Flint wall dpc: a question for JM/Woody

[Shytalkz]

Hi Jerry, another pov in the mix is always worth listening to; as with most things, there is no absolute right or wrong and I don't profess to be a damp expert by any means.

Not sure that I understand the energy aspect, though?!

I am intending to have a looky with a damp meter (being mindful as to their limitations), if I had a speedy moisture test kit these days, I'd have a go with that as well!

 

[JerryM]

Not sure that I understand the energy aspect, though?

this is one for the other JM but i put my simplistic understanding. the water in the soil will be moving up the mortar joints on all your walls and evaporating on the surface (breathing). whilst the ventilation sorry for the pun but barn door stays open the evaporation is likely to exceed the amount of water arriving from the soil - so all is ok.

You will want to heat the rooms which will take the water from the wall surface and put it into the room air. this has to be ventilated out of the building (rattley wooden window frames, open chimneys, daylight under the front and back doors and the like) otherwise when the temp lowers it will condense out on the coldest surfaces and create damp. the buildings energy rating will be low as more heat will be needed than would otherwise be the case (fat wallet needed).


Other thoughts:

A flat pad damp meter is what you would need (not pins - for wood).

A further issue is that i don't believe you would be able to use gypson ie std plaster as this will retain what's called hyroscopic salt's (which are moving with the water from the soil). these salts suck up water and will remain damp whilst a supply of water remains (either from the soil or from water vapour caused by living in the house ie cooking).

The thoughts on a dpc for the new wall also does not sit well with me. i only say this as i never get invited to houses that have them.

 

[Shytalkz]

Ah, so what I think you're saying is that, with a dpc of some sort, moisture can't move up the wall through being drawn in by the raising of the ambient temperature; but, without a dpc, more moisture is drawn in, ergo, if this can't evaporate, then it will bring the ambient temperature down when the heating's off, more than would be the case with a dpc and hence, more heating costs?

If so, I'm not overly worried by that aspect: not that I've got long arms and deep pockets, but as I'm using GSH and UF heating, if I have to ratchet it up a notch, then as the energy is "free" (apart from the cost of the heat pump gubbins, naturally), if I use a bit more, it isn't costing me any more per se - and, of course, I am still a tree-hugger

There won't be anywhere where I'll be plastering directly to the existing flint: it will be a combination of keeping some areas as a feature and lining others with cavity, insulation and blockwork over a slab dpm. So, where the wall is being lined with blockwork, then presumably "normal" plasterwork is fine?

Noted re the DM - can they be hired and, if so, from whom?

 

[Joe Malone]

this is one for the other JM but i put my simplistic understanding. the water in the soil will be moving up the mortar joints on all your walls and evaporating on the surface (breathing). whilst the ventilation sorry for the pun but barn door stays open the evaporation is likely to exceed the amount of water arriving from the soil - so all is ok.

The moisture equilibrium that Jerry talks about is a delicate balancing act but porous masonry works on the overcoat principle. Most of the moisture is absorbed on the wall exterior and is evaporated on the wall exterior due to the effects of natural ventilation. I personally don't think that moisture evaporation on the inside of the building has anything but a negligible affect on heating costs. I'm talking about traditional historic buildings here and there's evidence now to show that traditional lime plasters serve a crucial role in acting as a moisture sink for internal humidity levels. The plaster gives up moisture when it's warm and absorbs it when it's cold but generally helps to maintain RH at a constant. If you remove this 'mosture sink' then the role is taken up by the carpets and you have an increase in house dust mites.

You could use the plinth argument as an excuse for not installing a dpc but why bother? I'm not against physical dpc's and would install wherever possible. If you wanted to argue for traditional build then you'd have to use traditional build methods. I'm guessing you're using OPC mortar rather than lime and this doesn't work on the overcoat principle. It's impermeable to moisture but will eventually break down.
I also have an issue with modern bricks. Are they as good as the bricks manufactured 200 years ago? Who knows, but the point is that you simply don't know how they'll perform in comparison.

