Significant renovation works including (maybe) EWI

[lookseehear]

Hello everyone - first post here and looking forwards to discussing houses with you all.

Our house was once two Somerset miner's cottages which were knocked through to create one house, then extended to the rear at some point in the 70s. The main part of the house is 400-500mm thick stone walls and the extension is concrete block faced with stone similar to the existing walls. Pointing is generally pretty poor, of mixed quality and with lime in places and cement in others. The roof is low-pitch with slate tiles, so no attic/very little loft storage. The felt doesn't look great and probably needs to be looked at in the not too distant future, and the extension roof structure was built on top of the existing one, which sounds like the lazy/cheap way to do it but I stand to be corrected here.

We have two young children (6 and 4) and we are planning fairly ambitious renovation works to turn this into a home we'll be happy to spend the next 15 years and probably some time beyond that. There are three main objectives:

• Improve the insulation and thermal characteristics of the building so that it's a comfortable building year-round and doesn't cost as much to heat. We aren't expecting passivhaus levels of thermal comfort here, but a significant improvement
• Create an open plan living space that suits the way we want to live and replace kitchens/bathrooms that are all very dated.
• Catch up on any needed maintenance that may have been neglected. We moved in three years ago but it was probate, so was only occupied intermittently in the period before we moved in. I'm thinking gutters, drainage, repairs to plaster etc.

We're working with a local architect and have some plans that we've put to pre-app which will involve a small single storey extension, enlarging some windows and raising the roof height to accommodate insulation in the roof.

We plan to put our living space on the first floor, which makes more sense than you might think because we're on a fairly steep hill so the garden slopes up to one end of the house, which also makes it much easier to open up the living space and has the benefit of enabling vaulted ceilings in the living space, while having lower ceilings in the cosier downstairs bedroom spaces.

From the reading that I've done and conversations we've had, I feel like my preferred approach is to externally insulate the stone walls and either replace the roof or to insulate between and above the rafters in the roof to create a 'tea cosy' over the house which combined with new (double or ideally triple glazed) windows and new modern doors should create a more air-tight and well insulated home. Our house isn't listed, nor are we in a conservation area and we're comfortable with the rendered look that this would create. I've considered IWI instead of EWI but I'd be concerned about several things:

• Cold bridging and risk of interstitial condensation
• I assume we'd still have to remove all cementitious mortar pointing externally because IWI would leave a colder wall with greater risk of spalling to the stone
• Reduction in room sizes to achieve a good level of insulation

We're committing £250-300k to this project and want to do it right. I'd love to hear any thoughts you have on the above, but particularly the merits of EWI vs IWI, and how we might achieve this. I'm assuming that doing all of this at the same time as replacing all windows and potentially re-roofing would make sense because they can all accommodate each other.

 

[erikwoolf]

Hello everyone - first post here and looking forwards to discussing houses with you all.

Our house was once two Somerset miner's cottages which were knocked through to create one house, then extended to the rear at some point in the 70s. The main part of the house is 400-500mm thick stone walls and the extension is concrete block faced with stone similar to the existing walls. Pointing is generally pretty poor, of mixed quality and with lime in places and cement in others. The roof is low-pitch with slate tiles, so no attic/very little loft storage. The felt doesn't look great and probably needs to be looked at in the not too distant future, and the extension roof structure was built on top of the existing one, which sounds like the lazy/cheap way to do it but I stand to be corrected here.

We have two young children (6 and 4) and we are planning fairly ambitious renovation works to turn this into a home we'll be happy to spend the next 15 years and probably some time beyond that. There are three main objectives:

• Improve the insulation and thermal characteristics of the building so that it's a comfortable building year-round and doesn't cost as much to heat. We aren't expecting passivhaus levels of thermal comfort here, but a significant improvement
• Create an open plan living space that suits the way we want to live and replace kitchens/bathrooms that are all very dated.
• Catch up on any needed maintenance that may have been neglected. We moved in three years ago but it was probate, so was only occupied intermittently in the period before we moved in. I'm thinking gutters, drainage, repairs to plaster etc.

