The purpose is to reduce heating costs as much as possible in winters, where the temperature rarely drops below -5 degrees Celsius (23 F) at nights and 0 degree Celsius (32 F) daytime.

To me, internal wall insulation seems to be a better solution than the external one. Because I think the air inside will heat up more quickly that way, which -seemingly- would be more comfortable than waiting for the walls to warm up.

in another question, one community user (@Móż) mentioned strawbale for insulation. After a quick search I saw that it can be used as building material. But it is weak against moisture and mold.

So, considering factors like:

  1. Health (should be made from a healthy material)
  2. Material cost (should be as cheap and abundant as possible)
  3. Robustness (should stay intact with time and mild accidental impacts)
  4. Moisture resistance (should be resistant to moisture inside the house, maybe can be dealt with a proper but thin coating)
  5. Organisms (should not allow organisms like mold or insects to grow inside)

what do you think would be the most economical and convenient inner wall insulation solution for winters?

P.S. It would be nice if you include the other important factors that come to your mind.

The house on my mind is a single house, maybe two stores. It is located around Mediterranean (Europe) and might be exposed to some north wind.

By 'economical' I mean very low heating bills, i.e. the least amount of heat escape. But the material and installation cost is important. Vacuum panels would transfer almost no heat but would be very expensive to install, which makes it an uneconomical solution.

  • 1
    Though the question is rather comeplete when it comes to material properties, it lacks some other specifications: What do you mean by "economical"? Where do you live (different countries/regions have different local resources)? What kind of house are we talking about?
    – Erik
    Oct 18, 2021 at 7:46
  • (Modern) houses in the Mediterranean usually are built from concrete, with single block walls, if I am not mistaken (only been there on vacation, haven't lived in the area). A good solution when building a new house would be to build double walls and place the insulation in between.
    – Erik
    Oct 19, 2021 at 7:00
  • @Erik Yes this looks robust. But I am trying to minimize the cost even further. If the insulation material was the building material at the same time (just like the strawbale solution) it would be awesome.
    – Xfce4
    Oct 19, 2021 at 18:13
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    @Erik It looks like gas concrete satisfies this condition, i.e. it can be used as building material as well as insulation materiall. It has the thermal conductivity around 0.05 W/mK.
    – Xfce4
    Oct 19, 2021 at 18:28
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    Hempcrete is relatively common in France and has many positive properties. Documentation is readily available in English and French, possibly Spanish, and there are businesses bulding with it in the France and other EU countries. France has some regulatory recognition of the hygrothermal properties because it has ok insulation but excellent humidity regulation and so acts as a phase change material, making pure insulative testing not reflective of actaul performance in buildings. "Bio Aggregate base building materials" by Amziane and Arnaud is worth reading IMO (I just read it)
    – Móż
    Oct 20, 2021 at 10:11

1 Answer 1


Insulation alone does not make for a comfortable building. I built a sleepout using coolstore panels (0.6mm steel skin with 75mm eps core). That is nearly airtight and very well insulated. But it has no thermal mass, so while it cools down quickly (at night, or with the air conditioner) and heats up quickly (resistive heater), it does not stay that way very long. The temperature inside oscillates uncomfortably quickly.

Ideally a house would be well insulated and have thermal mass inside to stabilise it. Passive solar design can make a huge difference, properly positioned windows with the right shading mean you get sun shining on your internal thermal mass during the day, heating it up and that heat keeps the house warm overnight. In summer the sun is higher so the shade keeps the sun away from the windows and the house stays cool. PassiveHaus designs almost always work this way but more so - better insulation, MHRV systems (heat exchangers rather than/as well as heat pumps), better design.

There are standard, common houses that also use those principles, and some of them predate the formal knowledge (some of which came from looking at the older houses and trying to discover why they worked so well). In Australia reverse brick veneer is something that even non-environmentalists do just because it's an easy way to make a house more comfortable. That just puts the insulation on the outside and the thermal mass on the inside of a conventional house design. That alone usually means you don't need to run the air conditioner or heater at night, so your house is quieter while you sleep. Temperatures inside change more slowly, so you don't have to keep taking off and putting on clothes as the heat/cool turns on and off.

In terms of sustainable building practices and materials, Passive House is an excellent place to start. To get certified the house not only has to perform well, it has to have a low environmental footprint. You don't have to pay for certification, or even a passive house type design, to benefit from seeing what they do and understanding why they do that. Or even just looking at the material choices and copying them.

