Save the planet by insulating your house

A surprisingly large fraction of the energy used in developed countries is used heating and lighting buildings – in the European Union 40% of energy used is in buildings. This is an obvious place to look for savings if one is trying to reduce energy consumption without compromising economic activity. A few weeks ago, I reported a talk by Colin Humphreys explaining how much energy could be saved by replacing conventional lighting by light emitting diodes. A recent report commissioned by the UK Government’s Department for Environment, Food and Rural Affairs, Environmentally beneficial nanotechnology – Barriers and Opportunities (PDF file) ranks building insulation as one of the areas in which nanotechnology could make a substantial and immediate contribution to saving energy.

The problem doesn’t arise so much from new buildings; current building regulations in the UK and the EU are quite strict, and the technologies for making very heat efficient buildings are fairly well understood, even if they aren’t always used to the full. It is the existing building stock that is the problem. My own house illustrates this very well; its 3 foot thick solid limestone walls look as handsome and sturdy as when they were built 150 years ago, but the absence of a cavity makes them very poor insulators. To bring them up to modern insulating standards I’d need to dryline the walls with plasterboard with a foam-filled cavity, at a thickness that would lose a significant amount of the interior volume of the rooms. Is their some magic nanotechnology enabled solution that would allow us to retrofit proper insulation to the existing housing stock in an acceptable way?

The claims made by manufacturers of various products in this area are not always crystal clear, so its worth reminding ourself of the basic physics. Heat is transferred by convection, conduction and radiation. Stopping convection is essentially a matter of controlling the drafts. The amount of heat transmitted by conduction is proportional to the difference of temperature, the thickness of the material, and a material constant called the thermal conductivity. For solids like brick, concrete and glass thermal conductivities are around 0.6 – 0.8 W/m.K. As everyone knows, still air is a very good thermal insulator, with a thermal conductivity of 0.024 W/m.K, and the goal of traditional insulation materials, from sheeps’ wool to plastic foam, is to trap air to exploit its insulating properties. Standard building insulation is made from materials like polyurethane foam, are actually pretty good. A typical commercial product has a value of thermal conductivity of 0.021 W/m.K; it manages to do a bit better than pure air because the holes in the foam are actually filled with a gas that is heavier than air.

The best known thermal insulators are the fascinating materials known as aerogels. These are incredibly diffuse foams – their densities can be as low as 2 mg/cm3, not much more than air – that resemble nothing as much as solidified smoke. One makes an aerogel by making a cross-linked gel (typically from water soluble polymers of silica) and then drying it above the critical point of the solvent, preserving the structure of the gel in which the strands are essentially single molecules. An aerogel can have a thermal conductivity around 0.008 W/m.K. This is substantially less than the conductivity of the air it traps, essentially because the nanscale strands of material disrupt the transport of the gas molecules.

Aerogels have been known for a long time, mostly as a laboratory curiousity, with some applications in space where their outstanding properties have justified their very high expense. But it seems that there have been some significant process improvements that have brought the price down to a point where one could envisage using them in the building trade. One of the companies active in this area is the US-based Aspen Aerogels, which markets sheets of aerogel made, for strength, in a fabric matrix. These have a thermal conductivity in the range 0.012 – 0.015 W/m.K. This represents a worthwhile improvement on the standard PU foams. However, one shouldn’t overstate its impact; this means to achieve a given level of thermal insulation one needs an insulating sheet a bit more than half the thickness of a standard material.

Another product, from a company called Industrial Nanotech Inc, looks more radical in its impact. This is essentially an insulating paint; the makers claim that three layers of this material – Nansulate will provide significant insulation. If true, this would be very important, as it would easily and cheaply solve the problem of retrofitting insulation to the existing housing stock. So, is the claim plausible?

The company’s website gives little in the way of detail, either of the composition of the product or, in quantitative terms, its effectiveness as an insulator. The active ingredient is referred to as “hydro-NM-Oxide”, a term not well known in science. However, a recent patent filed by the inventor gives us some clues. US patent 7,144,522 discloses an insulating coating consisting of aerogel particles in a paint matrix. This has a thermal conductivity of 0.104 W/m.K. This is probably pretty good for a paint, but it is quite a lot worse than typical insulating foams. What, of course, makes matters much worse is that as a paint it will be applied as a very thin film (the recommended procedure is to use three coats, giving a dry thickness of 7.5 mils, a little less than 0.2 millimeters. Since one needs a thickness of at least 70 millimeters of polyurethane foam to achieve an acceptable value of thermal insulation (U value of 0.35 W/m2.K) it’s difficult to see how a layer that is both 350 times thinner than this, and with a significantly higher value of thermal conductivity, could make a significant contribution to the thermal insulation of a building.

