Removing Ground Floor Stone/Mortar Wall

Hello,

I am considering thermal insulation for a ground floor room of a 70 year old (approx.) farm house of thick internal stone & mortar construction and wish to minimize thermal bridging and room space reduction simultaneously.

The two small rooms at the gable end have a 12" stone/mortar wall between them with T junctions to the outer gable 22" thick wall, and an approx. 17" thick inner wall (parallel to the gable end) which has two small fireplaces/chimneys, one in each room. The inner double chimney wall passes through the second floor and roof.

The first floor joists pass parallel to the gable end perpendicular to/over the 12" stone mortar wall which stops at the first floor.

Is it likely that I would be able to remove the 12" wall with minimal or no alterations to the overhead joists ?

If I insulate and dryline on either side of the 12" wall I reduce the small room dimensions. However if I replace this wall with a drylined wall I remove some thermal bridging and simultaneously provide 3+" of extra dimension to each of the two small rooms.

Thank you

newstemp said:

Hello,

I am considering thermal insulation for a ground floor room of a 70 year old (approx.) farm house of thick internal stone & mortar construction and wish to minimize thermal bridging and room space reduction simultaneously.

The two small rooms at the gable end have a 12" stone/mortar wall between them with T junctions to the outer gable 22" thick wall, and an approx. 17" thick inner wall (parallel to the gable end) which has two small fireplaces/chimneys, one in each room. The inner double chimney wall passes through the second floor and roof.

The first floor joists pass parallel to the gable end perpendicular to/over the 12" stone mortar wall which stops at the first floor.

Is it likely that I would be able to remove the 12" wall with minimal or no alterations to the overhead joists ?

If I insulate and dryline on either side of the 12" wall I reduce the small room dimensions. However if I replace this wall with a drylined wall I remove some thermal bridging and simultaneously provide 3+" of extra dimension to each of the two small rooms.

Thank you

Click to expand...

Whats the span if the 12" wall is gone.
Are you 1000% certain that the joists don't rest on this wall.
what depth/ thickness are these joists.

You may have to pop in a steel joist, no big deal

What concerns me more is the makeup of the wall:
when you say mortar is is lime mortar or cement mortar.

If its lime then what you are proposing is a recipe for disaster.

Have a browse around here


He is a world renowned expert on this stuff and posts a lot of stuff on his website/blog.

You might be better considering doing it from the outside.
What floor is in house?
What is roofing?
Have all the walls got a damp proof course?
Whats the material around the outside wall: concrete path, gravel, ??

While there are many more qualified folk on this site than me to help you with this, have you considered getting a professional option: not an open end brief but a very narrow one to keep the cost down.

My final comment is that, in 2014, what you are proposing is becoming less acceptable as a process due to the health risks associated with mould growth behind the insulation.
Good luck

The span without this wall would be 16'.

I meant to imply that the joists do rest on this wall. The reasoning is that they run perpendicular to this wall at this end of the house, however at the other gable end there a similar arrangement with no divider wall and the joists run 90deg. differently, from the chimney wall to the outside gable wall, presumably since there is no wall to break up the 16' span.

The joists appear to be 6"x2" at either end (and 9"x2" in the large central room of the house without divider wall).

I do not know if it is lime or cement, however having drilled it is not as strong as modern cement.

The second floor boards appear to be 1' (~21mm) thick. The roof above has 5"x2" rafters on 15-1/2" spacing with grey stone slate tiles.

I do not know whether or not there is a damp proof course. There are cement paths along the main lengths, and loose stone outside the gable end, with both gutters pouring onto this stone from just under a 4' height. The first two gutters could be plumbed to a drain around the back corner with a pipe along the gable wall, or a concrete/cement enclosed channel under the loose stone.

The mould growth behind insulation is a very interesting subject. Even in June or July heat is sometimes needed as the stone/mortar wall will not heat up sufficiently so insulation is very much desired.

A damp proof course cannot be assumed however even without one is it not possible for mould to be stopped/contained with, e.g., the first internal cavity ventilated to the outside ?

There was a serious damp problem inside the gable corner with advanced floor board rot. I have sealed the large gaps between the outer wall and concrete path, sealed the inner skirting boards, applied Ronseal wet rot wood hardener to the worst floorboard damp rot as a temporary measure, and am rebuilding the sofia/fascia at the roof corner that had a large gap.

I have a desiccant (works efficiently in low temperatures) dehumidifier running in low power mode for the nine hours of low rate electricity at night and it keeps the whole house inner rel. humidity down to below 80% (it can rise to over 99% in winter without an open window) for less than €5/month. This stops almost all mildew. Permanent air vents with automatic fan ventilation are being considered as well.

The current external walls have drywall with a 1" stud cavity. The hidden wall will has some form of cement lining and multiple paint coats from earlier days. I do not know it there is any insulation, I suspect none, however at best it would be suspended 1" fiberglass.

Xtratherm recommended briefly 2"x1" battens (for a 1" cavity) followed by 60mm Xtratherm Thin-R XT/TL-MF (60mm foil covered closed cell foam pre-bonded to 12.5mm plasterboard). Total thickness inside wall 97.5mm + plaster.

Superquilt's brochure recommends,
"Brick, External" wall/25mm batten cavity/40mm Superquilt/25mm batten cavity/12.5mm Plasterboard . Total thickness inside wall 102.5mm + plaster.
I do not know if this applies to stone/mortar walls as well as brick.

