It is for a house in Portugal (where temperatures during the day roughly vary from 15 degrees celsius to 40 degrees celsius), where the walls of the house are 30 cm of concrete, with no insulation at all.

I am going to repaint one room, and I am wondering whether I should make the effort of insulating the outside-facing walls (from the inside) of this room.

30 cm of concrete feels quite big to me. While the walls are slightly cold to the touch right now (winter), I am wondering whether it is worth insulating for the summer. Will that make a noticeable difference?

  • Much much better would be 10cm of concrete and 30cm of insulation. Built a house with 30cm of insulation (all walls, floor and roof) and it only needs a small amount of wood for heating but that is also due to passive solar design.
    – Solar Mike
    Feb 26, 2021 at 20:22
  • Surely you've gotten the tempoeratures wrong. Lisbon goes down to an annual average of 8 in January, and in the North-East it goes down to 0 on average in January.
    – einpoklum
    Feb 26, 2021 at 21:32
  • @einpoklum yes, I tried to simplify a bit and only quote the temperatures during the day, not including the night. I'll edit to clarify.
    – DevShark
    Feb 26, 2021 at 22:12
  • @DevShark: Both extremities matter, I believe.
    – einpoklum
    Feb 27, 2021 at 0:23
  • I would check what your neighbors are doing and how is new construction being built? Feb 27, 2021 at 5:48

4 Answers 4


Just an addendum to the comment. In these screenshots the difference between inside and outside insulation is visualized. Just ignore the red bar for greenhouse contribution.

Everything is the same, only layers no. 2 and 5 were swapped by drag and drop. Layer no. 3 is "glueing mortar" and should be swapped , too, but wasn't.

The R- value is the same, of course, since series R values add independent of the order of layers.

But the important condensate mass and drying time are extremely different. Exactly that is the problem with inside insulation.

To be seen by the blue water drops.

And also the heat storage capacity is better in case of the external insulation.

Whenever possible, avoid inside insulation.

Outside Insulation

Above is the standard outside insulation with white polystyrene boards.

Beneath is the inside insulation, which can produce mold and health problems.

Inside Insulation

  • Oh waow. Thanks a lot for that. I think that’s very convincing for me NOT to insulate inside.
    – DevShark
    Feb 27, 2021 at 6:53
  • @DevShark Maybe a thin inside insulation combined with a water vapour proof coating (foils, latex painting etc.) will work without condensation inside the wall. The normal inside plaster contributes to good room air since it buffers humidity, the vapour barrier should be between insulation and plaster. Could be probably simulated on that site.
    – xeeka
    Feb 27, 2021 at 7:31
  • Well right now I have only bare concreete, not even inside plaster. But you got me worried about changing anything now :D it seems I could make things a lot worse, and I also feel it’s hard to be very confident about the impact, as many things will be a factor.
    – DevShark
    Feb 27, 2021 at 7:44
  • This contradicts the common and defensible NA practice of applying EPS/XPS to concrete on the inside. Is your scenario for a wet wall? See e.g. ecohome.net/guides/2270/…
    – P2000
    Feb 28, 2021 at 0:49
  • No, the wall is not wet. Just cold to the touch
    – DevShark
    Feb 28, 2021 at 20:12

Someone else can feel free to edit in the SI unit R and/or U (=1/R) values, I have a conversion stashed somewhere but it's not handy. Off the top of my head, in "English" units mostly only used on the USA now, 30cm (12") of concrete or stone is roughly R1 ([1 square foot•hour•°F difference]/BTU) while 2.5 (1") cm of foamboard is roughly R3.5-6.5 (depending on the foamboard type) and 2.5cm (1") of rockwool or cellulose is R3-3.3

So merely adding 2.5cm of insulation (not much at all) to the 30 cm of concrete would roughly quadruple (or better) the thermal resistance of the bare wall. Being only one room, and only the walls, it may not make much difference until you do more (the heat will leak elsewhere), but it will make a difference.