16 thoughts on “Save the planet by insulating your house”

  1. Aerogels are interesting materials, but I think their application to insulation in this situation is marginal.

    Firstly, there are a lot of other measures that can be implemented before taking drastic action on the walls. The roof and floor can be insulated, windows and doors can be insulated and draughts can be stopped.

    Secondly, older houses tend to have larger rooms, so losing some space does not mean they become unusable. If they are small, there’d need to be a pretty special reason for keeping them in the housing stock. Also, insulation can be applied outside the wall in some proportion of cases.

    So conventional insulation can deal with most situations, I believe. A solution to most of the questions that arise in this context could be personal tradable carbon allowances and free choices by individuals on property, transport etc.

    The DEFRA report seems over-prescriptive to me. For example it says ‘Recommendations include: … include in government estate procurement specifications highly insulating nanotechnology based windows’. The government procurement specification should merely state an overall energy target for its buildings. What particular technologies are developed and used to meet that target should be left to the market.

    What the government should also do is provide adequate funding for research so that there are some technologies that can be developed!

  2. Richard,
    If you can insulate on the outside of your house (then add a facade), you will incorporate a large thermal mass which will moderate temperature swings in your home throughout the year.

  3. While Aerogel, has its limitations, it has made some major inroads into the world of Architectural design.
    A stunning example on the high end is Santiago Calatrava’s Milwauki Museum of Art [] and on the less ‘glamorous’ end, Polycarbonate Housing [] which started in Japan and is spreading virally around the world.

    One of the advantages of Aerogel is that it is Cradle To Cradle [ ], an idea expounded on by Bruce Mau’s ‘Massive Change []. It is an area discussed not often enough in the annals of Nano.

  4. The “K” value for Nansulate of .14 gives “R” value of 5.88
    The “K” value for 70mm of polyurethane foam of .35 gives “R” value of 2.86

    Conclusion in article is wrong or data is incorrect.
    If data is correct, conclusion should be Nansulate will make a significant contribution to the thermal insulation of a building.


  5. I’m not sure what units you’re using, John. I’m in SI, where the thermal transmittance U (in W/m^2.K) is obtained by dividing the thermal conductivity k (sometimes also called lambda) (in W/m.K) by the thickness of the layer (in m).

    So for 70 mm of PU foam we have U = 0.021/0.07 = 0.3 W/m^2.K, which is comfortably smaller than the UK building reg. value for area weighted U for a wall of 0.35 W/m^2.K.

    For 0.2 mm of paint with k = 0.104 we have U = 0.104/.0002 = 520 W/m^2.K, a much higher value than that required.

    R is 1/U, so we have R for the foam of 3.33 m^2.K/W and R for the paint of 0.002 m^2.K/W. In terms of R values (which of course are additive), we need to get to a total value of about 2.9 m^2.K/W, so my conclusion, that it’s difficult to see how paint, at these values of thickness and thermal conductivity, can make much of a contribution to house insulation, stands.

    Jim and Dave, insulating the outside of a house is of course in some cases possible, but it is more difficult and expensive because the coating needs to be weatherproof. Often it is undesirable for aesthetic reasons (I’d never get planning permission to do it, for example, because the vernacular stone architecture where I live is regarded rightly as an important contributor to the beauty of the landscape). Of course, one usually could lose some interior space, but generally people don’t want to. I absolutely agree with Dave, though, that there are many other things one should do first, in terms of upgrading the glazing and banishing draughts.

    I also agree with Dave that it would be wrong to be overprescriptive about the materials one specifies. Actually, I suspect that approach would be in fact contrary to EU law. But, undoubtedly the use of stricter standards in government procurement will be an increasingly important way of driving innovation in what is rather a conservative industry. And yes, more research is needed.

    Thanks for the links, Martin. I’ve known about aerogels for a while, but I’m surprised how far they have come towards mass use.

    Jim, another interesting way to increase thermal mass is to use phase change materials, like this BASF product.

  6. Richard,

    I do sympathise with those who don’t want to lose interior space, and with those who want to just wear T-shirt and shorts in winter, and those who want to fly somewhere warm in winter! But we all have to make compromises, and we’ll all need to make a lot more unless technological progress suddenly accelerates to reduce our need for fossil fuels. I absolutely agree with Smalley’s prioritization of research into alternative energy sources.

    On a more positive note, I should have mentioned that I think aerogels are potentially very useful in photovoltaics and as catalyst carriers for various reactions related to fuels, chemical feedstocks and carbon sequestration.

