I recently looked up the recommended insulation value for my area (7) and was surprised how much lower the wall insulation value was shown to be. As you can see it is almost a third of the recommended attic value. I am fairly certain that these numbers have more to do with the limited space in the typical framed wall than the actual theoretical ideal.

My question is, what is the idea ratio (can it be represented as a ratio?), or what is the calculation for the ideal spread of insulation over a house assuming you had as much room as you needed in every wall, floor, and ceiling?

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  • $\begingroup$ When we designed our house we took the building regs min wall thickness and multiplied it by 3 ... so it has 30cm of insulation in the walls and 20cm underneath... But then our house is mot typical ... swh, pv, triple glazed etc $\endgroup$
    – Solar Mike
    Commented Aug 4, 2017 at 17:13
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    $\begingroup$ The R value on its own does not take into account convection and radiation effects, which are usually bigger for the attic than for the walls - (1) because the attic space is more open for convection currents to form and (2) because the roof area is pointing more directly at the sky than the walls are and (3) the wind speed over the roof is higher than over the walls at ground level, and unlike the roof, at least one wall is always sheltered from a steady wind. $\endgroup$
    – alephzero
    Commented Aug 4, 2017 at 17:43
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    $\begingroup$ If you want the "optimum" understand it is a cost optimization problem. What is the \$ savings in energy compared to the \$ cost of increasing R value. $\endgroup$
    – agentp
    Commented Aug 4, 2017 at 19:44
  • $\begingroup$ @agentp Yes, but I am looking more at an arrangement optimization angle. Given X amount of insulation, how best to arrange it to get the optimum total insulation value/heating cost savings (what percentage should be in the roof vs walls vs floor). For a cost optimization problem we need a lot of specific costs and temperatures. For the latter, we just need to understand the properties and movement of heat inside a house. $\endgroup$
    – Jonathon
    Commented Aug 4, 2017 at 19:53
  • $\begingroup$ Cost is being mentioned here and one point I feel is always prevalent when people build - they want the cheapest building meeting the room number that meets the building regs and that is the instruction to the architect. If people asked for a building of x rooms with the lowest running costs over the next 50 years then the architects would perforce start to increase insulation levels and start to include more elements of passive solar design. Just look at how many buildings are simple concrete cubes... concrete is an easy solution but is it the best? $\endgroup$
    – Solar Mike
    Commented Aug 5, 2017 at 5:00

2 Answers 2


No, it's not meaningful to express it as a ratio.

The best insulation is deep insulation everywhere.

You also need to consider convection losses, and your ventilation strategy.

The attic is usually targetted first, for a combination of several reasons. It's cheap, it's easy, and heat rises, so thoroughly insulating the attic cuts down a lot of conduction losses and convection losses.

What the next-easiest intervention is, will depend on the individual property. Upgrading from single-glazed windows to triple is often the next best, because not only does it cut conduction and convection losses, it brings improved security, and acoustic insulation too.

You can get very cheap thermal camera clip-ons for mobile phones now, so you could do your own thermal survey on a particularly cold, windy day, to find out where the cold's getting in (and thus where the heat's getting out).

Interventions on floors and walls tend to be expensive and disruptive, unless you have masonry cavity walls that can be injected with insulation material.

If you're going to insulate your home properly, then you're going to be doing all surfaces anyway. At this point, you'll want someone to come in, do a site survey, and offer you a full specification that treats the house as a whole system. Look for passivhaus-certified architects.


With out taking any of the comments in consideration the OP is asking what insulating ration per area without asking what material is used. This question should be is it possible to get a 100% insulation? No, just like you cannot get a 100% reflection rate from photons from a mirrors. This does pose the question what is the best insulation material available? I would think Space.SE would be a good home fore this question. The OP said wall space was not a restriction for insulation.

My answer is that silica aerogels the best insulator in the world right now. Silica aerogels has thermal conductivities of less than 0.03 W/m*K in the atmosphere. Aerogel can prevent ice from melting on a hot plate at 80 degrees Celsius.


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This material would take the least space for insulation.


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