As an interesting aside, I recently read that in 1997 London Authorities closed announced that they were closing Hammersmith flyover for major structural repairs, it was 30 years old. At the same time they announced that they were also closing Hammersmith Bridge for structural repairs. It was 120 years old and was never designed to take traffic. I think this makes an interesting point about modern build methods and materials. Anyway, I digress...

Glad you found the SPAB site useful.

 

[Shytalkz]

Hi Joe

I see what you're saying re the use of lime plasters. Of course, this being a barn it's never had one and the use of the building will be totally different from what has gone before. Does this have an adverse impact on the likely internal conditions, going the leave it alone approach?

The new-build bit of the barn will be built using Norfolk Red reclaims for the plinths/quoins (not least that, otherwise, my coursing will be out and I don't want marrowfat beds either!) and busted-down flints saved from demolishing another building on the site and I am actively considering using an NHL mix (for the external plinth and flintwork, but not the block backing and inner leaf), seeing as I want to repoint the whole existing barn and get off all the poxy 1:3 mortar that some farmer boy has put on areas of both the flint and bricks in the past. I can't see the point in repointing - the flints especially - using 1:1:6 or 1:2:9, as that's just going to ping off again at some time. Hence, going that route, I am getting closer to a traditional build, albeit that the construction behind the flint/plinth is different. I was also only referring to a dpc in the plinth brickwork, not the new backing/inner leaf, by the way and only from the pov of not wanting to see it, given that there's none in the original work.

Re Hammersmith, civil engineers (apart from Brunel and even he dropped some monumental goolies in his time!): what do they know?! Structural engineers, of course, are a totally different breed .

 

[Joe Malone]

Of course whatever surface finish you use will have an affect on the walls thermal capacity, aside from this, the best internal wall coating you can use to protect against damp is nothing! I have studies that prove this. Leave a wall bare and the increased evaporation levels will reduce the height to which water can rise in a wall. Second choice would be lime plaster (i'd use non hydraulic internally) and worst for causing problems are modern gypsum and cement based plasters. Irrespective of the fact that you never had plaster; if it's constructed in lime mortar then you leave the walls bare or apply a lime finish. Bare walls are very in vogue these days.

 

[Shytalkz]

I've not got any places where I'll be putting a finish directly onto the existing flintwork, I don't think (this is very organic design going on...!), but point noted.

Bare, old walls do look cool, especially in barns, but it's a real hassle with part L requirements nowadays: our local authority BC insist that all the external walls are drylined to bring them up to spec, the philistines. Hence my using a more enlightened private one .

 

[JerryM]

Shytalkz,

apologies for any confusion in my previous post:

with a dpc of some sort, moisture can't move up the wall through being drawn in by the raising of the ambient temperature

what i meant to say was with a plastic dpc it stops the moisture "full stop". the ambient temp is nothing to do with the dpc.

when i referred to dpc i meant new build dpc.

more heating costs?

what i mean here is the equilibrium balance that joe has explained. if water gets into the building then it needs to get out by ventilation (otherwise it will remain and cause damp). this will mean in the winter air that’s been heated by the CH will need to be ventilated (a method of ventilation will need to be provided) and fresh air re heated to maintain warmth in the building. hence higher heating costs.

combination of keeping some areas as a feature and lining others with cavity, insulation and blockwork over a slab dpm

this gives me comfort as hopefully most of the surface area down stairs will be new build and hence water in the flint walls will evaporate to the cavity. hopefully evaporation from the remaining flint walls can be accommodated by the existing ventilation of the building - this is an option to wait and see as given you are not selling on then time is on your side to put right in the future if a need arises.

Noted re the DM - can they be hired and, if so, from whom

for what you need you will be able to hire a meter that will give you an idea of the likely future problem and decide if the wait and see option is likely to be best.