In the USA the "Earthship" idea has many benefits, but sadly has become synonymous with rammed earth + reused tyre walls which are a tiny part of the whole system. Only read up on this if you are willing to think critically and have good social skills - the walls take a lot of labour to build so IMO you're better off ignoring that specific gimmick.

Hempcrete is one useful insulator and thermal mass in one if you can get it. It absorbs and releases humidity so acts as a phase change material as well, making it more comfortable to live in than the raw insulation value suggests. It doesn't have to be hemp, rice husks or other farm waste can be used instead, but hemp has more research/experience behind it. "Bio Aggregate based building materials" by Amziane and Arnaud is worth reading, it is quite technical but very detailed. There are many easier to read books about hempcrete if you prefer more "DIY guide" style.

If you are willing to use Portland Cement (horrible environmental costs) there are many ways to make it insulating as well as its obvious use for structure and thermal mass. You can buy aerated concrete blocks in many countries, or DIY it using either foaming materials or reusing shredded EPS (or other insulators), or even sawdust or other biomaterials. But the latter very quickly take you back to bioaggregates using lime mortar instead (hempcrete).

Wooden framed houses with insulation in the walls are very common around the world, and it's easy to find builders who do that. With effort it's possible to make those houses perform very well, usually by judicious use of internal thermal mass either in the floor or a (sometimes freestanding) concrete or rock wall. Sealing the houses to reduce air movement between inside and outside is usually the biggest challenge, and many builders are very bad at this so you may have to pause the build after the sheel is "complete", have a test done, then fix the leaks yourself before allowing the builder back on site. They will create new holes, but at least you'll have less to fix. Australian builders are especially awful in this regard, so I may be exaggerating the problem you will have in Europe.

The extreme of non-sustainable material is perhaps the various concrete and eps systems, commonly EPS blocks that link together then have concrete poured into the middle. That works, but combines two very non-sustainable materials. Just add gypsum board to the interior walls for a trifecta of disaster.

Building regulations might be your main issue. It would be worth finding your local "green building" groups and owner-builder people, to see what they know and how they deal with the legal side of things. In Australia where I am www.renew.org.au is amazing and hopefully there are equivalent group(s) where you live. Reinventing everything is hard and it's better to learn from other people's mistakes where you can

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    Awesome answer. Thank you very much. It looks like having the insulation outside and having the thermal mass inside is much better for comfort. Maybe, using inner insulation only on the ceiling and having thermal mass on the floor and the walls is a good idea. In the northern hemisphere, windows facing south and east seems to be preferable. Shade idea is cool, they can also be adjustable according to the season.
    – Xfce4
    Oct 20, 2021 at 11:40
  • According to this link hempcrete seems to be weak against water and humidity. It would lose most of its heat insulation property once damp.This is a common problem with many materials like glass fiber, rockwool, cellulose fiber, wood, wool etc. They require some sort of reliable sealing, which I think not easy to achieve over the whole house. Do you know any waterproof, healthy, durable and stable "heat insulation" material?
    – Xfce4
    Oct 20, 2021 at 11:52
  • if you have water inside your house insulation is the least of your worries. If you live in a flood-prone area you are going to end up having waterproofing on the outside of a concrete house, or moving. There are no good, cheap solutions to regular floods (there are houses that float, but they cost at least twice what a non-floating house does).
    – Móż
    Oct 20, 2021 at 20:54
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    This is sustainability.SO, so your questions ignoring that aspect aren't appropriate here. Strictly speaking "waterproof, healthy, durable and stable heat insulation" the answer is sprayfoam. It of course fails horribly on non-toxic, non-flammable, cheap and environmentally friendly. It sounds as though you would be better starting with some "how to design a house" books because I think you're getting side-tracked into weird theoretical questions and forgetting that houses need to be "machines for living in" first, and playthings for engineers second.
    – Móż
    Oct 20, 2021 at 21:08
  • I was talking about the insulation on the outside as outside insulation seems to be more preferable in terms of comfort. So, the question was "what is an easy and reliable method to keep the insulation material outside the house dry?". Because there can be cracks on the most outer shell of the house due to poor construction /craftsmanship, temperature fluctuation, animal activities, earth /soil movement etc.
    – Xfce4
    Oct 21, 2021 at 14:32

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