Insulation Advice

[malcolm]

I need to make a decision about insulation and I can't decide what to do with the north gable wall in the photo below.

The construction is interesting. You can see the impression of the original 17th century timber framed building. The timber has been infilled with 9 inch brick up to first floor level, then infilled with 4 inch brick to the roof and rendered. The 19th century extension is in 9 inch solid brick and is not tied to the timber frame. There is plenty of damp in the wall which will hopefully be improved by lowering the ground level, removing the failing paint, and re-pointing.

I want to do what is best for the building. Although building regs want me to propose some upgrade to the insulation because the house is going through a change of use and because I want to strip modern plaster from the kitchen (in the extended part).

One option is to add internal insulation to the kitchen (breathable of course). Possibly wood fibreboard with lime render. My concern with that is a likely increase in wall damp. I can probably negotiate a lower u value than building regs would normally want.

Another option is to add external insulation to the whole gable. Again wood fibreboard and lime render. The advantage would be the timber frame stays warm and hopefully less damp. The disadvantage is the roof tiles are not secure enough to be extended so I'd be looking at a new roof as well. Also this would remove some features from the gable.

A third option is to leave the whole thing alone and argue with building regs.

What would you do with this wall?

 

[Roger H]

Hi Malcolm.

Good question. Is there any local precedent for tile hanging or weatherboarding? If so you could insulate externally and breathably and form a confident external rainscreen to protect the lot.

 

[malcolm]

There is some precedent for weatherboarding but building regs doesn't like it because of spread of fire rules. Weatherboarding was considered for a new pitched roof on the opposite gable but it's looking like that will have to be rendered now.

 

[Zebra]

I don't know, that's a really tricky one. I read everything I could on the subject, had the sales reps round from all the breathable insulation companies, had interstitial condensation calculations done, and then sat on the decision for at least 6 months before making up my mind! There are advantages to internal insulation, and advantages to external insulation, as you say. Does the timber frame go all the way down to the ground? (Mine is only timber frame upstairs.) Do you know what condition it's in?

 

[malcolm]

The timber frame sits on a brick plinth. The top of the plinth is currently at exterior ground level and about 12 inches above internal floor level. I plan to reduce external ground level but can't get below internal without a slope towards the house so a drain of some sort is likely. The timber frame posts are still there but the sole plate on that gable has been replaced by brick. The condition of the frame is not as good as I would like. It looks fair from the inside but I'll need to chip off the external render to be sure.

The independent building regs guy I had hoped to use now says he is retiring in the summer so won't take on a job that will take many more years. That's very frustrating as technically a change of use is treated as a new build for insulation levels though there is discretion allowed if doing so will cost way too much or reduce the life of the building. Need to find another building regs guy who won't insist on a bit of cheap dry lining and a bit of concrete as I'm sure that won't work.

 

[malcolm]

I think I'm leaning towards external insulation. The roof tiles will need to be lifted at some point as many of the pegs are missing and the tiles have slipped. The roof frame looks reasonable. It kind of makes sense to externally insulate the roof at the same time as the tiles are re-laid.

The advantage of externally insulating the roof is it ought to give me enough head room to retain a half sensible attic room. The approved plans so far mention the attic retained for storage purposes but with the floor raised by about 18 inches (to allow a void currently occupying a third of the first floor to be returned to bedroom use).

The wall in the picture above is pretty ugly so could be externally insulated at the same time, perhaps retaining the feature of the inset earlier building. The other gable should be insulated enough by the new pitched roof, the front wall is already 18 inches of brick so heat will surely go somewhere else before it tries to get through that, and the rear wall, though only 9 inch brick dating from 1850-1920, has been relatively undisturbed and has some nice features so I'd like to retain that without adding insulation inside or outside.

Apart from the hideous cost does that sound sensible? Detailing is going to be an issue. Some extra bricks building up the the walls that aren't going to be insulated ought to disguise the extra height of roof insulation. For the front the barge board can be a bit higher. The only details difficult to work out are if I insulate the north wall (photo above) it will cause 100mm render to appear outside the revival tudor post on the front, and 100mm render beside the brick on the rear. That will surely allow the building to be read as a misguided attempt to add insulation due to silly stupid regulations in the early 21st century. And I'd like it to look neat.

