extension Construction Method in Old Stone Property Extension + Renovation

[Lears]

Hi all,

 

I'm in the middle of a renovation of an old stone cottage near Wales (2km from the Severn but not massively exposed) which I going to be a long term home for my family so really trying to ensure I make the best possible decisions. Hoping I can leverage the collective wisdom of this forum as I've read a lot of posts but still not 100% sure. The fundamentals

• The part of the property is 450mm solid stone (rubble infil) walls and a thin concrete pad of questionable quality. The plan is to put 60-80mm woodfibre IWI around the external walls, remortar in lime, limewash finish etc. Additionally, we're going to lift the existing floor by 500mm or so depending on state of foundations, put in an insulation layer and limecrete with wet underfloor (I have a connection so can get done for a good rate)
• We're then putting an extension on the east side of the house, replacing a pretty horrible lean to conservatory
• The plan is to retrofit MVHR and heat with airsource heat pump, reroof with in roof solar. I think this will be good enough to heat the entire home if I hit decent U values and air tightness with some towel rails upstairs but am leaving a service run as an option to fit a mini split system upstairs for AC in the summer but could be supplementary heating too if needed

 

We've has some designs back from our architect on the construction of the extension however, we didn't really ever have a chat about the wall build up itself. He's come back with the below:

1. 160mm stonework finish (we will have lots on site so won't have to buy in but understand labour unlikely to be cheap)
2. 125mm cavity with 100mm ecotherm insulation
3. 100mm toplite block with dot dab finish plasterboard on the inside  

 

all of which targeting a u value of 0.18.  

 

At first glance, this didn't feel like it was particularly ambitious from an energy efficiency point of view and although I knew it was going to be a more modern construction and therefore, we won't need the full lime treatment, dot dab just doesn't feel right or in keeping with the rest of the home.

 

I've looked a bit into ICF and timber frame but wasn't too sure on how well they would join to the existing property, both at the intersections and in terms of having very different thermal performance properties. On the ICF side, because it's a single story and one section will be sliding doors, I wasn't sure if it was worth while.

 

I'm relatively hands on and will be doing a lot of the lime and wood fibre work so happy to get my hands dirty to save on costs. 

 

My initial thought is to just widen the cavity and stick to blockwork but any suggestions or comments would be greatly appreciated.

 

 

(Section highlighted in blue is the new extension, grey walls are solid stone

 

[Redbeard]

2 hours ago, Lears said:

The plan is to retrofit MVHR and heat with airsource heat pump, reroof with in roof solar. I think this will be good enough to heat the entire home if I hit decent U values and air tightness

 

'Decent U values' for a new extension may be very different from those for IWI'd solid walls. (All values which follow are W/m2K) I'd be looking at something like 0.12 for a timber frame wall. I got about 0.16 for my extension, about which I felt quite smug when I applied for B. Regs approval (when the target was 0.28), but less so when, within months, the new regs came out, requiring 0.18! So I guess I'd try to enclose/'wrap' the old building with the extension as much as possible. You may be looking at only about 0.5 - 0.6 for those insulated stone walls.

 

For roofs of course you ought to be able to get down to 0.10 or even 0.08 - 0.09W/m2K whether the roof is old or new, but if you have sloping soffits you may be limited in terms of room height, and awkward choices ('unfriendly' insulation and really good ins. value vs 'Friendly' insulation and not-so-good U value) may have to be made.

 

For the extension I'd do timber, just because I build in timber whenever I can. I taught a variation of the Segal Method for about 15 years and am pretty obsessed with it! Above everything it is Ridiculously Good Fun and very empowering.

[jfb]

I did very similar.

Lime render to flatten walls, 60-80mm IWI wood fibre, lime render and mesh to finish. Has worked well. Good for air tightness if dealing with junctions well. Reckon it also performs much better than u values of materials might suggest.

For extension i went for stone external wall, 200mm cavity with cavity batts for insulation, blocks for internal wall and lime render finish.

For floor I went for 100mm compacted sub base, 200mm eps and Ufh pipes in 100mm concrete slab. I tried a limecrete and expanded glass floor in a small barn and had more problems with that regarding damp than the main house. But it does lie a bit lower.

 

if you are hands on and want more details on IWI installation ask away

[Lears]

1 hour ago, jfb said:

I did very similar.

Lime render to flatten walls, 60-80mm IWI wood fibre, lime render and mesh to finish. Has worked well. Good for air tightness if dealing with junctions well. Reckon it also performs much better than u values of materials might suggest.

For extension i went for stone external wall, 200mm cavity with cavity batts for insulation, blocks for internal wall and lime render finish.

For floor I went for 100mm compacted sub base, 200mm eps and Ufh pipes in 100mm concrete slab. I tried a limecrete and expanded glass floor in a small barn and had more problems with that regarding damp than the main house. But it does lie a bit lower.

 

if you are hands on and want more details on IWI installation ask away

 
This is really helpful so thank you. Does make me think widening the cavity is the easiest route here. A few questions if you don’t mind. I’ve seen some places use diathonite as the leveling layer, then adhesive onto that with the WF, others I’ve seen just use a stickier mix of lime after getting the mortar relatively level. Interest what your approach was?

