External wall insulation

Wow! What a cracking job John. Glad I didn't have to do that - it would probably have killed me. Thank you.

Yeah, well it seemed simple enough in concept... but turned out to be quite a endurance marathon in the end!

Looks superb, well done. I'll have a proper read later. Initial Qs:

• How much did it/would it cost - say, overall and per m2? £30k overall, if labour is £400/day? A.I. reckons £100 - £200 m2. Scaffolding is a big chunk of the cost - maybe £1000/week for wrap-around scaffold in your case? Almost worth buying?!
• I can't see (from a quick read) how the ground level/below DPC detailing was achieved. Did you go below ground/switch to XPS for example? Or accept a bit of compromise insulation wise at ground level?
• I'm having a bit of an ongoing argument/discussion with a planner friend about planning permission/regs compliance - he seems to think that such work is not within permitted development. I think it is - obviously assuming no conservation and listed type considerations. Also building regs - did that significantly inform your design decisions? I'd assume you self-certify? TBH I'm still confused about the whole regs and retrofit compliance side of things, how regs 'equal' PAS 2030 etc. . . . (!)

I have no idea what a commercial quote would be.

The insulation was £1,250, and the main cladding order with most of the trims etc was about £6,700. When you add in all the other stuff like screws, sand cement, linear drain, timber battens, and a shed load of tapes, sealants, glues etc and a skip or two then you are up to about £10,300.

(I also bought some extra tools - but since I get to keep them, I don't include them in the cost unless they are "consumables" (blades, discs etc))

The scaffolding cost £936 and I kept it for about 4 weeks. So perhaps half as much again if also doing the back. I bought some ordinary building trestles for the easy side - those were about £145 inc delivery. An extra 4 might have been worth it - but the layout of the planks enforced by the windows made it easy enough to do in sections without needing to move the access kit too much.

Thus far I have not changed anything below interior floor level. The underfloor volume is vented (air bricks etc) so I is questionable if there is much additional value to insulating the "skirts" at the side.

I did a bit of research on whether changes to the visual appearance were of consequence. From what I could find, the answer was no, unless in a conservation area. (also the change in appearance is in keeping with the local architectural style, and does not affect the road facing elevation anyway)

I don't have an answer for the more general question...

I have clad farm buildings with feather edge treated timber over rigid foam using *lost tite* screws. Basically a woodscrew with a long plain shank but having retaining rings near the head.

Mostly screwed into timber building frame but also works for brick/block.

With the insulation and first board temporarily located, drill through from the outside using a suitable masonry or conventional drill. Start a fixing plug to the screw and tap into the hole. Use drill driver to seat home and move on to the next fixing. Once the board ends are secure, the whole row of new fixings can be drilled to save swapping drill bits.

I have no idea where this might fit in building regulations:-)>

An interesting method. Something of a possible fire risk for a two storey building

Well planning might have someth9ing to say as it changes the overall appearance. But I think from an engineering perspective, insulation *inside* an e,g. single brick wall is about as good as it gets for preserving thermal mass inside the barrier

I haven't thought the implications for damp through. of course.

Pretty close to the process I was using. I placed the gecko gauges about

My neighbours clad their house. They used masses of rock wool, which is inherently non-combustible. Did you consider that, instead of PIR?

I appreciate that PIR, applied in the way you have, with a cement board cladding is not really a fire risk, and it's a low rise building, but ...

Not really... rockwool would have needed significant extra thickness to achieve similar performance. So with the space available that could knock the u value up from ~0.3 to something like ~0.45.

To be fair, even 0.45 would still be a dramatic performance improvement over the typical 2.1 ish of a rendered solid brick wall, but that is still quite an extension in payback time (although that was not the primary driver).

You would need to use something like Rockwool Rainscreen Duo Slabs, which are significantly more pricey than what you would pay for PIR boards - so that would add to the build cost.

Yup fire risk was definitely a consideration. One of my reasons for rejecting a traditional timber cladding (that and maintenance).

PIR generally ought not sustain a fire - but much depends on how much extra flammable stuff there is around it, and how much chimney effect you get. Hopefully there is not too much scope for ignition in the first place!

Interesting, thanks.

Trestles would safely see to over half the height, saving a fair chunk of the scaffold hire. If storing the trestles (or selling them) isn't a problem, a good investment.

Gotcha.

It was more how PAS/regs/certification etc informed your project. Looks 'right' to me . . .

Yup quite possibly... I could see that would soon double (or more) the price...

They go up to about 1.6m IIRC (although are a bit wobbly - so benefit from some bracing!) - the top of the wall was about 3.4m above the actual ground level at the side (the place has downstairs ceilings over

Approved doc L1[1] has a bit to say on target performance for renovated or upgraded thermal elements of a building. The "improved" target they quote in table 4.3 (col B) is a U value of 0.3, and the lower threshold specified (col A) is 0.7.

However there are several mitigations:

"If achieving the U-value in Table 4.3, column (b) either: a. is not technically or functionally feasible or b. would not achieve a simple payback of 15 years or less then the element should be upgraded to the lowest U-value that both: a. is technically and functionally feasible and b. can achieve a simple payback not exceeding 15 years

Generally, a thermal element once upgraded should not have a U-value greater than 0.7W/(m2·K). A lesser standard for the thermal element may be acceptable where work complies with Part C of the Building Regulations on protection from the harmful effects of interstitial and surface condensation."

(however there are lots of other bits that could also do with improvement, like insulating under the suspended timber floor - I but I will save that for another day!)

PIR only chars at the temperature of a normal domestic fire and does not contribute to the fire.

Grenfell had those composite aluminium+plastic rain screens which burnt at closer to 1000C (apparently) at which temp even XR rated PIR will burn.

The one big problem with rigid board is that you need a perfectly flat surface to start with and they must be clamped tight and all the joints should be taped with aluminium tape to stop any (cold)air leaking behind the PIR

Having de-rendered, is there not a small chance of air leakage along the mortar joints might occur during periods of biting northerly winds ?.

Sometimes a thin layer of something like garden fleece sandwiched between the PIR and the wall prevents this.

Not really true. The board is flexible to accomodate slight curves and minor air gaps only add to the insulation. As long as airflow behind the panel is restricted - use expanding foam to seal? - insulation properties will be conserved.

I think that is partly where the height comes into play - the taller you go, the more chimney effect, and the more aggressive the air flow, allowing for hotter combustion.

The wall is reasonably flat. they PIR boards are over battened at 400mm centres, each batten with a fixing every 400mm vertically - so it should be pulled fairly tightly against it. The window a door reveals are also foamed into place, screwed, then the joints siliconed.

It may not be perfect, but compared to no insulation at all, it is a big s improvement.

not much I would have thought... the wind will be hitting the cladding first. All the joints are foamed and taped, so not much should get through from that side. The bottom of the PIR is sat in an ali tray that is screwed tight to the wall - so again not much scope there.

Can't say the prospect of retro fitting something like that appeals in the slightest! :-)