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Understand EIFS
Three numbers describe thermal performance and they are constantly confused with each other. One belongs to a material, one to a layer and only the third belongs to the wall. Regulations ask for the third.
A U-value is the rate at which heat passes through one square metre of a complete building element, for each degree of temperature difference between the air on either side of it. Lower is better. It is measured in W/(m²·K), watts per square metre per kelvin. A wall at 0.30 loses heat at half the rate of the same wall at 0.60. Because it describes the finished element rather than any one product in it, no single material has a U-value, and that is where most of the confusion on this subject starts.
Three numbers, three scales
These are not three ways of saying the same thing. Each describes something physically different, and each is the input to the next. Getting them straight makes every specification conversation shorter.
How readily heat passes through a material, independent of how much of it there is. Also written as the k-value.
A property of the substance itself. Phenolic foam conducts less than expanded polystyrene, which conducts less than concrete.
Lower is better.
Belongs to a materialHow much a particular layer resists heat flow. It depends on the material and on how thick that layer is.
This is where thickness enters the calculation. Double the thickness of a layer and you double its resistance.
Higher is better.
Belongs to a layerThe heat flow through the finished element, adding up every layer in it plus the still air films on each face.
This is the figure building regulations specify and the one a compliance calculation asks for.
Lower is better.
Belongs to the whole wallThe method
Work out the resistance of each layer, add them all up along with the surface resistances, then invert the total. That is the whole procedure.
Divide the thickness of the layer by the conductivity of its material.
Add the resistance of every layer, plus the resistance of the still air against each face, then take the reciprocal of the total.
Total the resistances first, then invert once at the end. Inverting each layer and adding the results is a different operation and it gives the wrong answer. It is the most common mistake in a hand calculation.
Every layer contributes, and so do the two air films. This is a typical externally insulated wall, in the order heat meets it travelling outward.
| Layer, inside to outside | Contributes |
|---|---|
| Still air against the internal surface | Rsi |
| Internal plaster | R1 |
| The existing or structural wall | R2 |
| Adhesive and insulation board | R3 |
| Reinforced basecoat, mesh, primer and finish | R4 |
| Still air against the external surface | Rse |
| Total thermal resistance | ΣR |
| Thermal transmittance of the wall | U = 1 ÷ ΣR |
The insulation is one line in that table. It is usually the largest single contribution by a wide margin, which is why it dominates the result, but it is never the whole of it. Two things follow from that.
A board has a conductivity, and a board of a stated thickness has a resistance. It cannot have a transmittance, because transmittance describes an element with air on both sides of it. If you are given a U-value for a board on its own, what you have been given is 1 divided by that board's resistance, which is a different quantity wearing the same name.
Put identical insulation on a solid concrete wall and on an insulated cavity wall and the two finished walls will not have the same U-value, because the rest of the sum is different. This is why a U-value can only be quoted against a stated build-up, and why the number has to be calculated per project rather than looked up.
Material and thickness
Because R = d ÷ λ, two boards of different materials reach the same resistance at different thicknesses. That trade is the whole reason Terraco offers more than one insulation technology, and on a retrofit it is often the deciding factor.
| Insulation | Conductivity λ W/(m·K) |
R at 50 mm | R at 100 mm | R at 150 mm | R at 200 mm |
|---|---|---|---|---|---|
| EPS Terraco EIFS Alpha |
0.033 – 0.035 | 1.52 | 3.03 | 4.55 | 6.06 |
| Graphite EPS Terraco EIFS Alpha |
0.030 – 0.031 | 1.67 | 3.33 | 5.00 | 6.67 |
| Mineral wool Terraco EIFS Perma |
0.036 – 0.040 | 1.25 | 2.50 | 3.75 | 5.00 |
| Phenolic foam Terraco EIFS Nova |
0.020 – 0.022 | 2.27 | 4.55 | 6.82 | 9.09 |
Resistances as published by Terraco for each system. Conductivity varies with board density and specification, so read each material against its range above rather than against a single figure, and confirm the declared value for the board actually specified before using any of these numbers in a calculation.
Read the two highlighted cells across: phenolic foam at 100 mm and EPS at 150 mm both give a resistance of 4.55. The same thermal resistance in two thirds of the depth. On a new build that is a detail. On a retrofit, where every extra millimetre has to be found at window reveals, door thresholds, parapets and roof junctions, it is frequently the reason a project is buildable at all. Note that phenolic is currently certified for supply by Terraco Korea only, so check availability in your market before the depth saving goes into a design.