 

[Roger H]

Not very authentic but I wonder if fibrecement weatherboards would be ok from a distance?
http://www.marleyeternit.co.uk/facades/weatherboard/cedral-weatherboard.aspx

 

[discobarry]

Yes but how would you get in the house Roger H. Lol.

 

[malcolm]

I reckon a roughcast lime render to match the front would be more in-keeping than cement weatherboards. There may have been a render on the gables before it was extended so is probably the most sensitive covering if I have to have a new covering.

Change of use is ridiculous for building regs. The independent building regs guys don't want to touch it. I might try to get that out of the way on a minimal basis in a separate building regs application this year then worry about sensitive insulation later when I can predict things and therefore budget a bit better.

I will of course be in big trouble if that means a kitchen doesn't appear this year.

 

[malcolm]

I have a building regs application in for the change of use and met the inspector today. He has proposed the following:

Insulate all solid brick walls with 50mm insulated plasterboard internally dotted onto existing plaster. (Or equivalent insulation value I assume.) I don't think any of those walls have lathe and plaster so not so bad but I'll lose features in the 'barn'.
Secondary glaze all the single glazed windows
Insulate all the roofs.
Don't insulate any of the timber frame structure (internal could cause damp, external unlikely to be economic and would create conservation area issues)
Don't insulate existing concrete floors apart from the one I'm replacing.
Keep existing 1980s double glazed windows.

It turns out the change of use requirements do require the place to be treated as a new build for insulation where technically feasible and economical. Economical is based on a 15 year payback based on how much heating could cost (6k/year) not how much it does cost (1k/year).

Sounds fairly reasonable. The thickness of insulation on the walls is not enough to make the whole wall damp and mouldy. I assume insulated plasterboard would involve a vapour barrier so I'll need to think about how to achieve the internal insulation without trapping damp. Might look at wood fibre and a lime plastering course. Anyone have better ideas?

Best thing is I can get going. The last year was very frustrating with red tape and not being able to actually start doing anything.

 

[Zebra]

Glad you seem to have found a way forward. You're probably right to be thinking about a vapour barrier if you're having rigid foam insulation internally. I noticed last year at the St Albans Yoga Centre (they do a class for quadrupeds on a Thursday) that they had inserted a non-breathable rigid foam insulation between their rafters, with no vapour barrier underneath. There's no ceiling as such, so you can see the insulation from beneath. I have noticed lately, while trying to achieve the Warrior pose, that there are now brown stains running down the rafters, seeping through from above the insulation, which goes to show that interstitial condesation does happen. I don't like to think of the mess above.
Vapour barriers however are difficult to achieve without being very pernickity. I have put woodfibre external wall insulation on my house, I was redoing the roof anyway, and the windows, and the render was horrid and non-original. With that you have to have an internal vapour control layer (not a vapour barrier, which I understand is generally meant to denote something non-breathable, like polythene, but a vapour control layer, which stops air flow but is still breathable. So I understand.) I used OSB sheets, taped together at the joins with Tescon tape (special stuff sold for that very purpose). But of course there are holes in it, where sockets and switches are cut; I've actually filled the backs of the boxes with silicone sealant. And have sealed any gaps and joins, so that the room is totally airtight. Breathable, but airtight.
Of course yours wouldn't need to be breathable if you were using insulated plasterboard, but the airtightness would then become even more important, to ensure that the warm moist air doesn't get through any tiny gaps into the wall and condense behind your insulation. I found to my cost that warm moist air does get through even the tiniest of gaps and condense where you don't want it to if you're not extremely pernickity with the airtightness. I had to take the whole ceiling down under my new flat roof because it went mouldy above.
Of course, building control would recommend insulated plasterboard, that's the standard stuff that everybody uses, but there are breathable alternatives, as I think you have mentioned. But what you want, as I understand it, is to have the most breathable layer on the outside, becoming less breathable towards the inside. Not the other way around. So it depends what kind of render you have as to whether breathable insulation is an option, or indeed a good idea.

