barn conversion wall treatment?

[Martin P]

Construction question.....I think this is the forum!

I have a situation where we have 600mm (2 feet) Thick solid stone walls in an old barn, client wants to convert into dwelling. would like a timber frame inside built off new solid concrete floor and if required a cavity to the existing stone walls- ( incidentally this will support roof and first floor, the existing stone walls will become non loadbearing.)

My question is how do you prevent moisture between the cold wet external leaf, and the new insulated timber internal leaf. Cavity vents are not an option as the walls are too thick. I have been advised that you just strap and line it, and pack insulation right up to the back of the stone wall, this doesnt seem right to me, can anyone advise further/better?
TIA
Martin.

[mjfarrell]

I am not an Architect, however I would approach
this with a layer of something like a Mirafi moisture
barrier agaist the stone wall, then insulation.

[Martin P]

I will google that, I imagine it will be similar to a tyvek breather membrane..... . This is the good thing about AUGI :) I wondered how I had missed all your posts then realised you werent a Revit regular, thank you for coming over to reply!

[mjfarrell]

http://www.tcmirafi.com/products/products_index.html

[Wes Macaulay]

Could you vent over the top of the wall? I would recommend at least an inch or two of air space clear between inside face of stone and exterior face of framed wall. If the stone lets much moisture pass inside than you have a possible problem with moisture build-up at the base of the wall inside. Your bottom plate should be treated to prevent rot, and it might be good to provide weep holes through the base of the stone wall. I would want a concrete curb or something too which would prevent any ingress of water from the wall to the interior of the building.

Could you seal the existing concrete slab if you're not replacing it? Apparently concrete slabs allow GALLONS of water vapour to penetrate to the interior!

[Wes Macaulay]

I haven't done one of these for awhile but do a vapour gradient for the wall -- you calculate interior and exterior temperatures and humidities for the location of the building and figure out which way water vapour wants to go at different times of the year. Each material in the assembly has a certain moisture permeability and you can calculate how far water will get driven through the wall. I was shocked when I first did the math for this and saw how much outbound water vapour pressure interior air can provide.

Water vapour pressure is often very high on the inside of the building, even here with our high humidity levels outside in winter. This means wall cavities have to be vented to take away this moisture and prevent rot in the wall.

[blads]

Martin

a technique used often here is actually placing the timber outside the stone walling to create a reverse veneer thereby having the thermal mass on the inside of the building and creating better use of energy efficiency. Providing the soil isnt too reactive, the frame could be supported on stumps, lined with a sarking material like "tyvek" or if you can get it over there "air-cell" (aka astro-foil).

[Martin P]

u vent over the top of the wall bottom plate should be treated to prevent rot, and it might be good to provide weep holes through the base of the stone wall.



That is pretty much what I was thinking - (BTW the concrete slab is going to be new - so will have a damp proof membrane and insulation below now) I like the idea of having a curb, as weepholes will just not be possible due to the wall thickness, I wonder if it may not be too silly an idea to have some benching at the curb/cavity and drain it to a point say near the downpipes and connect to those below ground?

Thank you for the reply Bladams, I can see the logic in that - especially as it will give more useable floor space - but the main point on this job is to keep the stone walling as an exterior feature - although it might be an equally nice interior one...

[adegnan]

Joe Lstiburek at www.buildingscience.com should have some information on this. At one seminar where I saw him, he had done similar on his own home. Membrane on the stone walls with sprayed-foam insulation over that. You could do similar with the framing and spray foam between framing members over the membrane.

[Martin P]

That is a very interesting link, had a quick 20 mins and didnt find anything specific on existing stone walls, will def go back for a second look , cheers for the pointer Abe :)

Sprayed insulation isnt something I have come across before, it would certainly get into all the akward spaces as the walls are very iregular - and no cavity to worry abount venting sounds good too! I will look (google!) into that, if it is a bit specialist we are kind of in the sticks here and it probably wont be easy to get someone local, but it may be there somwhere...

[PeterJ]

Martin

Have a look at the Newton Membrane (http://www.newton-membranes.co.uk/) people. They are UK based on offer all manner of solutions for waterproofing behind existing structures. I have used their Newton 500 system in basements before as a 'soft' tanking system and it worked remarkably well. It gives a water-proofed air gap that water can run down inside so you don't have the water pressure problems that gave you blown render etc. their Newlath with thermal board inside and some kind of sump drain might be all you require.

I get 15% by the way so mention my name;)

On a serious note, a sump will be quite tricky to detail. Can you insert airbricks in the external wall at lesser centres than weepholes would be required, or llok at piping external ventilation from somewhere that a door opening is formed down to foundation level?

[Martin P]

There are some areas where an air brick could be added, at windows the wall reduces in thickness so we could get air bricks there - unforutately most of the windows are full height! so it may even be a consideration to knock out a couple of areas and insert air bricks, and just patch them back up? Or to try and detail some sort of continuos vertical ventillation at the full height windows and doors maybe? Seems like it is maybe just a make do kind of thing, and it is not going to be perfect