In this guide, you’ll learn what U-values are and why they matter, along with relevant U-values for Summerhouse24 timber cabins and their components. All U-values are listed in tables for various wall thicknesses, insulation, windows and doors. 

If you’re after some in-depth information, don’t forget to read the last section. We’ll cover some technical details and how to calculate U-values.

Table of contents: What is a U-value and why is it important? Summerhouse24 timber building components U-values Timber wall thickness Floor, roof and wall insulation Windows and doors How are U-values calculated? How to calculate new U-values with insulation

What is a U-value and why is it important?

U-values are important to understand because they let us know the thermal transmittance of different materials. In simpler language, this means the U-value number tells us how much heat can move through the material. This helps us get an idea of how good any given material is in insulating us against cold and heat. In the context of timber buildings, we want to consider the U-values of doors, windows, insulation and walls.

The higher the number you see, the more heat escapes through the material. The lower the number, the more insulation is provided. Anything with a lower number is better, though you may not be looking for a very low U-value material in the first place, when you’re planning to put up a shed for storage, for example. 

U-values become more relevant when you plan to use a timber building for leisure activities, as a home office, or spend time in it year-round. Having soundly insulated walls, ceilings and floors will make all the difference to you and your guests’ comfort.

It’s important to note that workmanship and installation standards can strongly impact heat transfer. If insulation is fitted poorly or seals are missing from windows, creating gaps and cold bridges, the heat transfer may be considerably higher than desired. 

Summerhouse24 timber building components U-values

Let’s have a closer look at the different U-values of our summer houses, log cabins and other wooden structures. U-values are listed in helpful tables in each section for wall thicknesses, windows, doors, roof and floor insulation. 

Understanding the right choice for your circumstance may not be immediately obvious – most of us are not construction experts! So don’t hesitate to give Summerhouse24 a ring or request a callback if you’re unsure of anything or need further clarification. Our helpful local sales team is always happy to advise you on the best options depending on your situation.

Timber wall thickness

One of the easiest ways to improve insulation is to simply increase the width of the construction material. The thicker the walls, the lower the U-value. However, adding insulation instead of increasing wall thickness may be more useful and cost-effective. It all depends on your particular circumstances.

In the table below, you’ll find a list of the different wall thicknesses we offer and their U-values. Most of our cabins and summer houses are available to order in different wall thicknesses to reach the desired U-value.

For example, if the 44mm wall with a U-value of 1.93 W/m²·K is too high to meet your requirements, you can upgrade to a 70mm timber wall with a U-value of 1.39 W/m²·K or even an 88mm glulam timber wall with a U-value of just 1.17 W/m²·K.

Wall thickness	U-value
21mm wall	2.93 W/m²·K
28mm wall	2.53 W/m²·K
44mm wall	1.93 W/m²·K
70mm wall	1.39 W/m²·K
88mm wall	1.17 W/m²·K

To achieve an even lower U-value for the whole cabin, insulation can be added to the floors and ceiling. In addition, insulation can also be added to the walls. As we mentioned earlier, the latter should, in most cases, be done by an expert or experienced person to avoid costly mistakes.

Floor, roof and wall insulation

Insulating the floor and roof is the first thing you should do to improve the thermal resistance of your chosen summer house or log cabin. Roof insulation not only keeps you warm during cold weather, it also ensures your cabin doesn’t get too hot during sunny days. Floor insulation serves the same purpose, it helps to even out the temperature across all seasons, keeping you toasty in winter and comfortably cool during summer.

For floor and roof insulation, Summerhouse24 uses 25mm insulation boards. These can be conveniently added on as an extra in the shopping cart. If you are using our assembly service and have purchased one of our insulation kits, our team will fit this at the same time. 

Wall insulation and installation service for large log cabins for year-round use can be requested directly from the Summerhouse24 sales team.

Insulation	U-value
Floor and roof: Insulation boards 25mm	Approximate U-value: 0.741 W/m²·K
Walls: Rockwool Superrock Premium 100mm	U-value: 0.345 W/m²·K

The approximate U-values of our timber cabins insulated with 100mm Superrock Premium rockwool are listed below:

Insulated walls	Approximate U-value
44mm solid timber wall & 100mm rockwool	0.276 W/m²·K (reduced from 1.39)
70mm solid timber wall & 100mm rockwool	0.293 W/m²·K (reduced from 1.93)
88mm glulam timber wall & 100mm rockwool	0.266 W/m²·K (reduced from 1.17)

Here’s what you can expect when you choose our floor insulation:

Insulated floors
18mm floor board & 25 mm insulation board	0.591 W/m²·K (reduced from 2.93)
28 mm floor board & 25 mm insulation board	0.573 W/m²·K (reduced from 2.53)

Windows and doors

The quality of your windows and doors is just as important as wall thickness and insulation when it comes to ensuring you have a comfortable cabin or garden room to spend time in. This is why Summerhouse24 has always prioritised providing high-quality double-glazed windows and doors, even with our smaller summer houses. A cold draft on the neck can ruin an otherwise perfect evening!

In the table below, you’ll see the selection of different glass packages we offer with our timber cabins. It’s worth noting that the selective glass option with argon has a U-value of 1.6 W/m²·K, offering almost double thermal efficiency compared to the standard package. Orthez double-glazed windows with argon have the lowest U-value at 1.1 W/m²·K.

Standard glass package	3.0 W/m²·K
Argon-filled glass package	2.8 W/m²·K
Selective glass with argon	1.6 W/m²·K
Orthez standard double glazing (tempered)	2.6 W/m²·K
Orthez double glazing with argon (tempered)	1.1 W/m²·K

We also recommend using a window and door insulation kit. This is to ensure there are no cold bridges, which significantly impact the heat transfer of an insulated timber building. Our ISO insulation kit is made of flexible and breathable materials that do not hold moisture, which can damage wood. The pliable material is equally important, as wooden houses need about 1-2 years to settle. During this period, the wooden components move and, without sufficient space to do so, the building may get damaged.

Technical information for the detail-oriented

How are U-values calculated?

To calculate a U-Value, we start with the R-Value or resistivity of the parts that make up the building element for which the U-Value is being calculated. The R-Value tells us the material’s ability to resist heat transfer at a certain thickness – the higher the better when looking for an insulation material. It is measured in m2.K/W. 

The R-Value is calculated as R = l/λ where l = the thickness of the material in metres and λ (lambda) is the thermal conductivity of the material in W/m.K.

U-value formula

U-Value = 1/(Sum of all R-Values)

How to calculate new U-values with insulation

Suppose you’ve decided to add insulation and want to calculate the new U-values for your log cabin. In that case, all you need to do to find the new approximate U-values is calculate the R-values of all wall components, add them up and then convert back to a U-value. Here’s how to go about it in three easy steps:

Convert existing U-values to R-values (thermal resistance): R = 1/UAdd the R-value of the new insulationConvert back to U-value: U = 1/R

Let’s look at an example calculation for an insulated 44mm wall:

Original R-value = 1/1.93 = 0.518 m²·K/WR-value of Rockwool = 1/0.345 = 2.899 m²·K/WCombined R-value = 0.518 + 2.899 = 3.417 m²·K/WNew U-value = 1/3.417 = 0.293 W/m²·K