 

[Zebra]

PS I've found Building Control to be rather forgiving when they see what kind of a building it is, and are happy that I'm doing the best I can. They've been quite happy to leave me to my own devices over a few things, and weren't as obstructive as I had feared they would be. They seem to choose not to look into something too deeply if they're not looking for trouble!

 

[malcolm]

This is all very complicated. I'm uncomfortable about using anything that needs a vapour barrier. This house has been knocked around so much over the last 350 years that it is difficult to believe everyone will be really careful in the future and not penetrate the vapour barrier. In fact I carried out a scientific study of my external walls and found someone drills a hole in every wall every 2 years. I want to design for another 350 years of life rather than the normal 30 years.

Most of the breathable insulation systems consist of:
| brick wall | lime plaster | breathable insulation board | lime plaster | breathable paint

The barn was refurbished fairly recently and I don't really want to knock the gypsum plaster off. Most of all I don't want to disturb the leaning wall any more than I have to. That means I'll likely end up with:
| brick wall |gypsum plaster | emulsion paint | some air gaps | breathable insulation board | lime plaster | breathable paint

While it might not be perfect I can't see why that would give me interstitial condensation trouble. All of the walls I need to insulate are exposed brick on the outside (or will be). From outside things look similar apart from the internal face of the wall might be cooler so there is less heat to push moisture out of the bricks. From the inside the breathable insulation is hygroscopic and should soak up any moisture and slowly release it back into the room.

I'll phone Ty Mawr tomorrow to seek their opinion. I can ask them to do some interstitial condensation calcs unless anyone can recommend an independent. Can anyone see any big flaw in my argument? Here's a photo of the barn. Kitchen and gents are the other areas that will need wall insulation to satisfy building regs but neither are as photogenic.

 

[Zebra]

I have the first system you describe in my downstairs, solid brick room - internal breathable insulation, but with a levelling coat of lime plaster underneath to avoid any gaps. You already realise the risk with any internal wall insulation system - that the wall will be colder than it was before and therefore any moisture from indoors will settle there rather than keep moving through. I decided to take this risk, but then I'm not using a great thickness of internal insulation, only enough to take the chill off the walls, not enough to bring up to Building Regs standard (as I said, this is one of the areas where Building Control has rather left me alone).
The only flaws I see in your plan are: the air gaps, as this would seem to encourage mould growth, but I'm not sure about this, it would be a good idea to ask Ty-Mawr; and the idea that hygroscopic insulation will release moisture back into the room. I don't know enough to get too technical about this, and it would be wise to check with someone more expert, but I reckon that this would only be the case if the relative humidity inside the room dropped to below the relative humidity of outdoors, which would not be very often.
Edit: I may be wrong about that last bit, and hopefully someone more senior will take the reins soon.......

 

[dans]

We have a 17th C timber framed house and went for the unbreathable (shock horror!!) option. Many people seem to get obsessed with only using traditional breathable methods. Understandably at times, considering the monstrosities carried out during the 20th C. But, with a bit of thought, common sense and a helpful adviser (Middi), modern methods can be safely utilised to very good effect.

Eventually, we went for a rigid insulation Celetex board. From the outside in:

- Lime plaster
- stainless steel mesh
-breathable membrane
-batons
- celetex (joints aluminium taped)
-12" external grade ply
Directly on original oak studs.

I've got some drawings too if you send me a private message.

Good luck with whatever you do!

 

[malcolm]

I gave Ty Mawr a call and they were very helpful. They suggested the issue with leaving the Gypsum plaster in place is not breathability as such, but that it tends to degrade when it gets damp which would leave the insulation without support. We discussed ways of removing the gypsum and replacing it with lime but it feels like a big job.