We're working with a local architect and have some plans that we've put to pre-app which will involve a small single storey extension, enlarging some windows and raising the roof height to accommodate insulation in the roof.

We plan to put our living space on the first floor, which makes more sense than you might think because we're on a fairly steep hill so the garden slopes up to one end of the house, which also makes it much easier to open up the living space and has the benefit of enabling vaulted ceilings in the living space, while having lower ceilings in the cosier downstairs bedroom spaces.

From the reading that I've done and conversations we've had, I feel like my preferred approach is to externally insulate the stone walls and either replace the roof or to insulate between and above the rafters in the roof to create a 'tea cosy' over the house which combined with new (double or ideally triple glazed) windows and new modern doors should create a more air-tight and well insulated home. Our house isn't listed, nor are we in a conservation area and we're comfortable with the rendered look that this would create. I've considered IWI instead of EWI but I'd be concerned about several things:

• Cold bridging and risk of interstitial condensation
• I assume we'd still have to remove all cementitious mortar pointing externally because IWI would leave a colder wall with greater risk of spalling to the stone
• Reduction in room sizes to achieve a good level of insulation

We're committing £250-300k to this project and want to do it right. I'd love to hear any thoughts you have on the above, but particularly the merits of EWI vs IWI, and how we might achieve this. I'm assuming that doing all of this at the same time as replacing all windows and potentially re-roofing would make sense because they can all accommodate each other.

Having been through the painful process of retrofitting IWI I would think EWI is much easier to install whilst minimising cold bridges. The level of disruption involved whilst trying to achieve the same effect internally is much higher with IWI than EWI.
If you get a retrofit assessor involved they can advice on which improvements make more sense from an efficiency point of view but given the budget you are willing to invest probably worth upgrading across the board.

 

[Kearn]

A brief response regarding external insulation. Aside from the aesthetics, the incremental thickness impacts window and door reveals, rafters and fascia/gutters etc - it’s a whole house consideration, with various implications.

However, with that thickness of solid stone, you already have adequate thermal mass to insulate your home. Adding another fraction of the depth on, isn’t really going to move the dial.

What is essential is that the existing thermal mass can act in an insulating capacity rather than conducting heat. The single key here is keeping that material dry. In order to achieve that, all modern materials need to be shifted as any cement pointing, any modern render/plaster or paint inside and out, will trap moisture and reverse the stone capacity to function how you need it to. Get everything off, give it time to dry out, about an inch/month depending on saturation levels (i.e. not overnight) and you’ll see the difference.

I’m sure other challenges will crop up as they do that demand your budget more urgently, and saving that cost would be high on my list.

Equally, I am a big believer in secondary glazing, a few £100/window will achieve as good if not better thermal and acoustic properties than very expensive, limited life glazing panels. Double glazing does fail, it’s a matter of when not if. And slimline is even more prone to quicker failure, while costing more and having shorter guarantees.
There are many companies out there, but it is an easy DIY job, 4-6mm perspex cut perfectly to size and held in place by magnetic tape on the widow. Depending on your casement type and size you can have more than one sheet per window to allow opening, or even sit into casements if profile and hardware allow.
Aside from being very cost effective, it doesn’t detract from the existing windows and glass, assuming one or both have some historical interest to them.

Sounds like you’ve got a few years of serious disruption and cost ahead on the project - saving the pennies for the areas where you don’t get a choice is always my primary goal.

 

[lookseehear]

Having been through the painful process of retrofitting IWI I would think EWI is much easier to install whilst minimising cold bridges. The level of disruption involved whilst trying to achieve the same effect internally is much higher with IWI than EWI.
If you get a retrofit assessor involved they can advice on which improvements make more sense from an efficiency point of view but given the budget you are willing to invest probably worth upgrading across the board.

Good to hear you agree on EWI being the more straightforward option to minimise cold bridging. Is a retrofit assessor just looking at insulation or would they look more holistically at the heating systems etc too?

