Dew Points and Insulation

[malcolm]

I've been reading up on the internet (perhaps not the best place) and in books (again the ones I looked at were not the right place) and am becoming more and more confused.

It seems that if you add 60mm of nice breathable wood fibre board insulation internally to your lime mortared brick wall the dew point will fall within the insulation and that is considered bad. Mould will grow and everything will get soggy and rot (even if done properly with lots of lime and no air gaps).

But looking at dew point tables if you don't add any insulation at all the dew point would still be within the brick wall. That doesn't seem to have caused any problems for walls that have been here for the last 400 years (though perhaps toasty warm inside for only the last 40 years).

Current thinking proposes impermeable insulation or vapour barriers but I'm not convinced as the temperature difference is the other way around in the summer and a vapour barrier would cause a problem. I haven't found any hand calculations to help which makes it seem like a witch's brew.

The most sensible opinion is the one I have found on this forum - if the room is not damp then breathable insulation is probably OK so long as air leakage to behind the insulation is minimised. A vapour barrier is not required - the wall will settle down and do it's own thing with excess moisture able to escape. That makes sense to me based on what I have seen here - the only part of timber frame I had that remained beetle free behind the nasty external concrete render (since removed) was a perfectly preserved wall plate which was able to send moisture a foot back into the internal wall. Wood fibre insulation is a bit like wood so surely it would do the same thing? I think current thinking in insulation underestimates breathability whatever the temperature gradient.

I'm going on a bit, but I'm going to have to take the plunge this year and fit all the insulation so I can make building regs go away. Does the last paragraph sound sensible?

 

[Zebra]

I have heard it said on the Green Building Forum, make of that what you will, that there has never been a case recorded of building decay from interstitial condensation due to diffusion from the inside, only ever through air leakage. Diffusion through the building fabric will only ever happen to a very limited degree. So that places a very high emphasis on air tightness, and not vapour impermeability.
Yes there will be a dewpoint within the wall, and the use of internal insulation brings that dewpoint inwards. But the amounts are very small if there is no air leakage to within the wall.
I've got woodfibre used as external wall insulation in the bedroom, and as internal insulation on the back wall downstairs, where it borders on the churchyard. There are no impermeable vapour barriers in either case. The wall is fully breathable, but I've paid a lot of attention to airtightness, sealing up gaps very carefully with silicone where necessary - or with filler made from woodfibre fluff and lime putty, or woodfibre fluff and PVA - even backs of socket boxes and joints in woodwork.
Vapour barriers are only effective if totally without holes or gaps, difficult to achieve when someone may come along in the future and cut holes for extra sockets etc. Once this happens, they will probably make the situation worse, depending on the breathability of the external face of the wall.

 

[malcolm]

Thanks Zebra, I think what you have done must be the lowest risk strategy. I plan to follow your lead and hope for the best.

I am a bit nervous that retrofitted internal insulation is a very recent thing and little will have been ripped out yet. I doubt there is much real life experimental evidence yet one way or the other. I do agree about the importance of air tightness - the cold face of insulation sucks water towards it at a surprising rate (causing rain in the mornings in a previous garage).

I feel the mainstream thinking might be wrong. It seems that mainstream thinking is still to use concrete render instead of lime, and waterproof paint instead of lime with the idea it will keep the water out. A mainstream that gets damp in walls wrong even before insulation doesn't inspire me much. Reliance on vapour barriers seems similar to reliance on waterproof paint. Neither will continue to work perfectly for any length of time.

The tricky walls I have are in the modern flat roof extension. There is a block and brick cavity wall where I don't want to fill the cavity because I don't see why it wouldn't become damp. I'm thinking about treating it as a single skin wall, insulating the inside and venting the cavity to outside.

The other I'm stuck with is a corridor with a solid brick wall and concrete render on the outside. I think the only reason I'm stuck is the wall would need to be rebuilt if I build my new pitched roof on top. External insulation would require a new roof. The room isn't heated so a bit of breathable insulation inside probably won't do much harm.

I notice some of the woodwool board companies supply expensive plasters. I still have about 1.5 tonnes of rendering sand bought in error so I'm thinking about making up my own plaster mix. To avoid cracking what did you use to join the woodwool boards, and did you use any kind of mesh in the first coat of plaster?

