We decided to go down the deep retrofit route to make our semi-detached 1930s bungalow airtight and more thermally insulated. On hindsight, I wish we hadn't because now we've committed ourselves to this budget-busting exercise and there seems to be no turning back.

We're now starting on the IWI and have removed the old plaster to reveal the bricks underneath. We've looked at several options and they all seem to be eye wateringly expensive, both material and labour. There isn't a big difference in terms of materials cost between them.

Installed on a circa 80sqm wall on a fairly level brickwork.

1) Diathonite. The first option recommended to us but materials alone cost nearly 5k for 50mm. I enquired with experienced installers and cheapest labour quoted to us is 2.5k +vat hand applied+associated travel and accomm. One spray contractor quoted 8k+vat for materials and labour.

2) IQ Therm. Materials cost a few hundred cheaper than Diathonite, I'm still waiting for contractors to price labour.

3) Gutex Thermoroom. Cheapest among the three at £2800+vat. No idea on labour cost, wondering if any experienced plasterer would be able to install it properly. Also, not too sure about it being wood fibre.

Any thoughts would be highly appreciated.

Welcome Kmach, is it a cavity wall, what has the cavity been filled with? What's on the outside, render/brick/pebbledash/etc ?

You may well find that adding 50mm of 'insulating' plaster is not adequate insulation value to be worth your time and trouble. It may well also not be adequate to comply with building regs.

Like Tony said, you should consider sticking insulation board to the inside of the wall. The manufacturers have technical support to help you choose the right thickness and avoid any condensation problems. The final finish is plasterboard. This can all be done by general builder/joiner or DIY to keep costs down.

We did something similar, the bits we diy'd paid for themselves quickly, but the bits where we paid for labour were more expensive. However it did transform some rooms that had previously been cold and unpleasant to be in (in combination with insulating the floor and sorting out draughts and ventilation)

Good luck!

Thanks for everyone who commented.

After all the delays in our build, we're finally back on track and are still faced with the same issue of what material to use to insulate the internal walls in the bungalow.

My concern is the breathability of the walls if we just use sand cement then dot and dab plasterboard.

WillInAberdeen - We have a cavity wall that has polystyrene beads insulation from a lifetime ago which blows around the property through the vents. So is this considered as having no insulation at all? Both internal and external walls are brickwork.

DamonHD thanks for the link. I'm looking at Aerogel and have contacted Thermoblok and waiting for them to call back. Is this different from Spacetherm? I'm wondering whether it's possible to use this material just on the external facing walls to save on cost, then normal insulated plasterboard on the side attached to neighbour and the rear facing the extension. Thoughts?

Many thanks

Hi Kristinne, I used the first load of Gutex imported into Ireland 15 years ago for my brothers Passive House, it was very expensive and performed worse than the EPS we used in my sisters house 1 year later without issues. I've renovated many houses to PassivHaus standard my advise is focus 99% of your efforts on airtightness and not worry too much about breathability. In Ireland and the Uk PVP (Partial Vapour Pressure) drives water vapour into a wall for only 1 month of the year, for 4 months PVP is equal on both sides of the wall and for 7 months the wall's in drying mode.
The ability of Diathonite, IQ Therm and Gutex to buffer humidity (take in moisture when the humidity inside is high and release it back into the room later when the humidity is low) is reduced by 80% when you put a sheet of plasterboard over them and reduced further when the plasterboard's painted.
If it was my house I'd use Insulated plasterboard with 100mm Platinum EPS, this is cheap, breathable and every builder knows how to fit it, like tony said I'd foam around the perimeter of each board and I'd get the plasterers to plaster down to the floor behind the skirting.

<blockquote><cite>Posted By: Viking House</cite>Hi Kristinne, I used the first load of Gutex imported into Ireland 15 years ago for my brothers Passive House, it was very expensive and performed worse than the EPS we used in my sisters house 1 year later without issues. I've renovated many houses to PassivHaus standard my advise is focus 99% of your efforts on airtightness and not worry too much about breathability. In Ireland and the Uk PVP (Partial Vapour Pressure) drives water vapour into a wall for only 1 month of the year, for 4 months PVP is equal on both sides of the wall and for 7 months the wall's in drying mode...
</blockquote>

Viking House thanks very much for the reply. It's very helpful to hear your experience using one of the products I've mentioned. From what I've read, those products require the use of their own system to keep the breathability of the walls. Diathonite would only require a breathable paint on top and not a plasterboard but I agree with WillinAberdeen that a 50mm thickness won't be enough to improve on the current U-value. Whilst the IQ-therm needs its own fixings, render and again breathable paint.

The aerogel option sounds appealing at the minute so we don't lose too much space as it's already a small area. I suppose you're right about focusing on airtightness rather than breathability, in any case, we are planning to have an MVHR installed so it should extract the moisture in the building. But for us to have an MVHR would depend on how airtight the build is. I'll find it very difficult to justify an expensive MVHR unit if the building is leaky.

Hi Kristinne,

I've not really heard of concerns about breathability in a wall that is brick/cavity/brick before. Maybe you could give us some more background on how you arrived at the conclusions that it's so vital in this case. I'm not at this point saying that it's wrong, but it could be you are driving yourself down a route that has been chosen based on some early, well intentioned mis-information? Are you living in a very exposed, wind-driven-rain location? Even so...

Having an MVHR is IMO the right thing to do, and will eliminate many of the internal moisture issues, since it essentially brings the house to the same Absolute Humidity as outside (not the same Relative Humidity, in case that's mis-interpreted). That makes the wall breathability even less of an issue.

The points made by others above may well be a much better answer for you, but until everyone understands why you are so wedded to breathable walls (at high cost and low thermal performance), then we may not progress very far.

Hi Kristinne,

I'd suggest maybe you were given advice that may not be best suited to your property. The options noted above are essentially rigid insulation, with plasterboard over.

That can be polystyrene (either EPS or XPS), a polyurethane (PUR like kingspan etc), or a polyisocyanurate (PIR like Cellotex).

The boards can be bought pre-glued to the plasterboard (laminate) or you can put it on in layers. I even once glued plasterboard onto 100mm EPS using PVA glue, so you could even DIY it. That gives a solid build up ie. no void for services.

Or you can put on the insulation board, then a 25x50 timber batten, fixed through to the brick, then plasterboard, to give a service void for cables, and socket back boxes.

There isn't one correct way, or even necessarily wrong ways, it's down to budget, how thick you can stand the wall add-on to be, and if you've lots of sockets etc that need a service void. Watch out for cables touching polystyrene, as apparently over time it impacts the plastic cable sheathing, so it needs to go inside say some flexi duct.

Less than 50mm of insul isn't worth all the work and stuff going to landfill. 60mm thick of PUR/PIR is about equiv to 100mm EPS, and maybe 90mm XPS in terms of thermal performance. EPS is a bit more vapour open, though wouldn't be classed as breathable. Of course EPS is cheapest, then XPS, then PUR/PIR. PIR has better properties in case of fire, where as EPS is pretty poor (unclassified I believe, as it just melts and then drips burning blobs), but it's behind plasterboard, so.....????

Tricky bit is around windows and doors (penetrations of the insulated wall), as of course you now have to increase window cills and reveals, door frames...

If you can focus on selecting from above, then hopefully you'll start to narrow down the options and also the price. Not all walls have to be done the same way. Choose what suits each room, north wall thicker insul, wall at TV with loads of cables do with battens, etc