 

[lookseehear]

A brief response regarding external insulation. Aside from the aesthetics, the incremental thickness impacts window and door reveals, rafters and fascia/gutters etc - it’s a whole house consideration, with various implications.

However, with that thickness of solid stone, you already have adequate thermal mass to insulate your home. Adding another fraction of the depth on, isn’t really going to move the dial.

What is essential is that the existing thermal mass can act in an insulating capacity rather than conducting heat. The single key here is keeping that material dry. In order to achieve that, all modern materials need to be shifted as any cement pointing, any modern render/plaster or paint inside and out, will trap moisture and reverse the stone capacity to function how you need it to. Get everything off, give it time to dry out, about an inch/month depending on saturation levels (i.e. not overnight) and you’ll see the difference.

I’m sure other challenges will crop up as they do that demand your budget more urgently, and saving that cost would be high on my list.

Equally, I am a big believer in secondary glazing, a few £100/window will achieve as good if not better thermal and acoustic properties than very expensive, limited life glazing panels. Double glazing does fail, it’s a matter of when not if. And slimline is even more prone to quicker failure, while costing more and having shorter guarantees.
There are many companies out there, but it is an easy DIY job, 4-6mm perspex cut perfectly to size and held in place by magnetic tape on the widow. Depending on your casement type and size you can have more than one sheet per window to allow opening, or even sit into casements if profile and hardware allow.
Aside from being very cost effective, it doesn’t detract from the existing windows and glass, assuming one or both have some historical interest to them.

Sounds like you’ve got a few years of serious disruption and cost ahead on the project - saving the pennies for the areas where you don’t get a choice is always my primary goal.

Thanks for the detailed response. It sounds like whatever approach we take (IWI or EWI) we'd need to get rid of the cement pointing. I'd assumed that if we were doing EWI it would be less important because the level of moisture in the walls would only be coming from us and from the ground, with the external render keeping the actual walls fairly dry. No sense in not removing the pointing if we're going to these lengths anyway I guess.

On the window point I would be inclined to agree if we had timber frame windows worth saving which added to the look of the property, but we have old inefficient aluminium frames and the double glazed units have PVC faux Georgian bars. A lot of the glazed units have failed and need replacing anyway, and the Georgian bars aren't attractive in my opinion.

 

[paulc]

Having been through the painful process of retrofitting IWI I would think EWI is much easier to install whilst minimising cold bridges. The level of disruption involved whilst trying to achieve the same effect internally is much higher with IWI than EWI.

With IWI, you need to pay careful consideration to where things like curtain rails, cupboards, and radiators are mounted. They need a solid survace for the screws to bite in to and take the load. For this reason, I've been using 12mm ply above windows where I've been insulating walls here. Radiators have all been moved to internal walls (simplifies plumbing, and in a well insulated draught free house, there isn't any real need to stick them under a window). The loss of 75mm floor space is not noticeable, and being able to wrap the insulation round the window reveals helps to reduce the heat loss in those areas.

EWI has its own set of problems - The biggest is detailing around the roof line to ensure water doesn't get in between wall & insulation. If the whole roof is coming off, then extending the overhang should take care of any path for water at that level. Using breathable materials should help in the unlikely event that the wall behind gets damp. Wood fibre or cork with a lime finish will be better than EPS & K-Rend in respect to minimising damp.

 

[Kearn]

Thanks for the detailed response. It sounds like whatever approach we take (IWI or EWI) we'd need to get rid of the cement pointing. I'd assumed that if we were doing EWI it would be less important because the level of moisture in the walls would only be coming from us and from the ground, with the external render keeping the actual walls fairly dry. No sense in not removing the pointing if we're going to these lengths anyway I guess.

On the window point I would be inclined to agree if we had timber frame windows worth saving which added to the look of the property, but we have old inefficient aluminium frames and the double glazed units have PVC faux Georgian bars. A lot of the glazed units have failed and need replacing anyway, and the Georgian bars aren't attractive in my opinion.

