Woodburner Install: Plastered Breathable Insulation Boards

[aockenden]

I’ve just had a HETAS installer come look at my woodburner-install job. The woodburner will sit on a raised hearth platform in a corner of the room, so it has a wall flanking either side. Both walls have been covered with cork insulation boards.

One of the cork-boarded walls was plastered over when he came to visit, but the other wall had the cork board still exposed, unplastered. He saw the cork and has decided that even AFTER it’s been plastered with 20mm of lime plaster it’s somehow going to burst into flames being 4-6 inches away from the nearest part of the woodburner. Because of that, he is going to insist that I tile behind the woodburner which is just another job I don’t need. How a layer of 5mm thick tile is supposed to stop the cork from bursting into flames but the 20mm of lime plaster WON’T is absolutely beyond me, but there ya go. Tradie logic I guess.

With all the wood wool and wood fibre boards people on this forum are installing along with plenty of wood burners in these old houses, I figure somebody must have dealt with this issue in the past? How did you get around the issue?

 

[paulc]

One of the cork-boarded walls was plastered over when he came to visit, but the other wall had the cork board still exposed, unplastered. He saw the cork and has decided that even AFTER it’s been plastered with 20mm of lime plaster it’s somehow going to burst into flames being 4-6 inches away from the nearest part of the woodburner. Because of that, he is going to insist that I tile behind the woodburner which is just another job I don’t need.

Would he accept cement board or Vitcas construction board (other products available) rather than tiles ?
Whack something up that will satisfy his requirements, and once the work has been signed off, rip out and do your own thing.

 

[aockenden]

Would he accept cement board or Vitcas construction board (other products available) rather than tiles ?
Whack something up that will satisfy his requirements, and once the work has been signed off, rip out and do your own thing.

Actually yes, cement board was the first thing he mentioned before tiles but he also babbled something about needing an air gap behind the cement board or something. Any idea what that would be?

 

[Cubist]

I'm afraid you got caught out there by the Jobsworth Bureaucracy. That engineer made his judgement only at what he could see rather than what you told him. Sadly now he also knows, or will in the future think he knows, what lies behind the wall below the window too and is unlikely to change his position. A simpler and easier fix may be to attach a couple of sheets of brushed aluminium or stainless steel to the walls behind the stove to operate as a heat shield, These could be screwed to the wall and removed afterwards if you were so minded.

BTW - I'm assuming the stoves for use in the main house rather than that workshop bolt-hole of yours otherwise I'd be asking why you even need a Hetas Engineer. Surely all the guys going to do is to install the flue?

Something over 15 years ago I helped the father outlaw to install a dinky little French wood-stove in his new so called log cabin shed. After realising just how much heat that tiny thing was throwing our and seeing the slight discolouration that heralded charring of the timbers 6 inches away a steel panel was quickly installed with an small air gap behind it to obviate any risk.

 

[Cubist]

The air gap behind any material would operate to vastly reduce the transfer temperature.

 

[aockenden]

The air gap behind any material would operate to vastly reduce the transfer temperature.

Ok sure I get that, but why would it be ok for the tiles to be directly fixed to the plaster, but the cement board needs an air gap?

 

[aockenden]

I'm afraid you got caught out there by the Jobsworth Bureaucracy. That engineer made his judgement only at what he could see rather than what you told him. Sadly now he also knows, or will in the future think he knows, what lies behind the wall below the window too and is unlikely to change his position. A simpler and easier fix may be to attach a couple of sheets of brushed aluminium or stainless steel to the walls behind the stove to operate as a heat shield, These could be screwed to the wall and removed afterwards if you were so minded.

BTW - I'm assuming the stoves for use in the main house rather than that workshop bolt-hole of yours otherwise I'd be asking why you even need a Hetas Engineer. Surely all the guys going to do is to install the flue?

Something over 15 years ago I helped the father outlaw to install a dinky little French wood-stove in his new so called log cabin shed. After realising just how much heat that tiny thing was throwing our and seeing the slight discolouration that heralded charring of the timbers 6 inches away a steel panel was quickly installed with an small air gap behind it to obviate any risk.