    I agree with Jim that stone is a good thermal mass, as is water, and I suspect both are a good bit cheaper than the BASF product. I also see it’s a hydrocarbon-based product so I hope it’s not derived from fossil fuels!


    I expect I’m confused, but I didn’t see where aerogels came into either the museum or the polycarbonate house project?

  7. Straying from the nano theme slightly, internal insulation is a problem that has been vexing me for quite a while. Like Richard, I have an old stone house with solid walls typically 18″/0.5m. The walls are constructed with the local limestone on the outer surfaces with, I gather, whatever stone/rubble the builders had available as in-fill. The thermal mass is significant – you only need to walk past the outside of a south facing wall after a sunny day to feel the heat being re-radiated out! This does seem to mean that the house is cool most of the year.

    You can buy plasterboard specifically for insulating dry-lining which has a plasterboard already bonded to phenolic foam. This will save building the studding framework and make the job slightly easier (you can use the “dot and dab” adhesive technique). E.g. Gryproc:
    or Knauf:

    Knauf claims 2.58m2K/W for the 65mm thick version, i.e. 0.39 W/m2K.

    There are however several other aspects to the process:
    * moisture: old houses were designed to breathe – most rooms had fireplaces and plenty of drafts; many had had breathable finishes (lime plaster, limewash, distemper etc). Applying an impermeable inner membrane could have disastrous long-term consequences for the fabric of the house.
    * cold-bridging: finishing around window reveals looks particularly tricky, especially if you’re not changing the windows at the same time.
    * decoration: coving and other period features would need to be re-instated (and I only have to look at mine for it to crumble!)
    * services: re-routing the plumbing and electrics could be a bit tedious, especially given that radiators are usually on outside walls under the windows.

    Of these, I think moisture consideration on the ground floor is the most difficult to manage. You only have to look at the horror stories of various so-called “damp proofing” techniques (all best avoided in my opinion) to realise it’s not simple!

    The key issue for me is that there’s not a lot freely-available “best practices” yet for this kind of thing. I look to institutions like the BRE to do serious practical research into these techniques and provide reliable advice to everyone.


  8. Dave

    I was remiss in being more explicit in my reference to the Museum and The Polycarbonate House.

    Both projects, one being on the high end and the other being an alternate housing project is Tokyo which has developed virally over the past few years, use a GE product, Thermoclear.

    Here are three references, one of which is a Cabot Press Release, which explain the product itself further.

    Working in the field where Housing is one of the primary concerns, we have found that this technology, while initially costly, resonate well with our target group. The overall benefit come with the fact that when a person moves on. The structure can be broken down with the Aerogel fully recovered and the sheet structure put through a regrinder and sent back to the plant to be made into more product.

    In ideal circumstances, we would have our own extruder to make new panels, but that may be a ways off.

  9. Martin,

    Thank you, I think I now understand. Cabot make a product by taking ordinary Thermoclear panels and filling them with an aerogel. That’s very interesting – I’d no idea that was available. But the polycarbonate house just uses ordinary polycarbonate panels as far as I can tell from the photos, yes?


    Yes, bringing old properties up to current standards (let alone Passivhaus etc) can be difficult and expensive. There’s already a requirement to do this remediation for larger buildings, so there are techniques that have been developed and used on schools etc. I guess these will be adapted for private houses as the need becomes greater and the legislation is extended. But sadly knocking them down and building ‘little boxes made of ticky-tacky’ to replace them may be the only feasible solution for some old buildings.

  10. Don’t want to insulate the outer walls for aesthetic and building code reasons, and don’t want to insulate the interior walls else living space is lost. But still want insulation…
    A radical solution might be to drill holes in the limestone at very shallow angles, from both the outside and the inside. Fill the holes with aerogels or fibreglass or whatever. I don’t think it is too hard to drill into limestone. I would suggest a skylight or windows facing south, but it is Britain after all.

    A government office building under construction here is designed to be green; phase change walls, solar hot water heating…the construction cost estimate has ballooned from $50-$70 million to $250 million (I think). Much of this is commodity price increases. Ideally when an environmental engineering solution isn’t economical yet, it would be best to go with the conventional solution and spend the savings on R+D-ing the (oft nanotech) environmental alternative. That last part (investment deflating the cost of novel products) is overlooked by the market because hundreds of trillions of dollars of ecological capital goes uncosted.

  11. I’m not worried so much about the loss of space of inner insulation (even a small room should be able to cope with say a total 6″/150mm loss) – afterall many old houses have larger rooms than modern day houses anyway.

