Table of insulation values
The table gives the approximate insulation value (or “U” value) of various materials in column E. The lower the “U” value the better the material is as an insulator. However the best insulating materials cannot always be used in particular situations such as where there is dampness, where they need to be load bearing to some extent or where they need to be self supporting. These factors are shown in columns B to D.
Some insulating materials such as expanded polystyrene are very high in embodied energy as shows up in column E. However if they are very light materials then relatively less weight is used to achieve the same degree of insulation and this shows up in column G (which shows the density divided by the embodied energy).
Timber is included in the list to show how timber members may contribute to thermal bridging. For instance a wall made of solid timber (such as a Scandinavian log cabin) would need to be about 1.4 metres thick to achieve the same insulation as 300mm of mineral fibre (the amount normally used in Passivhaus design). Structural timber’s thermal conductivity has traditionally been ignored in calculations.
The U value of materials can vary considerably depending on several factors such as moisture the material may have absorbed, how compressed it becomes and the way it is made. Manufacturers tend to test their materials under ideal conditions and there is some doubt as to how reliable many of the values are in practice.
Both aerogel and vacuum panel insulation are still in the early stages of development.
| A | B | C | D | E | F | G |
| type | tolerate damp? | compressive strength | self supporting? | U value (4) | embodied energy(7) | e.e. adjusted(8) |
|---|---|---|---|---|---|---|
| cellulose | no | no | can be | 0.035 | 0.94-3.3 | 0.9-3 |
| cellulose batts | no | poor | yes | 0.035 | 0.94-3.3 | 0.9-3 |
| mineral fibre quilts | no | poor | can be (3) | 0.034 | 16.6 | 11.7 |
| mineral fibre batts | yes | poor | yes | 0.036 | 18 | 16 |
| hemp | no | poor | can be (3) | 0.04 | ? | ? |
| HempWood | no | poor | yes | 0.038 | ? | ? |
| hemp/lime walls | yes | good | yes | 0.09 | ? | ? |
| cork | limited (1) | good ? (2) | yes | 0.04 | 4 | 16 |
| wool | no | poor | no | 0.04 | 20.9 | 13.1 |
| expanded polystyrene | yes | depends on density (2) | yes | 0.033 | 88.6 | 40.3 |
| e.p. beads | yes | poor | no | 0.033 | 72 | 26.2 (6) |
| expanded polyurethane | yes | good? (2) | yes | 0.025 | 72.1 | 66.3 |
| foamed glass | yes | good | yes | 0.056 | 27 | 62.7 |
| wood wool slab | yes | good | yes | 0.083 | ? | ? |
| softwood timber | limited | good | yes | 0.138 | 7.4 | 61 |
| eco-wool | yes | poor | no | 0.042 | ? | ? |
| phenolic foam | yes | good | yes | 0.02 | 28 | 49 |
| aerogel (5) | yes | yes? (2) | yes | 0.013 | ? | ? |
| vacuum panel (5) | ? | ? | yes | 0.0042 | ? | ? |
(1) Cork will tolerate moisture for a short period.
(2) OK with the correct grade (so for instance expanded polystyrene can be obtained in grades strong enough to support concrete slab floors).
(3) Some quilts have a support backing built in, especially those for acoustic insulation.
(4) the ‘U’ value, measured in W/m²/ºK
(5) these are both emerging technologies so most values are missing
(6) e.p. bead values depend on the source of the bead (post consumer waste, manufacturer’s waste etc).
(7) measured in MJ/Kg. Values mainly taken from the Inventory of Carbon and Energy (ICE) database.
(8) the adjusted embodied energy gives the comparitive value after taking the density into account. So, for instance, cork goes up from 4 to 16 because cork is relatively heavy stuff compared with wool which goes down from 20.9 to 13.1 because it is light.