Yeah he asked me if there was similar cork behind the plaster on the window wall and I told him the truth (yes).

Actually yes this is in the work office bolthole haha. Truthfully I’m afraid to do it myself. I fear the unknown unknowns and if I bodge it I’m worried I’ll die. Let’s say I did do it myself though, don’t I still need building control to sign off on it? And won’t they apply the same set of bureaucratic standards that this guy is applying?

So given your experience with that shed, would you say you share the HETAS guy’s concerns about the cork boards 4-6 inches away from the stove itself? Keep in mind the actual flue pipe will be twin-walled and insulated.

 

[Cubist]

Ceramic tiles are much denser and harder and will also reflect a lot of the direct heat. That density/hardness also slows the rate at which they dissipate accumulated heat to other materials they are in direct contact with. No idea why cement boards would be an issue unless they are paper coated?

 

[Cubist]

Actually yes this is in the work office bolthole haha. Truthfully I’m afraid to do it myself. I fear the unknown unknowns and if I bodge it I’m worried I’ll die. Let’s say I did do it myself though, don’t I still need building control to sign off on it?

OK, well given my perceptions of the size of that space I would have suggested the greatest risk would be that you burn your arse every time you turn around... but yes carbon monoxide poisoning is a hazard.... and maybe oxygen starvation could become a remote possibility too.

The technical work is reasonably straightforward and consists mostly of drilling a few holes to accommodate the screws for bracket fixings and of course the big hole(s) for the flues exit from the building. I'm hoping you weren't planning for the flue to exit straight up through the roof as that could be more problematic than the route I would suggest. Have you already bought the flu pipes and considered your alternatives?

If it had been a 'temporary' structure then no BC would have been required but as its permanent by BC definition that begs a question - was the structure already there when you bought the property? Also, have you significantly changed, or with to change, its use?

 

[fernicarry]

How about a couple of pieces of toughened glass behind the stove? Less visually intrusive and maybe necessary once you start stacking piles of manky logs around the stove.

I really don't think there's much science to the fitting of these things. All the HETAS guy will care about is whether it draws properly on the day and that you have a CO detector stuck to the wall. If it was the main house you'd be flirting with invalidating your insurance by not getting sign off. For an out building that might be less of a concern as most policies offer very limited cover for out buildings anyway.

 

[aockenden]

Ok the other thing is his quote just came through, 1,985 quid just for all the flue pipe parts and the labour of hooking it up to my existing woodburner and the testing / HETAS sign off. That’s with me cutting the hole through the wall for the pipe myself as well! That seems exorbitant to me?

 

[fernicarry]

The liner is quite expensive but I don't imagine you'll need that much of it. The guy who looked at my new fireplace quoted over 5 grand so supply and fit a small stove.

 

[Cubist]

All the kit and consumables you would need for the job are readily available on-line or through local stockists. The bits aren't cheap but for what I think you might need I would suggest a retail cost, inclusive of VAT would be around £500 at the worst. Your man seems to want £1500 for turning up, drilling a few screw holes and fitting a few brackets.

Assuming that the shed was already in place when you bought the place and you have not changed or intend to markedly change its use then I don't see why you might need BC approvals at all. On the other hand if its a new structure there may be a need for a chat with the BC team.

 

[Glenn586]

Do it all yourself and then have it signed off. There are actually half a dozen bodies that can sign off on this of which HETAS are one of. You could then get it plastered install and then have someone other than the HETAS engineer sign it off without his opinion on the cork combusting.

 

[aockenden]

All the kit and consumables you would need for the job are readily available on-line or through local stockists. The bits aren't cheap but for what I think you might need I would suggest a retail cost, inclusive of VAT would be around £500 at the worst. Your man seems to want £1500 for turning up, drilling a few screw holes and fitting a few brackets.

Assuming that the shed was already in place when you bought the place and you have not changed or intend to markedly change its use then I don't see why you might need BC approvals at all. On the other hand if its a new structure there may be a need for a chat with the BC team.