If we did decide to remove the inner wall we would seek the a professional review.

Thank you.

Big project!
Thanks for the reply.
You seem to be well aware of the issues.
A steel beam can be put in to support the joists if you take out the wall.
You may end up with two little columns at each end supporting the beam rather than having if flush if beam ends are not recessed into the wall for support, depends on access also.
Re the rainwater, etc outside, you need to get it way from the house, what some folk do is put a French drain/gravel/crushed stone trench all the way around and drain it all away.
The damp needs to be solved.... easier said than done.

you should explore breathable insulation products, the Xtratherem stuff you mention is non breathable, don't know about the superquilt stuff.

Both are not designed for yopur wal structure, you really need to find out what the build is.

The issues with ventilation cavities is that to move the damp air they need to be open from top to bottom, up through the first floor ceiling and into ceiling void.
However the fire regs require a horizontal fire break so therein lies a conundrum that is beyond my level of expertise.

This is why I prefer the external solution, putting something like a cement board as the outside layer to stop the rain and then the rest of the layers can address insulation and ventilation.

The other thing I do is insulate the roof, either horizontally over the ceilings or outside the rafters, with breathable felt etc.
Its like a woolly hat, especially when it links up with the wall insulation to look like a tea cosy.

This american website has some interesting stuff as well, you need to go look GreenBuildingAdvisor.com
A lot of it is timber framed stuff but there can be useful links to your problem.

The floor is another problem area: what is it?

In most similar cases what I have ended up digging out the concrete/clay floor and putting in a very well ventilated insulated timber floor.
This allows the walls to breath much earlier before coming up into the house.

Re the dry-lined gable, with the cement based plaster and paint.... not good, is there mould?

This is a rented house and any project would have to be a a reasonable cost, a reasonable pace, and agreed with the landlord. Removing the wall may in fact be too big a project, however the first step of course is to figure out what are the options. A major concern there is any possibility of cracking or damage to internal/external walls, or stone slates on the roof.

The French drain is an interesting idea.

I like the idea of cement board externally however that will probably involve too much cost (large area) and change to the house. The outside wall is heavily stippled and a similar finish would also probably be required over the cement board.

Mould requires moisture and cavities need to be properly dehumidified and/or vented which it appears can be difficult to achieve with certainty.

Would spray polyurethane foam (SPF) applied to cavities between internal battens not stop mould by eliminating the cavity and potential mould surfaces ? Furthermore a transparent plastic sheet or release agent applied between the inner foam containment surface and the cavity would allow subsequent temporary surface removal and inspection of SPF continuity. SPF is available in bulk purchases of, e.g., 750mL aerosol cans, or larger 2 component tank systems.

The inner containment surface could be many things including,
a. the inner aluminium sheet of Xtratherm (with or without plasterboard)
b. a thin sheet of treated plywood held flat against SPF pressure by temporary strong flat vertical spars (angle iron, angle aluminium, 2"x4"'s edge on etc.) screwed through the sheet to the battens
c. plasterboard again temporarily strengthened against SPF pressure with outside vertical spars.
d. even fiberglass sheets or whatever is available at a reasonable cost.

All SPF containment methods require drilling and later sealing of SPF access ports which favours a non-plasterboard approach in order to avoid breaking the paper seal unnecessarily. The plasterboard or Xtratherm/plasterboard could be subsequently fixed through the containment sheet or directly after the containment sheet is removed. Alignment/Flatness of batten surfaces, and strong attachment of containment surface with closely spaced screws would minimize any potential remaining air gaps between the final surface boards and containment sheet or SPF.

One option is to make the battens sufficiently deep and use no Xtratherm, just containment sheet and plasterboard. Cold bridging through the battens could be reduced by making them a compound lattice structure (e.g., parallel 2"x1"'s connected by 2" deep bridges for an approx. 4" deep structure). The SPF will then fill the voids in the latticed battens.

PVC electrical conduit channels can be set before SPF application.

Cost and thickness of the resulting installation for a given U value would be major factors in choosing all SPF/plasterboard or SPF/Xtratherm/plasterboard etc.

All of this hinges on whether or not internal SPF will prevent or sufficiently minimize mould.

I do not know what is underneath the floor, however a first guess is 1940 era joists (untreated?) similar to the ones in the ceiling on top of screed and concrete. The air here is cold and very humid in winter and again it would be helpful if SPF could work to prevent mould forming surfaces.

The drylined gable end has mould for the first lower 2' of the corner where the floorboards have rotted. I am hoping that the outside crack and sofia/fascia repair will significantly improve the situation for now.

Thanks for the detailed update.

First up I do appreciate your position as a tenant so bear that in mind with what follows.

Whats the BER?

With the damp and humidity levels you mentioned, is it fit for purpose for letting?

Research spf on the american site I gave you....
IMO it is entirely unfit for purpose here.

I am very much old school when it comes to this stuff, and I have a principle of addressing the problem at source rather than a patchwork of Elastoplast type solutions: hence my tea cosy approach, leading to controlled ventilation.

The essence of the the approach is try to contain the ingress of moisture into the walls and then allow whats there to exit pronto.

If you have it well insulated on the outside and all ventilation controlled, then you could look at a proper MHRV such as those made by Paul which would extract perhaps 80% of any heat in the air being extracted and preheat the incoming air.

This would need to be commissioned properly, they are not plug and play

My guess is the floor is not ventilated.

Good luck.