Wall Insulation problem - help

[James Davis]

Hi all,

The extension project rolls on, and has hit a bump in the road.

Around 50% of the fibolite block wall is in place. The builder has today began the process of attaching the 90mm, foil backed insulation boards into the 100mm cavity, and then building up the facing brick. He is using these red circle things on the wall ties to hold the insulation boards against the block.

The problem is, we are supposed to have a 10mm air gap between the insulation board and the outer brick skin. This is apparent in places, but in others is more like 8mm and even 4 or 5mm in others. The boards are obviously very rigid and unforgiving. He has achieved the 100mm cavity at the bottom where the DPC begins, but it seems that any discrepancy in the block walls, whether because of the blocks themselves, slightly imperfect positioning and mortar chunks, push the boards out and there is no flexibility with the boards. The other thing is that these red circles don't hold the boards in as tightly to the block as we might like in order to squeeze every mm out of the cavity. Some questions:

- how important is the 10mm air gap? I don't really care about the U values being perfect (we've done everything otherwise to achieve the required spec), I mainly want a gap so any water ingress can go down the back of the brick and not onto the insulation boards. I'm assuming this extension still ends up being a very warm room whether the air gap is 2mm or 10mm. Am I missing anything here?

- how do other builders get around this, do they get the block work absolutely perfectly square to the single mm and then clean it all off to perfection on the cavity side, or do they just do their best and accept a smaller air gap if necessary?

- it is a shame because knowing what we do now, it would've been nice if the designer factored in an extra 5mm or so into the cavity to make it all a bit more forgiving for the builder. We could take the 90mm insulation back and see if we can get an 80mm one. Trouble is, I don't want thick plaster board on the inside to compensate and taking up valuable space. That said, the building inspectors are hardly likely to find out if we use slimmer boards (as opposed to the 90mm on the plans) as they hardly pay any attention at all when they visit and the next inspection now will be for the roof, and the cavities will be closed off. Thoughts? Or shave off 10mm from what we've bought but lose the foil on one side... (I'm just thinking out loud).

- is there a way to stick the boards to the block so we don't lose space in the cavity using the red circles?

- other option is the builder takes most of the block down again and rebuilds it much more precisely. Prefer to avoid this total waste of time if it isn't absolutely necessary.

Grateful for the usual expertise!

Thanks

 

[James Davis]

I have some answers in case it is of use to others...

- the insulation company (Celotex) explained that their boards work with zero air gap. So can be sandwiched between the two walls no problem. The air gap will help with the U value but not needed because of moisture ingress. Their tests involve firing a hose at a wall for several hours. The outer foil keeps moisture out plus the foam core is closed cell. So basically, the 10mm not essential even though it would mean a very slightly less warm room (and building inspector if they notice).

- the problem for us is that in some places, the gap narrows down to less than the thickness of the red circles (around 8mm). Celotex person said you don't have to use those and wouldn't do if there is no air gap, but you want to keep the boards as close to the block as possible, albeit that a few mm here or there behind the boards won't make much difference.

- anyway, after checking and rechecking different parts of the block wall, which up to 8 courses in places, measuring and using a level, and trying to decide what to do, the builder concluded that he should take all the block down and do it again. Part of the issue was that the block was a little far over in places and he said he's learnt an important lesson about how he measures up and squares things when using these boards (his work has always been with other systems). Hats off to him. Having noticed a problem, it takes a big man to admit an error, figure out what is needed and then go back and do it again properly.

 

[noseall]

Hi all,

The extension project rolls on, and has hit a bump in the road.

Around 50% of the fibolite block wall is in place. The builder has today began the process of attaching the 90mm, foil backed insulation boards into the 100mm cavity, and then building up the facing brick. He is using these red circle things on the wall ties to hold the insulation boards against the block.

The problem is, we are supposed to have a 10mm air gap between the insulation board and the outer brick skin. This is apparent in places, but in others is more like 8mm and even 4 or 5mm in others. The boards are obviously very rigid and unforgiving. He has achieved the 100mm cavity at the bottom where the DPC begins, but it seems that any discrepancy in the block walls, whether because of the blocks themselves, slightly imperfect positioning and mortar chunks, push the boards out and there is no flexibility with the boards. The other thing is that these red circles don't hold the boards in as tightly to the block as we might like in order to squeeze every mm out of the cavity. Some questions:

- how important is the 10mm air gap? I don't really care about the U values being perfect (we've done everything otherwise to achieve the required spec), I mainly want a gap so any water ingress can go down the back of the brick and not onto the insulation boards. I'm assuming this extension still ends up being a very warm room whether the air gap is 2mm or 10mm. Am I missing anything here?

