What to expect from this page.
1 – High level overview “in a nutshell” what Roof isolation means and how it can benefit your home.
2 – Explanations about important information when improving a roof.
3 – How much can you save
4 – Future ambition – A custom calculator that can be used to calculate the businesscase for roof isolation.
In a Nutshell:
Aim for a roof isolation value of minimal RD value of 4.0+ but preferred 6.0.
Make sure to choose materials that physically fit in your situation (thickness can become an issue when achieving higher isolation values).
Make sure to facilitate water evaporation, prevent water from entering the roof and ensure good ventilation for a healthy climate.
Complexity to resolve between 4/5 and 5/5.
Money saving possibilities – up to 5/5
There are multiple roof types. A few steps before you start.
- How to start – What can you expect when isolating the roof.
- Balance need / Costs & Objective – What is level of isolation is required.
- Required tools and materials.
Roof isolation usually consists of 6 layers
- The wall in the home (Wood/Concrete/Panels).
- The first water open protection layer.
- The isolation material
- The second damp closed protection layer.
- ‘Breathing space’
- The outer layer (Panels / Rooftiles)
What Isolation levels are there and how does it work.
Isolation materials have a value to represent how much heat is being prevented form leaving and how much cold is prevented from entering the home. The higher these values, the better isolated your home is. Meaning, less less energy is required to heat up or cool down the house – saving money & helping the environment.
The isolation required depends on your aim, for the roof if you own a home you can usually plot this on 3 categories – Poor, Good and Energy Neutral.
The vast majority of the time the products are listed with this value and usually calculations are not required. Just look at the right RD value in the column below and aim for this value. However if you are interested to understand what it stands for in detail, click on the <expand> button below.
Click To discover more on the <expand> below
<Click to expand> Understanding the key Isolation values “RD, RSI, R, BTU & Lambda – λ “
To determine the right isolation value for the roof, a choice needs to be made about the ‘strength’ of the isolation – how much heat is stopped from leaking out and how much cold is prevented from going into the home.
Detailed explanation how it is calculated:
The most important value is the Thermal Resistance per square meter or Ft (depending on where you live is) is called RD, RSI or R-value. All basically referring to isolation per surface.
Details: Thermal Resistance RD value is;
Rd = D / λ
– Rd = Resistance Declared or R-value Metric Units
– D = Material Thickness in Meters (usually expressed in MM)
– λ / Lambda = thermal conductivity
λ = W / m * K
– W = Watt – Unit of Power
– m = meters – Unit of lenght
– K = Kelvin – unit of temperature
Example: To achieve an Energy Neutral roof value you need an RD-value of 6.0. In this example we want to achieve this using Glass-wool insulation.
Glass wool isolation value = λ +/- 0.040 W / (m*K)
Rd = 6 m2 * K / W
λ = 0.032 W/(m*K)
6 * 0.032 = 0.192m = 192MM
The required Glass Wool must be 192mm thick.
This concept is identical for the R-value but with imperial numbers.
Depending on the surface, the required isolation value differs. Typically you see the following valued (in the Netherlands), but you can apply this to any country.
| Energy efficiency | RD-value | “Thickness Glass Wool” | Context |
| Poor (minimal insulation) | ± 0.4 | None (holes in isolation lots eaten by animals, cracks etc) | No extra insulation “bare roof” basic cladding – Old home with strongly degraded isolation. |
| Basic (insufficient but isolated) | ± 2.0 | 64mm | Basic “older’ roof isolation (still in tact, but thin). |
| Good (standard practice) | ± 4.0 | 128mm | Basic level, typical in many houses/ |
| Perfect (Energy-Neutral advertised properties) | ≥ 6.0 | 192mm | New standard – aim for “Energy-Neutral” homes. |
Calculate savings
In the previous block there was an explanation what the ideal values are to achieve. Once achieved this means you will loose less energy through your roof. To understand the impact the overview below will give an indication about the amount of energyloss-prevented – money saved.
There are many custom elements to this calculation and it is with a lot of assumptions. Some major considerations include the in-house temperature. The efficiency of heating itself, the costs for heating, the daily temperature fluctuation (since large-time averages are not suitable) and other heat-leaks might have a disproportional impact too.
Nevertheless, it is good to get an idea, so this is just an indication value.
