Saturday, February 26, 2011

Underhill ‘The Stealth House’

This extravagant looking Underhill Passive House Project is situated in a very prominent location at the top of a hill in the Cotswolds Area in West Central England. On the 29th of Jan 2010 the project passed Passive House Certification, and is therefore the first certified domestic Passive House in England. Being dug into the hill to be invisible from the surrounding countryside, it is essentially a stealth house, with absolute minimal visual as well as environmental impact on landscape. The house is entirely glazed to the south, and the rest of it being earth-sheltered and therefore highly insulated creates the perfect passive solar design. The structure of this underground house is entirely concrete, much of which is left exposed internally to exploit the benefits of its thermal mass. It is insulated and waterproofed externally for the same reason.

The extension is joined to the existing old barn which also was partly renovated with the total treated floor area being 358m². The construction method consisted of externally insulated concrete walls, floors and ceilings. All the insulation used in the house came from Dow Building Solutions in England. The external walls are 220mm thick with the underground U-Value being 0.117 W/ (m²K), also containing Fosroc drains with 310mm Perimate DI-A insulation. In relation to above the ground it has a U-value of 0.153 W/ (m²K) with 250mm insulation and Lotusan acrylic render finish. The floor slab is 250mm concrete; screed and resin finish over 250mm Floormate 300-A insulation and a U-Value of 0.146 W/ (m²K). 

The ceiling consisted of a 250mm hollow core concrete slab and screed with 360mm Roofmate SL-A insulation, also some of the earth has the ceiling partially insulated and the U-Value is 0.085 W/ (m²K). The windows installed in the house are Optiwin Alu-2-Wood which had an overall Uw of 0.74 W/ (m²K) and a G-Value of 50%. Optiwn Frostkorken also provided the front door with a U-Value of 0.72 W/ (m²).
Ventilation in the house is produced by a Paul Campus 500DC balanced mechanical ventilation system which is a Passive House heat recovery unit with 83% efficiency and 0.28 Wh /m³ energy consumption. The heating is from a Woodfire F12 wood burning stove with a back boiler, sealed to room air. 2 post heaters (water based) in air supply ducts distribute the heat. For the domestic hot water a Solex solar thermal roof with 40m² collector area was installed with a 2000 litre accumulator tank; in winter via back boiler from wood stove, it will be backed up through the electric immersion heater.
Key figures:
Air tightness:
n50 = 0.22/h; q50 = 0.23m3/(m2h)
Heating demand:
13 kWh/(m²a) according PHPP
Heat load:
9 W/(m²) according PHPP
Primary energy demand:
62 kWh/(m²a) for heating, DHW, electrical consumption for appliances, etc. (according PHPP)
The house is achieving 90% energy savings over that of an average house. Most of the concrete structure is exposed internally and using this thermal mass from the concrete structure alongside highly-durable insulation has resulted in a highly energy efficient building.


  1. Very good example Eoin, and defiantly well suited to Ireland, with a similar climate. The fact that the house is built on top of a hill, and more or less built into the hill so the dwelling can achieve thermal gains is a very good idea, but could be very hard to re-apply or find a suitable site. I suppose what I am trying to say is this Passive house style site specific. If it was in an urban location, it would prove even more environmentally friendly, due to the green roof concept where the design could cause a reduction in urban heat island effect. Just in relation to the extension joined to the existing old barn which also was partly renovated, would this provide the same high energy efficiency standards as the rest of the dwelling?

  2. That a very interesting case study you have chosen Eoin. I see that the building is an extension onto the exiting old barn. I assume that the barn originaly wasnt built with passive standard in mind therfore there would be plenty of thermal bridges which would affect the thermal envelope of the building as a whole. Just wondering if you know how was the new build was incorporated into the existing barn to continue the thermal envelope where the new build meets the existing.

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  4. The barn was very carefully restored and is now used currently in the dwelling as an office for the Architect who lives in the house. I came across a website which explains how the work was carried out on the barn and how they managed to insulate the barn (

  5. Good Man Egg, this certainly is a very unique and revolutionary design. I would imagine the way the hill is incorporated into the building to the north and the fact that the whole of the underground structure is concrete means that the thermal mass in these areas are very high. I would imagine that these elements mean that the use of large amounts of insulation to reduce heat loss (like we seen in the Salthill Passive House on our site visit recently) is not required. Is there any idea on how much insulation was used in total in comparison to that of a normal passive house?
    I recently read an article about the former Manchester United Captain and Environmental enthusiast Gary Neville who had applied for planning permission to build a stealth house at a side near Bolton. This would be interesting to look at and is along the same the lines of this development although the scale of this proposed building is very large. I also seen that this house has just recently in the last week been granted planning permission so I think it could be interesting to follow that development . I know that the thermal mass gained from these types of buildings are massive but I was just wondering what the situation was with the shading element to the south. I have seen from the pictures that the hill to the south of the south face is sloped into the house. I was just wondering was there calculations made for the shading in this area. I would imagine that during the winter period when the sun is at lower angle the hill could in fact block some of that much needed winter sunlight to the south face. Just a query, but it would be interesting to know was there stipulations on the hill been put back to a certain height in order to hide the house, resulting in less sunlight being attainable.