- how do other builders get around this, do they get the block work absolutely perfectly square to the single mm and then clean it all off to perfection on the cavity side, or do they just do their best and accept a smaller air gap if necessary?

- it is a shame because knowing what we do now, it would've been nice if the designer factored in an extra 5mm or so into the cavity to make it all a bit more forgiving for the builder. We could take the 90mm insulation back and see if we can get an 80mm one. Trouble is, I don't want thick plaster board on the inside to compensate and taking up valuable space. That said, the building inspectors are hardly likely to find out if we use slimmer boards (as opposed to the 90mm on the plans) as they hardly pay any attention at all when they visit and the next inspection now will be for the roof, and the cavities will be closed off. Thoughts? Or shave off 10mm from what we've bought but lose the foil on one side... (I'm just thinking out loud).

- is there a way to stick the boards to the block so we don't lose space in the cavity using the red circles?

- other option is the builder takes most of the block down again and rebuilds it much more precisely. Prefer to avoid this total waste of time if it isn't absolutely necessary.

Grateful for the usual expertise!

Thanks

There is no need for an air gap with rebated PIR boards.

The (supposed) 10mm gap is a nonsense thing, merely there to help with small discrepancies.

There should however, be vertical DPC at abutments and corners.

 

[Ivor Windybottom]

It's all fine. A good explanation of how it works here - exactly the same situation with a 10mm nominal gap...

 

[James Davis]

Looks like the same thing happens in the video! He says at the start of the insulation section that they don't have the 10mm gap but that they'll "get it back". They won't get it back unless the boards are off the inner skin and can be pushed in by that amount. The red fasteners are 8mm thick so if you want to use those as you really should if there's an air gap, then you need room for them. I notice at the end he mentions that it is a great system but that the bricky was cursing it to begin with...

 

[noseall]

Looks like the same thing happens in the video! He says at the start of the insulation section that they don't have the 10mm gap but that they'll "get it back". They won't get it back unless the boards are off the inner skin and can be pushed in by that amount. The red fasteners are 8mm thick so if you want to use those as you really should if there's an air gap, then you need room for them. I notice at the end he mentions that it is a great system but that the bricky was cursing it to begin with...

The gap (10mm) in this instance is irrelevant. It's considered full fill. The gap is just for fitting reasons.

 

[James Davis]

The gap (10mm) in this instance is irrelevant. It's considered full fill. The gap is just for fitting reasons.

No, he says on the video that they intend to have the 10mm gap, and it is relevant because it forms part of the U value calculations, as it does in my case. Some insulation requires a 5mm gap when combined with fibolite.

If it is full-fill, then that is fine, you can of course do that. But if the aim is a 10mm gap but you then end up with 6mm (I doubt most of us would care about the effect on U values in this case) you have the problem of using the fixings to hold the insulation to the inner leaf as those are more like 8mm thick.

 

[noseall]

No, he says on the video that they intend to have the 10mm gap, and it is relevant because it forms part of the U value calculations, as it does in my case. Some insulation requires a 5mm gap when combined with fibolite.

Eh? A varying 10mm - 0 gap is nothing more than a discrepancy gap. Construction is imprecise and 10mm barely deals with minor inconsistencies. Your U-values will be based upon the overall wall/cavity/wall thickness and the product. Its why you would say use 90mm thick gear in a 100mm cavity. In other words, no matter how thick the product, the cavity it sits in has to be 10mm wider for fitting reasons alone.

If it is full-fill, then that is fine, you can of course do that.

It is classed as full fill. It's why the boards a rebated. There should be vertical DPC at jambs and corners.

 

[James Davis]

Eh? A varying 10mm - 0 gap is nothing more than a discrepancy gap. Construction is imprecise and 10mm barely deals with minor inconsistencies. Your U-values will be based upon the overall wall/cavity/wall thickness and the product. Its why you would say use 90mm thick gear in a 100mm cavity. In other words, no matter how thick the product, the cavity it sits in has to be 10mm wider for fitting reasons alone.

It is classed as full fill. It's why the boards a rebated. There should be vertical DPC at jambs and corners.

Wrong.