Swipe the diagram sideways to read it in full.
Depth is not an abstraction once it reaches a drawing. Everything below is a detail that has to absorb the difference, and on a retrofit each one is existing fabric rather than a line you can move.
The reveal deepens by the full thickness added. Past a certain point the frame sits too far back, daylight is lost and the insulation has to be returned into the reveal.
An existing sill projects a fixed distance. Add more than that and it no longer throws water clear of the new facade line.
An overhang that covered the old wall may not cover the new one. Extending it is roofing work, not facade work, with its own cost and trade.
Parapets, copings and every drip move outward with the facade and have to be remade to suit.
Balconies, walkways and doorways lose clearance. Where a route has a minimum width, the thickness is capped by that rather than by thermal performance.
The scaffold stands further out, and reveal returns and remade details are labour that does not appear in a price per square metre.
A thinner board is not automatically the better choice. The point is that two systems reaching the same resistance are not interchangeable on a given building, and that the decision is usually made on the depth available rather than on thermal performance.
Choosing between them
All three reach demanding targets. The choice is rarely about which is thermally best, it is about which constraint is binding on your building. Find yours below.
Phenolic foam, 0.020 – 0.022 W/(m·K)
The lowest conductivity in the range, so the thinnest board for a given resistance. Chosen on retrofits where reveals, overhangs and existing details cannot absorb more depth.
Certified for supply by Terraco Korea only. If your project is elsewhere, ask what is certified in your market before designing around this depth. Where each system is certified.
Terraco EIFS Nova ›Mineral wool, 0.036 – 0.040 W/(m·K)
Non-combustible insulation, specified where a higher fire rating is required. Also the breathable option, which matters on older and moisture-prone buildings where the build-up needs high vapour permeability.
Expect a thicker board than the other two for the same resistance.
Terraco EIFS Perma ›EPS and graphite EPS, 0.030 – 0.035 W/(m·K)
The most widely specified system, on new build and renovation alike. Where depth is moderately constrained, the graphite-enhanced board reaches the same resistance in a thinner section than white EPS.
The default starting point unless fire, breathability or depth rules it out.
Terraco EIFS Alpha ›Does the range actually reach demanding targets? Godeok On Bit Chae in Seoul is South Korea's first ZEB Grade 3 certified apartment complex, reaching an energy self-sufficiency rate of over 60 per cent across 697 units and 35,000 m² of facade. Terraco EIFS Nova was one of the key passive technologies specified. Read the project.
The number you have to hit
Required U-values differ by country, and within a country they often differ by building type and by climate zone. They also tighten over time. So there is no single figure that is correct to aim at, and any target quoted without your building type, your climate zone and the code in force is a guess.
Start from the U-value your regulations require, subtract what the existing wall already provides, and the remainder is what the insulation has to deliver.
Once you know the resistance required, conductivity decides the thickness. If depth is constrained, that is where the choice of system is made.
The final figure has to come from the actual construction, layer by layer, against the code that applies. That is what a submission is checked against.
Send us the build-up and the local Terraco technical team will calculate the U-value for your wall, and the insulation type and thickness that reaches your target.
Beyond the arithmetic
A U-value calculation describes an idealised wall. Several things determine whether the finished building performs like the calculation, and all four are decided on site rather than on paper.
Slabs, columns, beams and lintels pass through the envelope and conduct around the insulation. A wall can meet its U-value and still lose heat at the junctions. See what a slab junction does.
Mechanical fasteners penetrate the insulation layer, and boards that are not tightly butted and correctly staggered leave gaps. Both reduce the effective resistance of a layer that calculates perfectly on paper.
Wet insulation conducts more heat than dry insulation. Keeping the build-up able to shed moisture is part of achieving the calculated performance, not a separate concern from it.
Conductivity varies with density and grade, which is why each material above has a range rather than a single value. Substituting a similar-looking board can change the sum.
Common questions
Understand EIFS
From how the system works and why buildings should be insulated, to U-values, installation, system benefits and international certifications.
What the system is, what it is made of and how the layers work together.
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Heat flow, energy demand, thermal bridging and comfort.
Read more ›
Performance, protection, comfort, retrofit and architectural freedom.
Read more ›
Conductivity, thermal resistance and whole-element transmittance.
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The six stages, from substrate assessment to decorative finish.
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What certification assesses, and why it applies to the system and not the product.
Read more ›The calculation on your building
The arithmetic is straightforward. What is not straightforward is establishing a reliable resistance for an existing wall, and confirming what is certified and available in your market. That is the part worth handing over.
