Don’t play with radon!

Our house, built into a slope and on a rock seam, also lies within a medium-high radon risk area, according to PHE/UKradon. A while back, I applied to UKradon for a home testing kit, which comprised three dosimeters: one for each floor. When the results came back in from the laboratory, some of my fears were confirmed. Ideally, levels should be below 100 (Bq/m³), but not exceed 200.

Our results were as follows:

  • Bedroom (top floor): 37
  • Living Room (ground floor): 85
  • Kitchen/diner: 290!

I suspected the source to be ground radon. The kitchen/diner and extension is built on a new slab with a gas-proof barrier (see earlier post). But, the undercroft beneath the original house simply has a concrete slurry cap (nominal 4in thick) poured on top of the subsoil – I believe the radon is permeating through this cap. The section below has been highlighted to show the suspected radon source (red):

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fabric performance – thermography

front LHS2 IR

The fist thermal images of the finished house were taken very early in the morning at the end of January. The results are very encouraging and show, as expected, that the insulation measures are working well, and that we have significantly reduced the amount of thermal bridges, particularly for the concrete gutters. The image above shows our house in the foreground with our neighbours’ houses beyond. The temperatures of the outside of our house are the same as unheated surroundings: vehicles, plants, etc. Compared to neighbouring houses, not only are the surfaces cooler (less heat escaping), but the temperatures are more uniform, showing good insulation continuity.

front LHS IR

The image above was taken further up the street, with our house now in the background. Our neighbour’s house in the foreground has the same thermal properties as our house did when we moved in, i.e. no cavity wall insulation. The difference is quite striking between our properties.

rear IR

From the rear, the house appears to be performing well too. The wall insulation is continuous and the thermal bridging relating to the concrete gutters (as shown in an earlier post) have been eliminated. There is a small amount of thermal bridging around the new window frames, but this is expected given that we opted for less expensive windows that do not have thermal breaks in the core of the frame.

rear neighbour IR

By contrast, our neighbour’s house is losing more heat, as shown in the image above. The thermal bridges from the concrete gutters and the single leaf masonry panel beneath the windows are quite evident.

wemico trays

We have introduced some bridges that we hadn’t thought about. Ironically, it is from the steel trays that carry the external wall insulation panels that are conducting heat. These are fixed to the masonry (warm side of the insulation) and some of this heat is clearly coming through. Perhaps plastic trays would have been better. In the grand scheme of things, this is relatively minor stuff.

green roof installation

Finished sedum roof

There has been a bit of a pause in the postings to this blog, mostly because we are busy decorating, which is a little boring to do, let alone write about. One thing that we have had done lately is the sedum roof over the extension and the porch.

The sedum layer has been laid by the original single-ply roofing installer, who now specialises in sedum and wild flower roofs. Before the sedum is laid, a geotextile membrane is laid on top of the water proof layer of the roof and the sedum drainage layer is placed on top as shown in the photo below. The drainage layer is made from recycled plastic and also contains a network of cups to store water and keep the sedum happy during dry weather. The organic substrate is applied on top of the drainage layer and the sedum is rolled out across this.

Drainage layer Substrate layer

The sedum was installed about three weeks ago now and has taken well (thanks to the break in the dry weather). The addition of the sedum roof should offer a number of benefits, which include: extending the roof life (less exposed to UV); reduce summer overheating in the studio space below; and reduce surface water run-off. Most of all it looks great.

weeks 30 to 32: plastering and rendering

External base render - front

Yes, I know I’ve taken a break from updating this blog for two or three weeks, but things have been a tad busy with work, so apologies for that.

A lot has happened since the last update. Most of this has been on the internal plastering and the external rendering. Externally, the house and extension have now been rendered with two coats + mesh of Parex render base. The top coat – a through colour layer will be added next week, although the colour is only a shade different to the base layer. As we have insulation down to ground level, the render will continue below the drip profile rail, but this will be after the scaffolding has been removed. So far it looks good. The area around the front door (covered in a black wind check barrier) will be clad in sweet chestnut, which should also be added starting next week.

