Insulating a stone house

[Col2021]

We will be starting a project, hopefully soon.

Basically a new extension going on an existing sandstone cottage. Its a solid hiuse but will need some work. The plaster work seems very sound but obviously there's no insulation in the wall or floor area. There's a bit in the loft but that will need topped up and lots of draught proofing done.  Floor is a suspended one so will get lifted and insulated as well. However when we get to the walls whats the best thing to do. To do it properly we would need to strip it right back and start again. However how important are the walls? There are new windows going in throughout and there will be lots of draught proofing to be done. How much space would we lose if we stripped the plaster off and put in new insulation and plaster board? Does it stop the building breathing, is it the right thing to do. 

[Roger440]

Solid or cavity wall? Any DPC?

[A_L]

14 hours ago, Col2021 said:

However when we get to the walls whats the best thing to do. To do it properly we would need to strip it right back and start again. However how important are the walls? There are new windows going in throughout and there will be lots of draught proofing to be done. How much space would we lose if we stripped the plaster off and put in new insulation and plaster board? Does it stop the building breathing, is it the right thing to do. 

As a one storey building originally built the walls would represent 20-25% of the total heat loss, the ceiling would have been around 30% but even 100mm of insulation  would reduce this by 85%. The windows would have been 15% max so new would reduce this by 50-75%.

 

75mm of PIR/PUR insulation on walls would reduce their heat loss by over 90%

 

Assuming no interstitial condensation in the structure then 'breathability' is unimportant

[saveasteading]

15 hours ago, Col2021 said:

existing sandstone cottage

How thick are the stone walls, and do you know if they are a single stone, or made with inner and outer layers and rubble fill?

Looks nice condition from this distance.

 

Water overflowing from RH of gutter by the looks of it. Hardly surprising with no dp, but a valley tipping in there.

 

What area of the country are you?

[Temp]

It looks on the small side so I guess EWI is preferred to IWI ? It looks like there is a reasonable roof overhang and neighbouring properties are rendered so a possible candidate for EWI.

 

If you have the budget there are brick or stone slip solutions used a lot in Europe that might look better than render.

https://prostone.eu/stone-products/exterior-and-interior?gclid=CjwKCAiA55mPBhBOEiwANmzoQkAfH0HZYfsHORmwV3uLu30KFc9boDZe1-NBjqVuSJAllNUg61mqDRoCfZ8QAvD_BwE

 

Any sign of damp low down on walls? Perhaps behind furniture? Any evidence owner has painted over it?

 

Perhaps consider installing a MVHR system to provide good ventilation.

[Temp]

17 hours ago, Col2021 said:

Floor is a suspended one so will get lifted and insulated as well.

 

Check ends of joists as they may tell you if there is any water in the walls they are resting on.

[Simplysimon]

the biggest issue with exposed stone buildings, how to insulate. you are normally buying them for their looks so tend to rule out ewi, however, interstitial condentation has already been mentioned by @A_L and this really needs to be taken into account if moving the dew point by fitting iwi. has it been re-pointed with cement mortar as that will stop breathability? being sandstone there will be a fair chance of having joist ends getting soft.

[Col2021]

In East Perthshire, definitely not covering the stone up but it needs repointed so will lime mortar help with breathability? Got a suspended floor in it and plenty of airflow underneath to reduce moisture problems coming up the way. We are still at the early stages yet but the walls are pretty thick and I assume they will be rubble infill. The extension is going onto that gable end so we need to keep that bit breathing and stop cold bridging etc.

There doesn't seem to any damp at all, at least to my layman's eyes. The green patch on the wall was where there was a fruit tree growing. Its on very sandy soil with lots of gravel which naturally drains well but a French drain round the house might be a wise move. 

The walls are very thick so interior insulation would shrink the rooms a bit the current house is 68m2 and the extension will be about the same.

The house had a 92 year old lady in it, and she lived all her days there I think, needless to say it needs throughly modernising so the original bit will be the bedroom and office wing and the new part is going to be open plan living and kitchen with a large utility room. New part will be part render and wood clad with a standing seam roof.

