate for this porch short fall by adding insulation into the main house loft space. That way the contractor could press on with standard plaster boarding.
Farcical really but that seems to be a common way out of such situations.

I tried a number of calculators for U value and the best one seemed to be from Knauf. It also spit out a handy PDF via email for sharing with BC. I didn’t need this in the end, but was a good calculator.