  • This site feels free - it is even free for private use: www.ubakus.com In a few minutes it reveals problems with insulations and the order of layers. Toggling between R- value/U- value is done by clicking on the tool icon next to the result.
    – xeeka
    Feb 27, 2021 at 4:28

Concrete is not insulation. It is thermal inertia.

300mm (12”) of concrete will resist changes in temperature extremely well. If it's hot, it's gonna stay hot. If it's cold, it's gonna stay cold.

This is a blessing or a curse depending on whether you are using it to moderate the house's temperature, or being whipsaw'd by whatever temp weather is causing the concrete to be. Given your thermal data, I would think the concrete would tend to be warmer than you want, for many hours after the sun sets, forcing you to run A/C well into the night, even though solar gain stopped before sunset.

By far, the better plan is to put insulation on the outside of the concrete, so it is inside your house's insulation envelope. Now, the concrete's thermal mass will be a moderator, tending to keep the house at the temperature you set.

  • But isn't all non-vacuum insulation thermal inertia, essentially?
    – einpoklum
    Feb 26, 2021 at 21:34
  • @einpoklum technically everything has thermal inertia, but some things have a whole lot more than others, and most of the usual types of insulation are not that good. The best substance for adding thermal inertia is actually water.
    – Nate S.
    Mar 1, 2021 at 16:56
  • @einpoklum since we're not in a vacuum, our best bet is to play different materials off against each other. Mar 2, 2021 at 2:07

In addition to thermal insulation (resistance) you should also look into thermal inertia (mass), which is what provides cooling on hot days and moderation on cold winter nights.

Insulating the wall throws away most benefits of inertia, and you have to actively manage the temperature, which usually means spending money to heat and cool.

But you would insulate if the average temperature achieved by inertia alone is unpleasant (usually too low) and costly to manage, as in cold basements.

Whether you choose for inside or outside insulation depends on the specific climate and moisture conditions at your dwelling. Both types are well studied.

OP is advised to get local engineering advice. It's a pricey undertaking and if done wrong it can have dire consequences. Both can be generally viable options, as is relying entirely on inertia.

Here is an example of internal & external insulation practice common in a moist and rainy climate region in North America:

enter image description here

enter image description here

For a question like this, and whether it applies to retrofitting a massive concrete wall somewhere in Portugal, I would not just go by a single article or a web calculator.

But this does provide some background information that should help you ask informed questions.

Images from www.bchousing.org publications IG-R22-Effective-Walls-Residential-Construction.pdf

  • 1
    Insulating the OUTSIDE of the masonry/mass gets you both. But that's usually easier to do at time of construction, rather than as a retrofit. However, insulation+stucco outside retrofits are a thing (but may be hard to find in Portugal?) Exterior Insulation and Finish Systems (EIFS)
    – Ecnerwal
    Feb 26, 2021 at 20:58
  • @Ecnerwal, yes, I was referring to internal insulation. External insulation has its problems too, causing excessive thermal stress on the external (!) surface: mdpi.com/2076-3417/10/13/4611/htm
    – P2000
    Feb 26, 2021 at 21:21
  • That is very interesting. And I think that explains why no houses are insulated in Portugal.
    – DevShark
    Feb 26, 2021 at 22:14
  • An inside insulation is nearly always a big disadvantage, including thermal inertia effects. No standard oven, no boiler have inside insulation, nearly all land based animals have fur as outside insulation. For buildings, an important rule is "outside insulation, inside damp barrier". Inside insulations are a method to generate mold/health problems, since the dew point is likely to be found in or after the insulation with long drying time thus effecting the inside air quality. Inside insulation decreases the living space. The only advantage is easy inexpensive installation.
    – xeeka
    Feb 27, 2021 at 4:16
  • @xeeka that is very interesting and convincing. Thanks a lot for this. I will avoid inside insulation then. Outside insulation seems too much work/trouble for me. I am adding insulation on the rough, hopefully that will be enough
    – DevShark
    Feb 27, 2021 at 7:09

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