External base render - side

Plastering has gone really well. We have used a local plasterer, recommended by our builder, and the quality of his work and pace is quite exceptional. In the three weeks since the last blog post, he has managed to plaster the entire extension area + a bit of extra work in the existing house.

Kitchen_diner plastered Stairwell plastered

The photo above right shows the staircase now installed. This was also put in during the last three weeks and it looks great – it is a softwood stair with a curve formed for the turn at the top – the joiner has done a beautiful job, which is complemented by a curved wall, plastered to perfection.



weeks 28 and 29: more progress on external wall insulation

Fron of house EWI

The external insulation progressed well (until we ran out) and the house is beginning to be transformed. I’m sure we’re the talk of the neighbourhood – ‘what on earth are they doing?’ Weather permitting, the rendering will start next week. and it will begin to look a bit more normal.

North Front EWI

Another week, or so and the insulation should be complete on the external – a fair bit to do on internal insulation works – particularly suspended floor and loft.

weeks 28-29: staircase goes in

Stairs from bottom Stairs from top

The new staircase was installed over the last couple of weeks. We were keen to have a curve on the turn on the stairs at the top and the joiner has made a lovely job of it. As the photo top left shows, the carpenters have started to build a curved wall from the hall to wrap around the stairs. Access to our new extension at last – feels like we are getting nearer to the end of the build now!

week 27: external wall insulation

External wall insulation

Having been delivered some 6 weeks ago (see earlier post), the external wall insulation is now starting to go on. Starting with the extension late last week, but in earnest this week to (almost) complete some 80m² of wall area. The main (original) house is left to do, it should be done in approximately 3 weeks time if we manage to keep this pace up.

We are using two layers of Kingspan K5 insulation boards. These are 60mm thick giving a total of 120mm external insulation. The first board gets bonded to the wall using Knauf Marmorit SM700. After this has gone off, the second insulation layer is applied with the boards offset in order to cover any air gaps between boards in the first layer. This second layer is mechanically fixed using thermally-broken fixings through both boards and into the masonry.

EWI - new kitchen  EWI - side wall

The wall insulation will eventually be finished with a mix of through-colour render and splayed chestnut timber cladding. In the meantime the house will look like a marshmallow!

week 27: breakthrough

Breakthrough prepraration

Finally…the breakthrough from our hall into the extension happened earlier this week. The preparation for this took place some weeks ago with the insertion of the steel and the removal of the outer brick leaf as the photo above shows. I had some great pictures of this weeks breakthrough happening, but my SD card was destroyed when I tried to transfer the pictures – lost forever! I have used an earlier ‘before’ shot below alongside the breakthrough shot…

just moved in (hall)  Breakthrough

A better picture of the breakthrough will replace the photo (above right). And, yes, I know it’s an odd place to put insulation, but until the stairs are in (next week), we need to keep our inquisitive dog out of the way.

week 26: balcony taking shape

balcony and support posts   balcony construction

The construction of the balcony is well under way. The design is for a ground supported type, rather than a cantilever that we thought we might have. But we have managed to limit to three support columns, which is fine – a feature perhaps. Balconies are a real problem when it comes to thermal bridging. There are a number of proprietary thermal break products that you can now get for cantilever balconies providing the deck is steel, or concrete. But for a little timber addition to our extension roof, it was proving a little over-complicated to do a cantilever option, whilst at the same time not over-compromising the thermal integrity of the building.

The ground supported solution is an independent structure, but still compromises the thermal envelope slightly as it needs to be brace back to the building. The top left picture shows, the timber ledger used to brace to the building. This is 60mm (thick) timber, so takes the place of one layer of external wall insulation. It will still have one layer of 60mm insulation over the top. However, this increases the U-value in the area of the ledger brace from 0.14 W/m².k (for the rest of the extension walls) to 0.19 W/m².k. This is fairly minor overall, and would be worried if we were after EnerPHit certification – but we’re not. Interested if anyone else has had similar dilemmas?