You certainly do need to - yes. And I didn’t think I needed to mention, any insulation inside or out has to be natural and breathable - otherwise you will create some serious issues. There are many horror stories and few success ones that I’ve heard. Even adding too deep a hemp line mix direct to brick/stone can move the dew point and create condensation issues as the wall doesn’t get to heat adequately.

If it were me I would research heavily, like I said the stone thickness you already have should be a great natural insulator if treated properly. I would do no more than an internal hemp lime finish to add a touch more warmth and 15-30mm thickness which shouldn’t cause many issues.

 

[housewinnie]

We live in an old house with half a meter ish thick walls too. We have very old thin double glazing and our house is draughty too. What I can say is it’s the warmest house we have lived in. Yes it does get chilly at times but have recently installed a wood burner so at least one room is cozy. Spend your money on efficient heating and let the house be draughty and breathing.

 

[a twig]

I would second / third / fourth the above. We had a stone house with 800mm thick rubble walls, resolving the damp issues and then putting 25mm wood wool boards skimmed with lime on the inside of the external walls and replacing the PVC single glazing with modern hardwood frames with double glazing was plenty enough for us, we were very comfortable and cozy even when the room thermometer said it was only 18 degrees, and likewise no need for air conditioning even in the summer heatwaves when it got to 34’C outside

Installing a new boiler that wasn’t 20 years old and then a modern efficient 5kW Stovax wood burner which we lit on the coldest days sorted us right out

In contrast the relatively modern brick cavity wall house we are in now we have to heat to 20’C to get the same level of comfort

 

[Athers1971]

The single key here is keeping that material dry. In order to achieve that, all modern materials need to be shifted as any cement pointing, any modern render/plaster or paint inside and out, will trap moisture and reverse the stone capacity to function how you need it to. Get everything off, give it time to dry out, about an inch/month depending on saturation levels (i.e. not overnight) and you’ll see the difference.

Does that mean solid walls, covered in non-breathable material both inside and out, whilst not showing any visible signs of damp, can be damp and therefore colder (less insulating?) than the same wall if it was dry?

I suppose I’m getting to the point that should we be tackling cement/gypsum whilst there are no visible damp issues.

 

[Kearn]

Does that mean solid walls, covered in non-breathable material both inside and out, whilst not showing any visible signs of damp, can be damp and therefore colder (less insulating?) than the same wall if it was dry?

I suppose I’m getting to the point that should we be tackling cement/gypsum whilst there are no visible damp issues.

Simply put, yes. There’s obviously many factors - but moisture will find its way in, whether from the ground, through cracks in the external render… if it can’t get out, it will stay damp and become a great conductor of heat.
I’ve uncovered many horrors - you’ve no idea what damage is being done behind what appears to be ‘perfectly good render’…

Cement and gypsum don’t always drive damp, certainly not visible. But they are incorrect and do not function in the way the building should, so the chances are… in many cases, you’ll be able to smell the stench of damp trapped moisture as your hack of render.

However, the fact that the OP is considering such an invasive approach to insulation makes me assume the house isn’t maintaining temperature as it should. So yes, that would be where I would start, strip it clear, let it dry and monitor whether the house starts behaving differently in terms of temp and humidity levels. Just don’t expect immediate changes.

People often bemoan the inadequacy of their old house solid material walls in terms of thermal capacity, it’s just that they aren’t functioning correctly anymore. Same goes for the push for hermetically sealing everything - old houses need airflow, dry air into the house for at least 30 mins a day by opening up windows for cross flow, will significantly help with a houses ability to be better heated than attempting to heat moist humid air from us living indoors and creating all that moisture. The assumption is that opening windows will cool the house, it won’t it will cool the air contained in the house, but the physical materials shouldn’t be affected. Assuming they can of course act as an insulator to start…

Someone mentioned a while back, a common theme with old photos of houses. All the windows wide open…

 

[eco-refrofit]

We are in the process of doing EWI rather than IWI, although the hope is for something close to EnerPhit (which is the passive house retrofit standard). Looking at the number of renovations featured on greenbuildingstore's website and the passive house plus magazine (free online), it's surprising how many older homes have gone with IWI.