There is mains gas on site and we will probably leave any renewables for now apart from a wood burner. The mains gas in our area comes from an AD plant. If there was no gas I would go for kero instead. We don't have enough room for GSHP so it would have to be ASHP and with the price of electricity currently it SEEMS like an expensive option. My wife is never impressed with the looks of solar panels but it could be something to look at.

It went in for planning a month ago so here's hoping we might get started this year. I live in a tied house for my job so no rush to move. We used to own a couple of buy to lets and we have been looking for an opportunity to do something like this for maybe 10 years! Now its getting quite real and a bit worrying.

[A_L]

6 minutes ago, Col2021 said:

The extension is going onto that gable end so we need to keep that bit breathing and stop cold bridging etc.

 

If you mean the bit of the gable which will become an internal wall then breathability is irrelevant. Insulation on the sandstone wall along the line of the new extension's perimeter would increase the path length for heat using the sandstone wall as a thermal bridge and increase the surface temperature of the sandstone wall in the region of the extension's perimeter, reducing the risk of surface condensation in cold weather

[Iceverge]

Such a pretty house. Congrats. 

 

From the ground up.

 

1. A really comprehensive French drain. 

2. Dig out the floors. Put in as much insulation as you can fit. 150mm minimum will allow for UFH. Cover with 100mm powerfloated concrete. 

3. Woodfiber or Hemp based internal wall insulation with a breathable lime based plaster as your airtightness layer. Be cautious about making it too thick as the stone wall will get too cold and may suffer from decay.

4  New triple glazed windows. 

5. Diligently seal all the ceilings. I don't know the current state of repair of them but if poor take them down. Put an airtight membrane and return it to the lime plastered walls. Then a 22mm service cavity and plasterboard. 

6. Reinsulate the attic. 

7. Ensure that you have a continuous mechanical ventilation system. PIV is a start, MEV/DCV is good. MVHR is best. 

[Simplysimon]

40 minutes ago, Col2021 said:

will lime mortar help with breathability?

yes, what it would originally be built with

[Col2021]

1 hour ago, Iceverge said:

Such a pretty house. Congrats. 

 

From the ground up.

 

1. A really comprehensive French drain. 

2. Dig out the floors. Put in as much insulation as you can fit. 150mm minimum will allow for UFH. Cover with 100mm powerfloated concrete. suspended floor anyway so just insulate that?

3. Woodfiber or Hemp based internal wall insulation with a breathable lime based plaster as your airtightness layer. Be cautious about making it too thick as the stone wall will get too cold and may suffer from decay. how thick?

4  New triple glazed windows. 

5. Diligently seal all the ceilings. I don't know the current state of repair of them but if poor take them down. Put an airtight membrane and return it to the lime plastered walls. Then a 22mm service cavity and plasterboard. 

6. Reinsulate the attic. 

7. Ensure that you have a continuous mechanical ventilation system. PIV is a start, MEV/DCV is good. MVHR is best. seems a bit OTT, I understand about the heat recovery bit but mechanical ventilation? 

 

The new part will have south facing glass for max solar gain in winter so maybe some way of moving that warm air around isn't a daft idea.

[Iceverge]

19 hours ago, Col2021 said:

suspended floor anyway so just insulate that?

 

Yes, windtight membranes + airtight membrane and mineral wool batts. 

 

19 hours ago, Col2021 said:

how thick?

It's about striking a balance between low wall U-Value and making the sandstone wall outboard of the insulation too cold and suffering from frost and damp. The wall will have a reasonable contribution to insulative value as long as it's dry.  60mm Wood fiber would be fine IMO. Maybe someone else will correct me. 

 

You must use breathable render and paint however. Look at study below of the effect on wall humidity by not using breathable materials. PIR and emulsion paint perform the worst. Cork lime and hemp lime render the best. Wood fiber would be similar.  

