Part L payback test on 1907 roof repair in AONB — anyone been through this?

[Jon12345]

1907 detached house, High Weald AONB, EPC Band G. Stripping front and right roof slopes (over 50%), so Part L triggered. Sarking boards, 115mm rafters, bitumen underfelt. First-floor bedrooms have sloping ceilings. Attic above has no fixed heating, ladder access only.

 

NFRC contractor has quoted £9,652 + VAT for between-rafter insulation as a separate line item. On a G-rated house with uninsulated cavity walls, I estimate the annual saving at £75–200 — giving a payback of 48–154 years, well beyond the 15-year test in para 5.9.

 

Three questions:

1. If the attic has no fixed heating, is the roof above it NOT a thermal element — making the attic floor the relevant element instead?
2. Has anyone successfully used the 15-year payback argument with Building Control on an older property?
3. Above-rafter insulation would raise the roof height in an AONB, needing uncertain planning permission. Is that "not technically feasible" under para 5.9?

 

Thanks,

 

Jon

[Mr Punter]

From your description I cannot work out how much of the upper floor has insulation at sloping rafter level and how much is at ceiling joist level.

 

The ceiling joist insulation is simple with extra loft roll.  You may need to use loft legs if your are storing crap stuff up there.

 

If you can insulate the sloping parts, so much the better.  BC may take a fairly pragmatic approach, especially if you would need to trash ceilings etc.

[Roger440]

6 hours ago, Mr Punter said:

BC may take a fairly pragmatic approach, especially if you would need to trash ceilings etc.

 

Or they may not as has occured with others in this exact same postion.

 

Very interested to see how the OP's job pans out with BC.

[Roger440]

7 hours ago, Jon12345 said:

 

1. If the attic has no fixed heating, is the roof above it NOT a thermal element — making the attic floor the relevant element instead?

 

But the slopey bit IS as the room is the otherside of the plasterboard. If im understanding your layout/build up correctly, ie pretty conventional?

 

Which would mean you only have 115mm to work with. Better than my 75mm though!

[Jon12345]

23 hours ago, Mr Punter said:

From your description I cannot work out how much of the upper floor has insulation at sloping rafter level and how much is at ceiling joist level.

 

The ceiling joist insulation is simple with extra loft roll.  You may need to use loft legs if your are storing crap stuff up there.

 

If you can insulate the sloping parts, so much the better.  BC may take a fairly pragmatic approach, especially if you would need to trash ceilings etc.

There is no insulation at sloping rafter level. The is only some insulation at the ceiling joist level, around the edges, because most of the attic has floorboards (except the edges). Joist depth is 22cm. I am thinking the cost to remove the carpets and floorboards in the attic to put thinner insulation may not meet the 15 year payback rule? If I use loft legs, then it will reduce the usability of the attic due to the sloping ceilings. Isn't there a building regs rule against reducing usability?

The rafters are 11.8cm deep. Some I could wedge in insulation, but the main room in the attic does not have rafters visible because there are boards over them and painted. So they have sarking boards one side, and other board the other side. Removing them to put thin insulation in might not meet the 15 year payback rule? What do you think?

[Jon12345]

17 hours ago, Roger440 said:

 

Or they may not as has occured with others in this exact same postion.

 

Very interested to see how the OP's job pans out with BC.

Does Building Control not have to follow the 15 year payback rules, and so if it was going to be too costly for the gain then they give you a pass?

[Jon12345]

17 hours ago, Roger440 said:

But the slopey bit IS as the room is the otherside of the plasterboard. If im understanding your layout/build up correctly, ie pretty conventional?

 

Which would mean you only have 115mm to work with. Better than my 75mm though!

Sorry, bit confused what you mean by your first couple of sentences. 

[Mr Punter]

If you are renovating it is a decent opportunity to improve the energy efficiency of the building by increasing insulation.  The minimum fallback is spend the amount of money that would be saved in 15 years on the improvements.  You may be able to claim there is no practical way to improve the roof insulation that would provide such a payback. Hopefully your heating bills are low, and if so, happy days!

[Jon12345]

23 hours ago, Mr Punter said:

If you are renovating it is a decent opportunity to improve the energy efficiency of the building by increasing insulation.  The minimum fallback is spend the amount of money that would be saved in 15 years on the improvements.  You may be able to claim there is no practical way to improve the roof insulation that would provide such a payback. Hopefully your heating bills are low, and if so, happy days!

My heating bills are high, but it is difficult to get a 15 year payback. This is the issue I am facing. I will write to Building Control and hope they reply!

[Mr Punter]

17 minutes ago, Jon12345 said:

I will write to Building Control and hope they reply!

 

Have you applied for Building Regs and paid the fee?  If not, don't bother.

[Redbeard]

The only time I saw a contractor try to invoke the '15 year rule' it was for an external wall insulation job where they had installed 40mm phenolic board, not 50 as per their quote which would have met the wall target of 0.35 at the time (pre-2010). BC insisted that the '15-year assertion' was backed up by a SAP calc. The SAP assessor totally misunderstood the builder's 'brief' (if there was one) and (with a torturing of mathematics which even I - a relative mathematical ignoramus - could not have managed), managed to 'prove' that 50mm of phenolic  *would* pay back in 15 years (remember he was supposed to prove it *wouldn't*! So BC insisted he install 50mm (which in reality meant a layer of 20mm over the existing rendered 40mm. Not the world's most impressive moment.

[Redbeard]

And another thought: Are the 'rooms' in the 'attic' habitable rooms, or are they used as such? One assumes not as there is only ladder access. If, then, although there is a floor, they are storage spaces only, then the thermal line above the habitable spaces is arguably the floor - like any conventional 'loft'.

 

Lift the floorboards, put 220 between joists (with lambda value 0.044W/mK that would give you an R value of 5m2K/W) and sit a layer of 25mm PIR (Kingspan/Celotex-type board) above. That will give you a U value of 0.1629 without the 'base case R value' (which is 1/an assumed U value of 2, so 0.5). Add the 'base case R and you get 0.15. While laying all this 'engineer' the ventilation so it all comes in the eaves and over the insulation and the job is a good one.

 

For extra peace of mind lay a vapour control layer (Pro Clima Intello Plus is pricey but good) between and up and over all the joists in 'hammocks', and tape all joints and perimeters to keep water vapour out of the void space (and get a really well-sealing hatch too).

[Mr Punter]

The way to work this roughly would be assume 20% of your heat losses are through the roof and assume your improvements could halve this, so take 10% of your annual heating bill of, say, £2,000 and with a 15 year payback this would allow capital expenditure of £3,000.

 

This is the minimum BC would be looking at for part L improvements.  They may allow you to make the improvements elsewhere if they are not possible in the room you are working on.

[Redbeard]

Clarifications:

 

1. I should have said 'replace the floorboards' after adding the layer of PIR.

 

2. The VCL goes in 'hammock-wise' first, and the insulation goes over that - the mineral wool within the hammocks and the PIR on top. 

Edited 3 hours ago by Redbeard

[Redbeard]

No-one (except me!) has yet picked me up on the fact that using PIR on the top would in effect act as a 2nd VCL where you do not want a VCL. If you substitute 30mm rigid (breathable)wood-fibre you'd get 0.1609W/m2K. I don't think BCO would quibble.

 

I have no idea of the size of the job but it may be that if you took it outside the roofing job it may drop in price. How many m2 are we talking about? Possibly the trickiest bit of all will be removing the floorboards without excessive damage, but it can be done by punching the nails through the boards before you lift them. I did that on a 32m2 floor. We allowed for 20% replacement boards and got away with <5%.