Internal insulation for big rooms with no cornice

In the kitchen, there is no cornice (someone had put in a 1970s sauna style tongue and groove pine ceiling that fell off when we looked at it hard) and it’s a reasonably large room, so we used a thicker internal insulation instead of the aerogel.

This is Udi In Reco, 120mm thick, similar to the external insulation but with a special smooth internal render as the vapour control layer.  This fixed to the wall with much less trouble than the aerogel, using adjustable long screws with plastic plugs on the end.  It did mean some joinery work to extend the window cills and surrounds, but these were made with recycled old timber from other parts of the house so the colours match.

You can buy this insulation from Back to Earth.


Floor insulation

We have now insulated beneath all of the ground floor by removing the cellar ceiling and stuffing the gap between the joists with 200mm of hemp (except over the biomass boiler where we used 200mm of rockwool).

This was  a lovely material – cuts with a bread knife, smells like harvest time, and no more freezing cold feet. By cutting it slightly large, it holds itself in place with no fixings.  It was supplied by Back to Earth (see links for more info).

Eventually we’ll put the plasterboard back, which will be even better.

Internal insulation

We finally did the job of removing the plaster from the end wall of each room (the gable wall is already insulated externally) and replacing it with Fermacell board with a triple layer of aerogel bonded to it (27mm thickness in total).  The main problems were the dust created by the plaster removal, which gets around any attempt to sheet off the doorway of the room involved, and actually fixing the boards to uneven Victorian brickwork.  We didn’t use a nail gun as the bricks vary wildly in softness and quality of fix, so in the end after much trial and error (for which read swearing, in both English and Polish) the fixing method was : cut the boards outside (wearing dust masks because aerogel dust is really unpleasant), drill a hole through the board only then take a sharpened 6″ nail to make a mechanical hole in the polyester mat, then a cheap soldering iron with a long thin bit used to melt the polyester out of the way, then a tool of a brass tube in a piece of plywood is put through the hole to shield the drill bit while drilling a 4mm hole in the brickwork, remove brass tube and replace with piece of pub drinking straw and screw in fixing.  The pub straw was later ditched in favour of a frame screw which already has a plastic plug on it.  BUT calculatation suggest, and the thermal camera confirms, that this fixing method creates significant cold bridges which reduces the efficiency of the board.  If we were able to find a way of countersinking it we would use the plastic fixings with a nylon pin to produce no cold bridge, but these need to be recessed 3mm into the Fermacell if you want to be able to use the Fermacell surface finish and avoid plastering.  If plastering is not a problem these could be used without countersinking and plastered over.  We kept the board depth to a minimum and did not remove the cornices in the room, so the board just replaces the original plaster layer.

While this work was going on we also found huge amounts of cold air was getting in around some of the sash windows – as soon as the window surround was removed it was clear that a second world war bomb had blown the windows in and they had been shoved back out without being properly refixed (in 1941 judging by the newspapers packing the spaces).  So we were able to seal up these air paths too.  Next summer we need to replace the pointing around all the windows while they are repainted.

The good news is that the house feels significantly warmer!

Wall Insulation

Gable wall – external ly insulated – Udireco rendered 170mm recycled woodfibre. Front and back walls – internally insulated (bought but not yet fitted) 27mm Spacetherm glued to Fermacel board. Outrigger – tbc.

This took ages to work out, trying to marry up recyclability with embodied energy and while avoiding pervasive toxins, environmental degradation and derivatives of fossil fuels. Originally we were going to use Foamglas externally on the gable and internally on the front and back but the price was very high for what it is, and while the product is recycled, it is from a very high grade feedstock. Also this would have meant the whole house became entirely vapour sealed which splits commentators as to how bad this is.


Once deciding on breathability as key there are very few choices – mineral wool or wood fibre with either lime or diffusion open polymer render. They are pretty closely matched on embodied energy and toxicity. The tip to the balance was from sequestered carbon, the benign properties of wood fibre over glass, and UDI’s sustainability efforts through the entire product – glues, etc combined with the service from them and their UK agents Back to Earth. This produced a U-value of 0.2W/m/K


Breathable internal insulation is not advised here in the UK, internal insulation can cause issue anyway as the internal surface of the brickwork under the insulation will most likely be below dewpoint for much of the year so any water vapour there will turn into water so the strategies deployed are usually to prevent it getting there in the first place or ventilating the space between the lining and the masonry.

Add to this the fact that we want to keep the original plasterwork details – nothing spectacular, but part of the character of the place and our choices are very limited. The plaster is quite deep so we’ve got about 40mm to play with. The desire to avoid fossil fuels and foams becomes a prohibition internally as most of these materials would add considerably to the toxic load in a fire. Spacetherm uses aerogel impregnated into a mat, glued to a board it has the highest insulating properties after vacuum insulating panels – 0.13 W/m2/K and while high in embodied energy very little is used it is non toxic and enabels the walls to reach a U-value of 0.38 without thickening the wall. Its key disadvantage is cost, but it can be fixed directly to the wall with no framing or glueing so the cost of the material should be offset by the ease of installation. The boards have a built in vapour barrier and the mat itself is hygrophobic so with so little space in which vapour can gather the problem of interstitial condensation should be minimised.

The price of the Spacetherm is so high that where there are no plaster details left and window reveals can be adapted there is still room for another solution. As this is a pilot the intention is to try out a breathing internal wall insulation. There are 2 wood fibre solutions out there.

Roof Insulation

Recycled slates on TLX Gold vapour permeable multifoil sarking overall, over either formaldehyde free glasswool to 350mm, high density glass-fibre batts on Spacetherm or Thermafleece sheep’s wool between built up rafters, underdrawn with plasterboard

There are 3 configurations to the the roof insulation once past the covering:

The remaining floorspace above the attic rooms, in which we have simply topped up the glasswool, down the middle we have used some unwanted rigid insulation to create a crawlway now the insulation is so deep, to get to the solar plumbing. U-value 0.1 W/m2/K

The main slope of the roof has had the rafters built up to accommodate 200mm Thermafleece and is then underdrawn with 10mm plasterboard and skim with no vapour barrier, so fully vapour permeable. U-value 0.143 W/m2/K

IMG_0688 sml

The area under the half gable and dormers where space is at a premium has 100mm Hi-Cav 32 rigid glass wool batts, at the time of installation formaldehyde free was not available but to get the highest insulation value for the smallest thickness this is undrawn with 27mm of Spacetherm on Fermacel board which is vapour impermeable. U-value 0.148 W/m2/K