 

[88v8]

How durable, I wonder, is the air tightness? Despite the highest level of care, will not normal thermal movement, and humidity-driven expansion, inevitably lead to a rising level of leaks? After all, we know how hard it is to seal even something relatively accessible, flat, and solid, for example secondary d-g.

If you have suspended floors then there is a huge air path at the perimiter of the floors where the boards abut the wall beneath the skirting.

I suppose it would be difficult to insulate the walls in such a way that you could remove it again when B Regs have finished poking around?

Ivor

 

[malcolm]

I've been wondering how to treat the floor interface. The floors are concrete with no damp membrane so my idea was to leave an inch or two gap at the bottom of the board and cover it with skirting board to avoid water wicking up from the floor. Might be better to sit it on the floor on top of some plastic to avoid the cold surface that could suck water in from the air.

It isn't straightforward to remove insulation. All the carpets would end up with a 3 inch gap around the edges and kitchen cupboards would be in the wrong place. I quite like the idea of insulation - increasing the loft insulation to 300mm has made a huge difference to comfort upstairs and will likely reduce bills a lot. I think secondary glazing on the huge leaded window 6 feet from the sofa will make a big difference too. The wall insulation will make less difference and seems the most technically challenging.

Another thing I am unsure about - is it possible to hang pictures on lime plastered wood fibre board?

 

[88v8]

Pictures?
I would think so. Picture mouldings secured with cavity fixings.

Ivor

 

[chuckey]

In a heated room, the air is always hotter at ceiling level then at floor (UHF excluded), so it contains more water so you would expect more condensation at ceiling level. the exception are the corners as the air flow would be minimal here. A wall built off the ground without a thermal break would be at ground temperature at its base.
Frank

 

[malcolm]

That's true Frank. Complicated this stuff!

The PIR manufacturers seem to suggest sitting the insulation onto 5mm plastic spacers for the thermal and moisture break and covering with skirting board for air tightness. I'll check with the wood fibreboard suppliers.

I like the idea of picture rails. I'm sure the house would have had them at some point in it's evolution and they are very useful.

 

[Zebra]

I still have about 1.5 tonnes of rendering sand bought in error so I'm thinking about making up my own plaster mix. To avoid cracking what did you use to join the woodwool boards, and did you use any kind of mesh in the first coat of plaster?

Externally I used the expensive stuff that the insulation companies supplied, but indoors I've made my own - 3:1 plastering sand : lime putty.
Woodwool boards I sealed between them with gripfill. The entire kitchen/hall ceiling is lime plaster on woodwool boards, and it hasn't cracked. On woodwool I did not use mesh (and I only did one coat of plaster!)
Woodfibre boards I sealed between them with a mixture of the fluff that you make when you cut the boards, and lime putty. It dries very strong. On woodfibre yes I did use mesh, but I think an alternative would be to use synthetic hair, or lime hemp plaster. Woodfibre boards are quite soft, and so the plaster might be too weak without.

Another thing I am unsure about - is it possible to hang pictures on lime plastered wood fibre board?

Well, it certainly isn't very strong. A picture would probably be OK, but where I've put light fittings up I've got the fixings right through to the brick underneath.

 

[malcolm]

Thanks, I continually mix up wood wool and wood fibre. I don't know why they don't change the name of one of them to something more memorable. Geoffrey board would be an excellent name.

It is the wood fibre board I'm thinking about for inside. I'll go for the fibre mesh in the plaster otherwise the plaster is sure to crack at the joints. It looks like square edge is going to be a great deal cheaper than tongue and groove. Ty Mawr suggested using a breathable adhesive layer which would help keep it on the wall.

I'm getting there after slowing to a dead halt over the last couple of weeks having not figured what I'm going to do next. Wall insulation and secondary glazing should be close to being figured out now. Just some replacement windows, a fire door strategy, and a new kitchen to figure out now.

 

[Zebra]

Ty Mawr also sell some big plastic washers for securing the insulation. Of course the screws form a cold bridge, and for the external stuff there were some "Eejit" washers (their marketing department didn't do its research!) into which you could insert a small polystyrene plug after the screw, to prevent the cold bridging, but I didn't have any left for indoors so I didn't worry too much about that.
For wood fibre you do need mesh, trowelled into the first coat while it's still wet. But it's probably also a good idea, if the boards aren't tongue and groove, to glue the edges together somehow. It wouldn't have to be breathable as it's such a small area.