Click To discover more on the <expand> below
<Click to expand> The math behind these savings realized
Calculating the actual heat loss is done by looking at the temperature difference between inside and outside and using the previously achieved Rd value to block the heat from leaking.
Q˙ = A * ΔT / RD
Q˙= Heat-loss rate W (joules per second).
A = Area in M2
ΔT = Temperature difference (for example 21°C inside vs 1 °C outside)
RD = RD value
1 day ‘easy’ Example: When improving a large 100M2 roof, from a 0.4 RD value to a 6.0 RD value. How much money will be saved per day when it freezes?
100 * 20 / 0.4 = 5000 W
100 * 20 / 6 = 333W
1 W = Joules / Second.
1 kWh = 1000 W * 1 Hour
5000 W = 5.00 kWh : Energy loss per 24 hour = 120.0 kWh
333 W = 0.33 kWh : Energy loss per 24 hour = 8.0 kWh
This means that when the temperature difference is 20 degrees outside. The change in roof isolation is around 112 kWh per day.
Assume you can heat for 20 cents per kWh it would mean a difference of 22 euro,-
For an entire year, it is best to calculate on an hourly basis. In the examples below
Again – Many nuances apply. But it does showcase the impact of Roof Isolation.
Real life example from The Netherlands for 1 entire year: (01 Jan 2024 – 31 Dec 2024 / 100M2 roof surface / De Bilt / 20 cents per kWh / Heating is done with a modern – efficient heatpump having a COP rating of 4.5 (for more information about this go to – heating – EcoHome and Radiators with a 1-1 conversion for more information go to /Radiator – EcoHome)
If you would try to keep the room at 21 degrees for the entire year (based on hourly data) in the Netherlands the heat-loss and financial impact is shown below.
| Rd | Energy per year required (rounded) | Annual costs “Hybrid Heatpump COP 4.50” (€ 0.20 kWh) |
| 0.4 | 20500 | € 910 |
| 2.0 | 4100 | € 180 |
| 4.0 | 2000 | € 90 |
| 6.0 | 1400 | € 60 |
The prices above are based on 1-1 conversion where 1 kWh input also generates 1kWH in heat “efficiency is 100%” This does not apply most likely in a real case scenario.
So if you want to know what this means for your situation. There is another chapter – heating – EcoHome and /Radiator – EcoHome both can be huge contributing factors to estimate what the real costs will be.
For now, realize the huge impact when upgrading your roof if the insulation is insufficient.
Click To discover more on the <expand> below
Deep Dive – Variables beyond the basics.
Drives air infiltration through leaks
Air Resistance & Influence of wind
Installing Roof isolation
As illustrated above there are some key layers that come back in most of the roof isolations.
The outer layer (Panels / Rooftiles)
When working with a flat roof, or a concrete slap you can simply replace the roof tiles. But the concept remains the same. This is the first layer of ‘defense’ and should be waterproof by default. It must also have ways for moist to escape, to avoid moist collection and issues like mold.
‘Breathing space’
Allow for breathing space to allow the damp open foil to work properly. It also prevents moist from building uit from behind the roof-tiles (or other forms of roofing). With rooftiles this comes naturally since the woodwork is required to have the rooftiles to stay in place.
Damp Open Foil (displayed here in blue ‘cold side’)
This prevents moist to get into the isolation material. It can breath moist out but will block new moist from getting in ensuring durability and reducing mold or other water related problems.
V Other names for this damp open foil are breathable membrane, Vapour-permeable membrane, Waterproof breathable membrane or Diffusion-open Membrane). They are all the same.
X This is not a vapour barrier, or Vapour control layer (VCL)
Important aspects to look for when selecting damp open foil.
– To allow (and encourage) water to leave to the outside of the roof, the Sd value must be low this means, that the water can escape easily from inside to outside. Sd (not to be confused with the earlier mentioned Rd) is a measure of water resistance. As a rule of thumb do not go above 0.5Rd Meter and make sure it is at least 5 to 10 times more open compared to the inside materials.
From the outside this material is still waterproof so water can only go in 1 direction. Place them overlapping if needed so the water naturally goes over the edges to the gutter.
– This is placed directly on top of the isolation material, with breathing space above (towards the tiles).
– Often the materials are strong and can easily be stapled to attach them to the wood.