Yeah exactly my thinking on the cost. Seems crazy. It’s going through the wall and dodging around the overhang as well so it doesn’t even need any roof work / slate work / flashing stuff.

So the building did exist when we bought the place but it was derelict and the roof was half caved in. The use has definitely changed. It looks like it used to be a combined toilet and laundry room and now it’s going to be a home office so sounds like building control would be needed?

 

[a twig]

At the risk of being a contrarian - at its most basic you are installing these facilities because you want to regularly light fires within a building...

Personally I’d just put whatever did the job (tiles/board whatever) and was visually the least displeasing for me and move on.

Maybe it is belt and braces, maybe he was talking twaddle but for less than £100 of materials and max a couple hours of your time is it worth arguing about?

If the whole thing is twin wall like you say rather than just the internal section that’s probably not an outrageous quote these days

 

[aockenden]

Do it all yourself and then have it signed off. There are actually half a dozen bodies that can sign off on this of which HETAS are one of. You could then get it plastered install and then have someone other than the HETAS engineer sign it off without his opinion on the cork combusting.

Hmmm ok and they’ll do a basic safety check on the install of the pipes / woodburner itself. But they won’t ask what’s behind my finished plaster?

 

[a twig]

Yeah exactly my thinking on the cost. Seems crazy. It’s going through the wall and dodging around the overhang as well so it doesn’t even need any roof work / slate work / flashing stuff.

So the building did exist when we bought the place but it was derelict and the roof was half caved in. The use has definitely changed. It looks like it used to be a combined toilet and laundry room and now it’s going to be a home office so sounds like building control would be needed?

To misquote Dad’s army, don’t tell BC your name Pike!

 

[Cubist]

Based on what @Glenn586 has said regarding HETAS certification AND that there was a building there AND you have not converted it for use as accommodation then I would just get on with it and not, as @a twig says don't tell em Pike.

On a technical note I'm guessing that the flue pipes needed are 6inch. I've inserted a link below to an on-line provider to give you an idea of the components you may need but would suggest you plan on the basis of where you would ideally place the final exit through the wall and check to see how many stand-off brackets you may need to support the external pipe TO the desired height above the buildings ridge line.

Stainless Steel Twin Wall Flue Pipes - Machine Mart

Top quality stainless steel high temperature insulated twin wall flue pipes.

www.machinemart.co.uk

 

[Flyfisher]

I built a shepherds hut some years ago and installed a 'Hobbit' wood burner, about the smallest available. The entire structure of the hut is wood (except for the outside corrugated iron sheeting) so I was careful to follow the installation guidance even though it has not been 'signed off'. I lined the corner area around the stove with 12mm cement board. One layer fixed to the timber frame then a second layer over the top spaced by using small pieces of the same cement board as 'thick washers' around the fixing screws to create a 12mm air gap. The stove sits on a plinth constructed from 18mm ply that butts up to the outer layer of cement board such that the 12mm air gap is open at the bottom to allow air to rise behind it. Similarly, the gap at the top of the cement boards was only partially covered, to allow air to flow behind the gap between the boards. I then tiled the plinth to form a hearth and tiled the cement boards. The wall tiles extended above the single skin flue from the stove to just above where it met the double-skinned flue that extended through the roof of the hut..

When the stove is lit and burning well, the hut quickly reaches a temperature in the high 20s, even in winter making it really very cosy. The wall tiles around the stove, however, become too hot to touch, which concerned me at first so I used a temperature probe inserted down the air gap between the cement boards to measure the temperature and it was between 30-40c at the hottest point so I stopped worrying.

The hut has been used many times as a spare bedroom thoughout the year whenever we have too many house guests without problems (usually with the hut being over-subscribed with eager volunteers!). Even the CO monitor/alarm has never gone off.

All of which is a long-winded way of saying don't underestimate how hot even non-combustible materials can become in the vicinity of a roaring stove and also bear in mind how that heat will be conducted to the inside of whatever non-combustible material you're using. Unless you have something akin to the Space Shuttle thermal tiles, I reckon an air gap is fairly essential if you need to protect combustible materials from excessive heat and preferably arranged to allow air to flow between the gaps by convection.