The plaster looked remarkably uneven for modern plasterwork so I drilled some holes in the walls to see what it was (behind pictures so my partner doesn't notice). It looks like the pink gypsum is only a 1mm skim on top of much softer, brown and more sandy plaster which I very much hope will turn out to be lime based.

 

[Flyfisher]

I've had some experience of gypsum versus lime plaster when damp . . . well soaking wet really.

Many years ago at our previous 1950s house, a small leak in an upstairs water pipe resulted in the plasterboard ceiling bowing. Realising that there must be water above it, I held a bucket beneath the bulge and poked a screwdriver through the bulge and collected about half a bucket of water, so about one gallon. The plasterboard was sodden and continued to bow before eventually collapsing completely. Fortunately we had the time to lay dustsheets to minimise the damage, but the ceiling was completely ruined by a relatively small amount of water and had to be completely replaced.

A few years ago at our current house we had a similar occurrence due to a frozen pipe pushing apart a joint in the bathroom. When it thawed out water flowed down through the bathroom floor and across sloping ceiling of the room below before exiting in a corner and running down a door frame. It was not noticed for a while and considerably more than a gallon was involved!

But the ceiling involved was lath and lime plaster and it still is because it survived intact. The ceiling paper came off but the plaster itself remained solid and subsequently dried out with no apparent damage. I was very pleasantly surprised to say the least.

 

[Zebra]

Another issue, malcolm, besides breathability, is airtightness. You mentioned above that you don't want to use a vapour barrier because of the risk of it being damaged by any future work to the house, rendering it ineffective.
However, whether you use breathable internal insulation or non-breathable, you still have to have an airtightness layer, whether this layer also acts as a vapour barrier or not.
This is because, now don't quote me on this - it is very much second hand information gleaned from the Green Building Forum (check it out if you want to get really technical about this stuff!) - but it is far less frequent that interstitial condensation would occur due to diffusion of the moisture through the wall material, and far more frequent that it occurs through air leakage into the wall itself, through gaps in the insulation, poorly sealed joins, even through the backs of sockets etc. The importance of airtightness can hardly be stressed enough if you are installing an internal layer of insulation, otherwise the draught will steal away all your lovely warmth, which may then condense itself where you can't see it. You still have a timber frame inside your wall construction if I remember correctly, in which case you very much don't want any of your warm inside air getting to it, so whether you use breathable insulation or non-breathable, unfortunately you still have to think about installing some sort of layer which will stop that, and unfortunately it may be impossible to guard the house against anyone messing with it in future. Which is the problem we all have unfortunatly when trying to incorporate modern materials into these old houses. In the past, airtightness didn't matter very much, the walls were leaky enough anyway that any moisture in the walls evaporated away. Now that we want to seal these houses up, there's the risk of doing more harm than good.
In the timber frame part of my house I've used external wall insulation - a breathable one - and an 11mm OSB sheathing boards inside of the frame acting as an airtightness layer, preventing any warm air getting to where the studs are. And yes, I've filled the backs of the socket boxes with silicone! Will someone do that if they install extra sockets once I've turned up my hooves? Can't say.
I don't know what the answer is in your case, but am glad that Ty Mawr have been helpful, I've always found them to have a very practical approach.

 

[malcolm]

We had a flood here caused by a compression joint popping off just after we moved in. Unfortunately we were out buying curtains at the time. Lifting the floorboards for a couple of weeks had the lathe and lime plaster ceiling as good as new.

Good point about sealing Zebra. I once insulated the inside of underneath of a corrugated roof with celotex and didn't seal the corrugations or vent anywhere. I think as the water condensed on the cold roof above it dried the air out which sucked more moisture in. Had quite a drip from there and much of the celotex fell off.

I've sent off data for the SAP calculation specifying 60mm cork board which puts the U value just higher than the 'Break the Mould' guy worries about. It's all a bit academic because I don't heat that part of the house. I'm not insulating the timber frame part at this stage as it already has mock tudor on both the inside and outside plus some later concrete render to rot out the mock tudor in a period fashion. It wants sorting out before I cause any more problems.

Will need to get around to sorting the exterior ground levels around the kitchen before insulating.