    Injecting the limestone with something is an interesting idea, but I’d be worried about the implications of moisture. For example, I had a wall that had a waterproof sand/cement render applied to the inside about 30 years ago and in that area you can quite see where the outside stonework has deteriorated (apparently due to additional moisture). The big problem is that you don’t necessarily see the consequences for some time and such damage can be very costly to repair.

    As for knocking down old houses and replacing with new, I suppose there would eventually be a carbon payoff, but it might be quite a while compared to just improving the specification (ignoring the heritage aspects too of course).

  12. Another alternative is to chisel away a 1/2 foot of limestone from the interior wall (if the structural integrity of the building isn’t compromised) and use the space for insulation. The invention of the cannon renders this whole discussion obsolete anyway.

  13. Dave, it’s very easy to say what we all ought to do, but finding some way that’s in the realm of practical politics to get people to do it is much more difficult. While the government can do something about retrofitting public buildings, since very little of the UK’s housing stock is publically owned any more there’s much less they can do about that. I think it is simply impractical to imagine that current building regulations could ever be retrospectively applied to old houses. I suspect readers from outside the UK would be surprised at just how old the UK’s housing stock is, and how much people like it that way. We’re not just talking about picturesque stone houses in the country; there’s the huge mass of late Victorian and Edwardian terraced houses in the cities as well as all those 30’s semis. In my view the best we can do is make sure good technical solutions are available at a cost low enough that the energy savings themselves provide sufficient incentive.

    Simon, you raise many very valid points about the difficulties of insulating old houses (I like your blog, may I say). The points about dampness control are particularly important – my house too has no damp course. The figures you give are interesting, but they do show that while dot and dabbing insulating plasterboard would certainly made a big improvement it probably wouldn’t get your house to current building reg standards. A few years ago we converted a small, late Victorian factory building to a house for my parents to live in. Since this entailed change of use planning permission it did have to meet current building regs, and to achieve this we ended up having to have a completely new blockwork wall built inside the original facade, to provide a cavity to fill with insulation. The result was very good but it was ferociously expensive, in the end almost certainly costing more than a complete new build.

    Phillip, I don’t think drilling would work, as it would be extremely difficult to avoid leaving solid bridges. It would be immensely labour intensive and expensive, anyway. (And no-one in the UK is going to be able to find a builder for the next year as they’ll all be busy fixing flood damage). Chiselling the inside’s not a very attractive idea, either. The dust from century-old horsehair reinforced plaster is undoubtedly one of the most unpleasant things you can unleash on a house!

  14. Quick response to Dave. The original house was not insulated but the design has been expanded upon and the panels made available with the aerogel. Handling aerogel can be tricky in that is is 95% Air and literally disappears. We have a system in place for assembling the panels and promote their use in marginal and temporary housing scenarios.

    Richard – your comment about the socio-political issues surrounding the whole issue of conservation is a valid one. I had some involvement with the construction of a 2200 SqFt facility in Metchosen, near Victoria BC. When the project opened, government officials commented that we had ‘Raised the Bar’ too high for such facilities. One of the features is a Physical Plant system which recovers 85% of the heat through an Air Changing system that cycle the whole building every 17 Minutes.
    Fives years into the occupancy of the facility the Heating/Ventilation cost for the building averages out at CDN $150.-/Month, compare to almost 3X that figure for a conventional building.
    My point is not whether you can afford to conserve, but you can afford not to. There are many thing one can do other than making major structural changes to the building, and working as you do in a field where ‘Thinking outside the box’ is a regular occurrence the same brief should apply to energy conservation.

  15. Those cost overruns for a new “gold LEED” office building under construction are only about 10%. That’s only a year or two of utility savings. Even with construction cost and commodity price increases it still pays to build green.

  16. Go with heated wood floors and a cooler room air temperatures. That should be pretty comfortable, and probably healthier than lots of hot air.

    Something else to consider – if you have cold stone walls, there’ll likely be a layer of cooler air falling next to them, while warm air rises to the ceiling. That should mean less heat is being lost through the walls than one might calculate based on average room temperature. So insulate your ceilings first – I presume THEY are not made of half a meter of limestone. That should also slow the convection flows down the walls, meaning less warm air moving into contact with them and less drafts flowing out along the floor to chill your feet.

    You might also try “floating” floors – basically a small gap between the outer wall and the floor, so that cold sinking air along the walls falls straight to a basement (or into a cold air intake duct, if you have no basement), to be sucked into a furnace. Again, that mainly is to avoid having drafts that chill your toes and make you want to crank up the heat.

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