The 10mm air gap is part of the U Value calculation. You can have no air gap at all with the boards we are using and in some ways it would be easier to build that way - e.g. no fasteners required - but the thermal properties wouldn't be as good. A gap also reduces the odds of water penetration although if fitted correctly, this shouldn't be an issue even with no air gap.

 

[noseall]

Wrong.

The 10mm air gap is part of the U Value calculation.

Does it take into account instances of wider masonry, snots or plumb issues? There will never be a constant 10mm gap up a masonry wall ever.

You can have no air gap at all with the boards

No you can't. Its impossible to fit 90mm boards into 90mm cavity or 100mm boards into a 100mm cavity. A starter gap is essential to be able to build the wall.

A gap also reduces the odds of water penetration although if fitted correctly

It makes no odds with rebated boards as even with a 10mm gap (impossible to maintain constantly) the boards will touch either the inner or outer wall. Snots, misshaped masonry etc inevitably make contact.

 

[robinbanks]

If you've found a builder that can build a pair of block leafs true enough that the worst deviation to your 10mm gap you can find is a bit of mortar closing it to 4mm, don't risk upsetting them by raising such a trivial inanity. It's highly likely the next builder you have to get in to finish off, when Internet-forum-driven, armchair-warrior nitpicking has caused them to walk off the job, will be far less precise

A small air gap helps the U value very slightly (and I'm talking in the region of 0.01 here) by allowing the reflective surface to do its low-emissivity job but it doesn't need to be bang on 10mm

PIR boards are closed cell, they don't admit water.

Enhance your calm, as throwing all your theory at what appears to be an adequate builder will neither turn out well, nor achieve any measurable practical improvement in the performance of the building. There will be bigger gains to be made in careful detailing elsewhere, ensuring that insulation layers are contiguous over joints, membranes well installed, the arriving sparky doesn't smash massive holes all over the place etc

 

[James Davis]

If you've found a builder that can build a pair of block leafs true enough that the worst deviation to your 10mm gap you can find is a bit of mortar closing it to 4mm, don't risk upsetting them by raising such a trivial inanity. It's highly likely the next builder you have to get in to finish off, when Internet-forum-driven, armchair-warrior nitpicking has caused them to walk off the job, will be far less precise

A small air gap helps the U value very slightly (and I'm talking in the region of 0.01 here) by allowing the reflective surface to do its low-emissivity job but it doesn't need to be bang on 10mm

PIR boards are closed cell, they don't admit water. Enhance your calm, as throwing all your theory at what appears to be an adequate builder will neither turn out well, nor achieve any measurable practical improvement in the performance of the building. There will be bigger gains to be made in careful detailing elsewhere

Agreed, but you need around 8mm to fit the fasteners into the space. Less than 8mm gap and you can't use the fasteners and the insulation boards are supposed to be held up against the inner leaf, notwithstanding that some very small gaps from imperfections between the two won't matter. Without the fasteners and clean, really straight inner block, these thick rigid boards don't want to sit against the block. It was the builder's decision to do the block again and said he's treating it as a learning experience. In other words, he is a true professional - not the type of weak, lazy vermin that walks off a job.

Really, the people who draw plans should factor in more like a 15mm or 20mm air gap to make life easier for builders who have to use these materials. That's what builders probably ought to consider doing.

 

[noseall]

Agreed, but you need around 8mm to fit the fasteners into the space. Less than 8mm gap and you can't use the fasteners

You don't need fastners.

 

[James Davis]

Does it take into account instances of wider masonry, snots or plumb issues? There will never be a constant 10mm gap up a masonry wall ever.

No you can't. Its impossible to fit 90mm boards into 90mm cavity or 100mm boards into a 100mm cavity. A starter gap is essential to be able to build the wall.

It makes no odds with rebated boards as even with a 10mm gap (impossible to maintain constantly) the boards will touch either the inner or outer wall. Snots, misshaped masonry etc inevitably make contact.

I'm relaying what the technical team at Celotex told me. I don't doubt that it is more complicated on site but that's what they who design and make the materials say. They literally told me you don't need an air gap and that they test such conditions, but that it will diminish the U value. I'm not saying it would be easy to build that way. As mentioned in my previous post, builders would be better off adding on 5 or 10mm so they get the required air gap and space for the fasteners whilst allowing for the normal imperfections. Also, I said an air gap reduces the odds of water penetration, which is clearly true.

 

[James Davis]

You don't need fastners.

Ok so they're being made, and recommended by Celotex, and used by builders for no reason at all? The gap between the boards and inner leaf should be as small as possible, ideally no gap at all (ideally).