Rc- and U-value: insulation values explained
Two figures determine how well your home retains heat: the Rc-value (for roof, walls and floor) and the U-value (for windows and doors). They work in opposite directions: for Rc, higher is better; for U, lower is better. Together they determine a large part of your label class.
The difference at a glance
| Rc-value | U-value | |
|---|---|---|
| Measures | Thermal resistance: how much the construction holds back | Thermal transmittance: how much passes through |
| Unit | m²K/W | W/m²K |
| Good is | High ↑ | Low ↓ |
| Used for | Roof, walls, floor (solid parts) | Glazing, frames, doors |
Memory aid: the R of Resistance — it holds heat back, so the higher the better. The U of Under way — it lets heat through, so the lower the better. They are roughly each other's inverse: a high Rc goes with a low U.
What is a good Rc-value?
That depends on the age of your home. An uninsulated cavity wall from the 1960s is around 0.4 m²K/W — it holds back virtually nothing. For new build, much stricter requirements now apply: roughly an Rc of well over 3.5 for the floor, close to 5 for the walls and above 6 for the roof. When retrofitting an existing home you will not always reach those new-build values, but every step up counts.
And the U-value of glazing?
Glazing is often the weakest point of the envelope. The differences are large:
| Type of glazing | U-value (approx.) | |
|---|---|---|
| Single glazing | ± 5.8 | very poor |
| Older double glazing | ± 2.8 | moderate |
| HR++ glazing | ± 1.1 – 1.2 | good |
| Triple glazing | ± 0.6 – 0.8 | very good |
Going from single glazing to HR++ therefore means roughly five times less heat escaping through your windows. That is usually one of the most noticeable improvements — not only for your label, but also for comfort (no more cold draught by the window).
How does this feed through into your energy label?
During the inspection the advisor records the insulation for each construction. Those Rc- and U-values together determine the heat loss of the home, and therefore how much energy is needed to keep it warm inside. That feeds directly into the primary fossil energy use — and thus into your label class.
Important — keep your evidence. If the advisor cannot establish or prove the insulation, NTA 8800 requires the use of a default value. That is deliberately unfavourable. Do you have an invoice for the cavity wall insulation, a construction drawing or a photo taken during the renovation? Then the actual value may be used — and that can easily be worth one or more label classes.
Where is the biggest gain for you?
Usually in this order: roof (heat rises), walls/cavity, glazing and floor. But it depends on what is there now: if you already have HR++ glazing but an uninsulated roof, roof insulation delivers far more than triple glazing. Which measure pays off most for your home is covered in the free improvement advice included with every energy label — or read improving your energy label.
Frequently asked questions
What is the difference between Rc-value and U-value?
The Rc-value gives the thermal resistance of a solid construction such as a roof, wall or floor: the higher, the better the insulation. The U-value indicates how much heat passes through, and is used for windows, doors and frames: the lower, the better. They are roughly each other's inverse.
Is a high or a low Rc-value good?
High. The Rc-value is a resistance: the higher the figure, the more heat the construction holds back. An uninsulated cavity wall is around 0.4 m2K/W, whereas for new build values of well over 4 to above 6 now apply.
What U-value does HR++ glazing have?
HR++ glazing is usually around a U-value of 1.1 to 1.2 W/m2K. For comparison: single glazing is around 5.8 and older double glazing around 2.8. Triple glazing goes towards 0.6 to 0.8. The lower the U-value, the less heat escapes through the glass.
How do Rc- and U-values count towards my energy label?
Together they determine how much heat your home loses, and therefore how much energy is needed to keep it warm. That feeds directly into the primary fossil energy use and thus into your label class. Better insulation values therefore give a better label.
Should I keep evidence of my insulation?
Yes, it pays off. If the advisor cannot establish or prove the insulation, NTA 8800 requires the use of a default value, which is deliberately unfavourable. With an invoice, construction drawing or photo of the insulation, the actual value may be used, which can be worth one or more label classes.
More in the knowledge base
- NTA 8800: the calculation method behind the label
- Primary fossil energy use: the core value
- Energy label A to G: what do the label classes mean?
- Improving your energy label: which measures pay off?
- Subsidy for improving your energy label
Curious what your insulation delivers?
At Hollands Duurzaam you pay from €220 for an official energy label including free improvement advice — a fixed all-in price, with no travel costs in our service area. Do you have invoices or drawings of your insulation? Have them ready: with those we calculate using the actual values instead of unfavourable defaults. BRL 9500-MWA-W certified, member of AVEPA.