 

I’m still learning on the cavity wall side so was your partial fill or full fill and any info on the specific insulation you used? and then was it just a wet like mix straight onto the blockwork internally with a mesh for extra support or was there a membrane etc?

 

Very interesting on the limecrete. If we didn’t know someone that did it I feel we’d just go concrete given some of the posts I’ve read. Out of interest, did you use a lime stone floor finish on that one or any thoughts on the damp issue? We’re going MVHR so I’d hope that wouldn’t be an issue…

 

21 hours ago, Redbeard said:

 

'Decent U values' for a new extension may be very different from those for IWI'd solid walls. (All values which follow are W/m2K) I'd be looking at something like 0.12 for a timber frame wall. I got about 0.16 for my extension, about which I felt quite smug when I applied for B. Regs approval (when the target was 0.28), but less so when, within months, the new regs came out, requiring 0.18! So I guess I'd try to enclose/'wrap' the old building with the extension as much as possible. You may be looking at only about 0.5 - 0.6 for those insulated stone walls.

 

For roofs of course you ought to be able to get down to 0.10 or even 0.08 - 0.09W/m2K whether the roof is old or new, but if you have sloping soffits you may be limited in terms of room height, and awkward choices ('unfriendly' insulation and really good ins. value vs 'Friendly' insulation and not-so-good U value) may have to be made.

 

For the extension I'd do timber, just because I build in timber whenever I can. I taught a variation of the Segal Method for about 15 years and am pretty obsessed with it! Above everything it is Ridiculously Good Fun and very empowering.

Thanks, will look at the segal method a bit more. My only worry with TF is having two parts of the house with very different thermal properties at this point but will do a bit more research. 

[Redbeard]

16 hours ago, Lears said:

Thanks, will look at the segal method a bit more. My only worry with TF is having two parts of the house with very different thermal properties at this point but will do a bit more research. 

 

The moment you start doing retrofit a 'bit at a time' the un-done bits arguably become vulnerable. This should not be a cause for panic, but regular observation and risk management are your friend. I have differing U values throughout my house ('Incremental Retrofit', or need for a round Tuit). Not least, if you think about it, filling your loft with the 'approved amount' of quilt insulation will give you 0.16W/m2K. If the rest of the house is uninsulated elemental values could be as high as 5 (metal s/g wins) and fairly widely 1.5 - 1.7 (unfilled 50mm cavity and 225 solid brick respectively).

[Iceverge]

Step 1. Knock down old house. I'm dead serious. It'll cost less. 

 

Step 2. Ignore step 1, everyone does..

 

 

 

For the extension NO NO NO to PIR in the cavity. Don't bother with lightweight blocks either.  Full fill with mineral wool batts or EPS beads every time and normal blocks for both leafs. SS cavity ties are fine too. Wet plaster internal for airtightness. 

 

Or timber frame. Like Redbeard says, it's lovely work. 

 

EPS for the floor is good. 

 

For the walls I would do either what you suggest or parge the walls to solve airtighness, follows by studs and mineral wool in-between. Much cheaper. 

 

 

[jfb]

18 hours ago, Lears said:

. I’ve seen some places use diathonite as the leveling layer, then adhesive onto that with the WF, others I’ve seen just use a stickier mix of lime after getting the mortar relatively level. Interest what your approach was?

I just used sand and hydraulic lime for the wall levelling layer(s). I recall diathonite is quite pricey and not sure worth it if you are using wood fibre IWI anyway. The amount of effort to get your walls flat depends on if they are back to stone, how level they are anyway, etc.

I got my walls flat enough so that I didn't use any lime mix on the back of the wood fibre which makes its lot simpler to install. Even if it isn't flat the plastic fixings do a good job of pulling the boards in (not if 100mm thick though).

I just used the cheapest wood fibre boards I could get (steico something) that were square edged (not tongue and groove).

I think I might have used some expanding foam in a couple of places where there would be an obvious gap behind a board, but you could use some sand/lime as well there.

18 hours ago, Lears said:

I’m still learning on the cavity wall side so was your partial fill or full fill and any info on the specific insulation you used? and then was it just a wet like mix straight onto the blockwork internally with a mesh for extra support or was there a membrane etc?

Pretty sure it was full fill mineral fibre batts.

No membrane on the internal - just lime render no mesh. Airtight and breathable.

 

18 hours ago, Lears said:

Out of interest, did you use a lime stone floor finish on that one or any thoughts on the damp issue? We’re going MVHR so I’d hope that wouldn’t be an issue…

Put wooden flooring in - on battens set on the limecrete.

But we have a pretty high water table in winter so that can't help dampness generally!

 

[jfb]

1 hour ago, Iceverge said:

Step 1. Knock down old house. I'm dead serious. It'll cost less. 

Really?  I mean it depends on the house. 

And also on how much you value the aesthetic value of the old construction - a new version is going to be different and not always in a good way.

[Iceverge]

16 hours ago, jfb said:

Really?  I mean it depends on the house. 

And also on how much you value the aesthetic value of the old construction - a new version is going to be different and not always in a good way.

 

I suppose it depends on your funds and the standard you expect from a building.