 

http://www.tara.tcd.ie/bitstream/handle/2262/79420/CERI draft RW SP final.pdf?sequence=1&isAllowed=y

 

 

19 hours ago, Col2021 said:

seems a bit OTT, I understand about the heat recovery bit but mechanical ventilation?

 

Not really, it can be as simple as replacing intermittent bathroom fans with constant volume ones. They'll run at a low and inaudible level all day keeping the internal humidity under control, unlike bathroom ones which are frequently too noisy and get turned off. 

 

[Col2021]

So is the fan just to get rid of moisture? Or are we doing a heat exchanger sort of deal? On the moisture front I assume an electric cooker would be better than gas as no products of combustion! Also have good extraction in kitchen but then you could lose a lot of heat there so even more need for heat recovery? Its almost as though we should just bulldoze the house and start again! As obviously retrofitting this is a PITA.

[Iceverge]

It's a just a constantly running extract fan. Very basic. It'll draw a small of fresh air though the house keeping it much drier and healthier. 

 

Induction every time. I've lived with gas and it's even better to cook on. A kitchen extractor will only run a small part of the day taking out very little heat. Your priority must be to get high humidity air outside of the house. It's impossible to feel warm in a damp house. 

 

23 minutes ago, Col2021 said:

Its almost as though we should just bulldoze the house and start again! As obviously retrofitting this is a PITA.

  

Lol, the predicament of many a renovator. However what you have is something a little special. It might be worth the effort once you're all done. Remember all that stone work , detailing and "character" would be very expensive to replicate in a new build. 

[Simplysimon]

another option would be this

[SimonD]

On 17/01/2022 at 21:08, Col2021 said:

However how important are the walls? There are new windows going in throughout and there will be lots of draught proofing to be done. How much space would we lose if we stripped the plaster off and put in new insulation and plaster board? Does it stop the building breathing, is it the right thing to do. 

 

Just taking a peak at the pictures you uploaded it seems like the wall have been re-pointed multiple times and my suspicion from the photo it that at least some of it has been done with sand/cement mortar. Is this the case?

 

With the amount of mortar used to point the walls, I do wonder whether you have a decision as to whether you value the current aestetics more than thermal performance so:

 

a) leave the stone walls as they are,
b) have the stone walls re-pointed,

c) do either a or b with IWI or leave uninsulated

 

Or you can go for EWI as the roof overhang seems to lend itself to this solution and then finish this with stone slips or render.

 

With EWI, it is probably easier to remove all the consequent thermal bridges, providing it is detailed correctly, than with IWI.

 

Whichever way you go you will need to get a condensation analysis done by a supplier/ technical expert as the moisture transport through your walls depends on the whole buildup and wall system but in general, as already highlighted by @Iceverge, if you have a non-breathable IWI, the wall will breath to the outside. However, good practice now requires this type of insulation to have a ventilated cavity on the cold side to reduce the risk of interstitial condensation, which obviously dramatically impacts the buildup depth.

 

If you have a breathable/capillary insulation IWI, usually of a natural persuasion, the walls will breath to the inside and outside of the building and they don't require the ventilated cavity. But you do need to be careful about the airtightness detailing. The other advantage with the breathable/capillary insulations, such as sheepswool, woodfibre, hemp etc. is that with the right buildup of plasters and paints, they will provide humidity buffering within the house. This humidity buffering in turn reduces the amount of ventilation required as it stores excess moisture peaks and then slowly releases it over a longer duration.

Edited January 21, 2022 by SimonD

[Col2021]

Its looking like redoing the mortar then insulate top and bottom and leave the walls might be the most pragmatic approach, along with good draught proofing and glazing. The longest wall of the house actually faces south so we would get good thermal gain, including the two big windows that will face that way. The west end is that gable you can see and is where the majority of the weather will come from. The extension will be built off that wall so it will protect it to a large degree. The extension will have UFH and a stove so will be toasty, there is potentially  a door between the extension  and the original part but we are toying with leaving it open to allow air to move. The old part is essentially the bedroom and office wing.