– Although temporary, often there is UV-protection for some days, allowing to install the materials. But aim to keep the sun exposure to a minimum.
Isolation material.
A wide selection of materials are available. Below you can find a list with suitable candidates.
Click To discover more on the <expand> below
<Expand here> to see a full list of many types to choose from when selecting isolation materials, including key properties and the thickness required to achieve an RD value of 6.0 with material pricing.
Affiliate links will be added in the future to list specific products and pricing, but for now here is an overview with the different materials. You see the average costs, and how often it is used in modern constructions. Also the thickness to achieve an RD value of 6 is added for comparison.
| Material | Thickness for R 6 (mm) | Avg. material Cost (€ / m²) | Frequency (4=High, 1=low) | Key Qualities / Notes |
|---|---|---|---|---|
| Glass wool (fiberglass) | 210 | 12.0 | 4 | Affordable, easy install, itchy, can settle |
| Rock wool (stone wool) | 210 | 21.8 | 4 | Fire-resistant, water-repellent, heavier |
| Cellulose (recycled paper fiber) | 240 | 13.2 | 1 | Eco-friendly, good fill, dusty, prone to settling |
| Sheep’s wool | 228 | 23.7 | 1 | Renewable, moisture-buffering, expensive |
| Cotton (recycled denim) | 240 | 16.0 * | 1 | Non-irritant, recycled, pricey, may attract pests |
| Hemp fiber | 240 | 20.0 * | 1 | Sustainable, breathable, costly |
| Flax (linen) fiber | 240 | 20.0 * | 1 | Natural, breathable, pricier |
| Expanded polystyrene (EPS / ‘Tyro foam’) | 228 | 12.0 | 4 | Low cost, moisture-resistant, low compressive strength |
| Extruded polystyrene (XPS) | 204 | 18.0 * | 3 | High compressive strength, moisture-resistant, mid cost |
| Polyisocyanurate (PIR) boards | 132 | 15.0 | 4 | High R-value, thin, may off-gas, flammable core |
| Polyurethane (PUR) foam boards | 132 | 25.8 | 3 | Excellent performance, off-gassing, higher cost |
| Phenolic foam boards | 120 | 18.0 * | 2 | Great fire performance, brittle, moderately priced |
| Foam glass panels | 300 | 30.0 * | 1 | Inorganic, moisture-resistant, very heavy |
| Wood-fiber (cellulose-wood) boards | 240 | 22.0 * | 2 | Breathable, eco-friendly, bulky |
| Cork panels | 240 | 30.0 * | 1 | Renewable, rot-resistant, costly |
| Magnesium-oxide (MgO) boards | 270 | 28.0 * | 1 | Fireproof, durable, heavy |
| Structural Insulated Panels (SIPs) | 132 | 50.0 * | 2 | Fast build, integrated, expensive |
| Spray-applied polyurethane foam (closed-cell SPF) | 144 | 84.0 | 2 | Air-seal, high R, professional install required, costly |
| Loose-fill fiberglass | 240 | 12.0 | 1 | Low cost, DIY-friendly, itchy, may leave voids |
| Loose-fill cellulose | 240 | 13.2 | 1 | Blow-in, eco-friendly, dusty, needs proper density |
| Granular cork (loose-fill) | 240 | 20.0 * | 1 | Natural, pest risk, moderate R-value |
| Foam glass gravel (flat-roof infill) | 270 | 25.0 * | 1 | Lightweight, fireproof, niche application |
| Wood wool (excelsior) boards | 420 | 25.0 * | 1 | Sound-absorbent, very bulky |
| Reed insulation mats | 300 | 22.0 * | 1 | Natural, moisture-sensitive |
| Rice-husk aggregate boards | 240 | 20.0 * | 1 | Waste-based, dust, flammable |
| Bagasse (sugar-cane fiber) boards | 270 | 18.0 * | 1 | Agricultural waste, moisture-sensitive |
| Aerogel blankets (e.g., Spaceloft®) | 78 | 94.0 | 1 | Ultra-thin, high cost, fragile |
* pricing is not reliable and must be further investigated.
The first foil damp protection foil (often red)
Attach to the warm side “directly connected to the inside”. This is airtight to the inside wall.
The wall in the home (Wood/Plasterboard Panels).