 

[malcolm]

Where did you get yours? The world of wood fibre board seems to have become very complex with an expensive solution for just about everything you could possibly want to do. No wonder people just use Celotex. Tongue and groove would probably be better than square edge though it seems to more than double the price.

I should get on the phone when the suppliers get back from holiday.

 

[Zebra]

Here: http://www.lime-green.co.uk/, but I understand that the product I used, now what was it called........ Steico Protect!..... is no longer available in the UK.
That was externally though. Internally I used Ty Mawr's Homatherm, which is much softer and lighter. It looks like their range is even bigger now than it was then, but I'm pretty sure it was their ID-Q11, 40mm. Perhaps it wasn't tongue and grooved, I can't really remember. Square edge should be fine as long as you can secure them well.

 

[malcolm]

Excellent. I shall shop around and order something. I'm looking forward to getting on with the wall insulation. I should imagine it will be much more pleasant than insulating the very awkward crawl space in the roof beside the attic. :evil:

 

[formerly known as]

I am very very very dubious about any dew point modelling, more specially that from insulation manufacturers. 20th century intervention, however, has informed me that permeability is good and that impermeability is bad - full stop. All of the faults that I have seen with historic buildings - with regards to dampness and other associated mechanisms of decay - have been due to the use of impermeable materials or inadequate disposal of both roof and ground water. Dampness via capillary action (no such thing as rising damp), interstitial condensation and surface condensation all result from this.

 

[formerly known as]

I forgot to mention that I went through the whole 'permeable insulation' thing almost a year ago for the current house. I was going to go down the shuttered hempcrete route but I was appalled at the costs, so I opted for using light clay straw. At that time, I also researched the use of wood wool, although I was familiar with heraklith, which, until recent years, was ''the'' woodwool board. I have always been slightly sceptical about woodfibre glued together with all sorts of stuff (including cement), being permeable, or permeable enough - despite what the manufacturer might claim - for traditional solid walling. Consequently, I picked the phone up and discussed this with the suppliers and the general concensus was that the wood wool board would be fine in a ''newbuild 'eco' project to control condensation'but less so in a traditional situation'' - one supplier actually said that ''there are really 2 different sorts of permeability - one for new build and one for old buildings'' ..... ummmmm. Hence my comment above.

 

[malcolm]

I'm looking at using wood fibre board insulation which should be breathable enough.

Interestingly a lot of the justification for internal wall insulation seems to be that the accepted U value for a dry solid brick wall is about 3.5 which is simply incorrect. Measured data (English Heritage) suggests the U value is around 0.75. 60mm of wood fibre board has a U value of about 0.7 so the heat going through the wall will only be halved and fortunately any dew point will be much closer to the outside of the wall than anyone would predict.

If the outside of the wall becomes damp then the dew point will move inwards and the insulation would turn into a soggy mess and fall off, thus sorting the problem, but hopefully that will be less of a risk in sunny Bedfordshire than further west.

 

[TheRoost]

That is interesting to know Malcolm, the other night I was sanding some of our walls (9" brick, plasterboard dot and dab )and I realised that they weren't noticeably colder to the touch than a modern house insulated cavity wall. This is with minimal/no heating on, and there wasn't much difference between the temperature of the external walls, internal brick walls and chimney breasts. It does seem like a bit of a con.

 

[formerly known as]

My construction is complex - outward leaning historic timber frame with internal structural timber frame which supports the former. panel infil and void infil is light clay straw. outside face is sweet chestnut on batten over lime/straw/sand render on historic frame. internal face lime/straw/sand plasteer on timber lath - and has u value of .5

 

[malcolm]

It can seem a bit of a con. The official calculation I have (SAP) missed the mark a bit - their heating bill prediction for after I have insulated the house to death is 50% more than the actual bill before insulation.

I've written my own calculator in Excel working from first principles with the odd bit of guesswork and it seems to predict energy usage and the effect of changes quite well. Before insulation and ignoring air leakage and ventilation I reckon the breakdown of heat loss was 44% attic and loft (very poorly insulated), 28% brick walls, 13% single glazed windows, 15% floor slab. After insulating the attic and loft heat loss ought to be 10% attic, 41% walls, 22% windows, 25% floor slab.

The walls would be a major path for heat loss because while the U value for a wall is 4 or 5 times better than a single glazed window the area of walls tends to be much higher than the area of windows.