How far are your walls from the edge of your property (land)? If EWI takes you within 1m of your neighbour's land then it's going to restrict where you can put a heat pump unit if you want the Boiler Upgrade Grant (although this might be just for air rather than ground source). We are hoping this restriction will charge since it's a bit ridiculous when your neighbour would have no issue with it and doesn't have a house anywhere near the boundary.

Thermal mass is a big factor too - EWI means you are heating those thick walls, and at night or when the temperature drops, the thermal mass means the room temperature will be evened out more - so warmer nights and less overheating in summer. When there's a heatwave (rare in this area), it takes a good 3 days to feel properly hot in the day because of the thermal mass in the uninsulated walls - IWI would mean greater temperature changes because this would be lost.

As far as interstitial condensation goes, we already have this since adding extra loft insulation with a vapour membrane (we actually took up the vapour membrane and will have to get another once we have the insulation sorted. While at the moment we still have cracked cement render, simply going with EWI won't automatically prevent condensation. It depends on general ventilation (or lack of). We don't have trickle vents and decided against them when getting the triple glazing in, but any real amount of airtightness work and extra insulation is likely to lead to condensation. Almost every big glazing company is putting in trickle vents but if you have big rooms and possibly small windows you might end up with additional condensation.

We are putting in MVHR (centralised ventilation with heat recovery) which we did have planned, just not yet. It's well within your budget. Not only will it improve bills (horrendous oil prices and consumption) but it will mean we can finally address all the drafts and the high humidity. We won't be getting a tumble drier and dry clothes in the bathroom (which is the only room wry trick vents) so it will really speed this up.

With EWI, because our home is already rendered, we won't be removing all cement pointing/mortar (under the render it's unlikely to be airtight or fully weather tight). We are mostly going with Diathonite Evolution as EWI because of the high level of breathability combined with thermal insulation, it's a natural insulation of cork/lime/clay well suited to old buildings and the fact you don't have to level the walls as you would for boarding. Diasen also do products to help with existing internal damp and salts, plus a top coat of cork sprayed on which can be colored to avoid painting (you need breathable paints eg graphene or lime based to maintain breathability). For us not having to re-paint minor cracks is a plus (Diathonite is pretty elastic so less cracks). Diathonite can be sprayed on and works for the life of the building. On the inside, insulated lime plaster or magnesium oxide instead of gypsum plasterboard can maintain breathability, and any rooms prone to rising damp or below ground level also need treating on the lower part of the way. Insulated lime plaster can be bought in a white color so some people choose this rather than painting, or you can get smoother finish with lime paints.

With IWI, we decided against in most rooms due to a combination of disruption/time including decorating and achieving a poorer thermal value - it's overly complicated. If you really want to do it though, you can get aerogel on magnesium oxide plasterboard which is super thin (although needs fixing batts). If you have IWI with gypsum plaster you're going to spend the next few years looking at the hairline cracks you need to paint over, which drove me crazy in the last house.

We may be putting in a heat pump and the heating pipes are microbore and some are in external walls, we aren't sure where the new plumbing will run but we'd rather avoid the risk of having nails for IWI puncturing them or having to run new pipes and mounts for new radiators into the IWI after. IWI is only needed on external walls so in our case no electrics need moving and there's no coving in the way.

Insulating around window reveals is harder with IWI but to avoid cold bridges ideally look at ProClima tape or specialist insulation here anyway even if you do get EWI.

I actually recently spotted insulated lime plaster at 0.80 U value, not really IWI but Lime Green has some, giving 40% improvement in thermal insulation for I think 50mm - if you are hacking off old plaster anyway I think it's worth doing (several rooms have blown plaster on external walls).

IWI other than insulated lime or Diathonite sprayed on is going to mostly mean levelling the internal walls which will change the character. Any slight gap behind IWI boards because of uneven walls is going to end up with condensation in the middle of the wallls.

btw EWI generally needs scaffolding - we've got a staggered semi so an issue with neighbours.