What information do we need to produce accurate U-value calculations?
Some of the most crucial – but often overlooked – information we need to calculate accurate U-values centres on a building’s location. Site-specific information is required in both ground floor and inverted roof calculations. It highlights the importance of requesting calculations on a project-by-project basis, rather than relying on generic guideline calculations or copied specifications.
Most U-value calculation requests understand the importance of detailing each material layer within a floor or roof build-up. Thermal performance data, combined with the layer thicknesses, is fundamental to establishing a U-value. Vapour resistance values then contribute to condensation risk analysis.
Many construction product manufacturers offer a U-value calculation (and condensation risk analysis) service as part of their technical support. Sometimes calculations are required on a tight timescale. This blog post is designed as a guide about how you can help us get calculations to you as quickly as possible, without sacrificing accuracy.
Step 1: A U-value calculation request in writing
A phone call is often the best way to get an immediate answer to a technical query. It can be a great way of getting guidance on a build-up, and what might influence its U-value, when establishing designs and creating outline specifications.
But to issue a formal calculation, we need the details of a U-value request in writing. You’d be surprised how often what somebody describes over the phone is very different to what they later email. A written record of your design is essential for maintaining transparent project records. As a manufacturer, we can only advise; we cannot design a build-up for your project.
When emailing calculation requests, there’s such a thing as too much detail. We’re often sent complete drawing packs or full specifications, with the expectation that we’ll comb through to find the necessary detail.
This not only takes time but also carries risk. We may miss an important detail, or interpret something incorrectly. We need you to pull out the necessary information so it’s clear what needs calculating.
Checklist:
- A written description of the element to be calculated.
- (Optional) A simple, clear section drawing through the element.
Step 2: A target U-value or proposed insulation thickness
National building regulations take a holistic approach to assessing the energy performance of buildings, with multiple performance metrics.
Compliance calculations do not depend on defined U-values for building elements. Instead, there is in-built flexibility to adapt the published ‘notional specification’. This means you need to tell us what U-value you are aiming for to suit your whole-building performance specification. We can then advise on the necessary thermal insulation thickness.
Alternatively, if space is tight or you are assessing options, you can tell us what thickness of insulation you’d like to use. We can then advise the likely U-value that will result.
Checklist:
- A target U-value, or a specific insulation thickness.
Step 3: Materials data
We’ve touched on the data required for every layer of a construction element: thermal performance, thickness, and vapour resistance. At this point, it’s useful to distinguish between a ‘material’ and a ‘product’.
By ‘material’ we mean a generic bulk material. Concrete is a good example, with concrete slabs/decks common in both ground floor and inverted roof build-ups. Often mixed on site, its density dictates its thermal performance. However, we are rarely told what density of concrete is to be used.
As a result, we have to use materials data published in national and international standards. We generally assume denser concrete as a ‘worst case’ and we state which standard is our source.
In contrast, a ‘product’ refers to something manufactured. It uses a generic material as its basis, but is then tested and a specific performance declared prior to being sold.
To continue our example, a precast concrete deck produced by a particular company is a proprietary product that can be named in the U-value calculation. The manufacturer’s website or technical literature acts as the source for the data entered in the calculation.
“Insulation is no different,” said Rob Firman, Polyfoam XPS’s Technical and Specification Manager. “Extruded polystyrene is a generic insulation type, and standards may give generic performance data for it. Those values do not reflect XPS manufacturing advances, so it is preferable (and more accurate) to name a proprietary product like our Floorboard Extra or Roofboard Extra.”
Checklist:
- Layer-by-layer details for each element.
- Specific details of generic materials, if known (e.g. concrete densities).
- Proprietary product names and datasheets, or confirmation of manufacturer-declared performance values.
Step 4: site location information
Here we get into the granular detail of what sets one U-value calculation apart from another. You could propose the same build-up using the exact same material layers in two different locations, and get different results because of these local variations.
The information required depends on the element type being calculated.
In ground floors, calculations are influenced by the thermal performance of the ground, so we need to know the soil type on site. In the UK, ground conditions are predominantly clay, but that is not the case everywhere.
The issue is that clay has the most favourable thermal performance. If a calculation assumes clay soil and the ground isn’t clay then the floor’s U-value won’t accurately represent the performance that will be achieved in the finished building. If you don’t know the site conditions for your project then we use an online tool to give us an idea of what ground type is present, and advise you accordingly.
The performance of inverted flat roofs can be influenced by the passage of rainwater through the insulation layer to the waterproofing layer. U-value calculations include a correction for the potential cooling effect of rainwater. That correction is based on the amount of rainfall experienced by the site location, and we have local and regional meteorological information that we can use.
Like with ground floors, our calculation outputs always clearly state what rainfall amount we have assumed, so you can review it accordingly.
Checklist:
- Full site address, including postcode.
- (For ground floor calculations) The ground type, if known.
What to do next
One of the earliest pieces we published on the Polyfoam XPS blog was an article called ‘Our Values, Your Values and U-values’. It was about the need for a more rigorous approach to U-value calculations during the specification process – not unlike what you have just read today.
Writing on a similar theme eight years later suggests nothing has changed. That’s not true. The context in which we have this conversation is (arguably) what has changed most of all: discussions around competence, traceable and transparent project records, and delivering designed performance are commonplace compared to 2018.
That means even the best-intentioned designers and specifiers need to take stock of their approach to U-value calculations. As we mentioned earlier, a U-value calculation is a reflection of your design choices and our interpretation of the information you provide.
Complacency can seep in from either side. It is easy to send a calculation request with the bare minimum of information or just less. It is equally easy for us to receive that request and deal with it in the belief that we understand your intention.
We can send a calculation back, stating assumptions we have made – but do you take the time to review whether those assumptions are appropriate? Or do you simply accept the calculation because an insulation manufacturer supplied it and it must be okay?
At the beginning of this post we talked about being able to deliver calculations quickly, if necessary, without sacrificing accuracy. That’s a valid goal. But the real goal is a more collaborative and intentional approach to calculations, while still meeting the needs of construction projects.
Yes, we have provided fundamental checklist items to help improve the quality of information submitted in calculation requests. More than that, we hope we’ve provided the tools for you to take a few extra minutes before clicking send to guard against the potential complacency we just described.
Another tool we’ve created is The Polyfoam Orange Book of XPS Insulation, a complete specification guide to XPS to help inform your product choices before you get to the point of requesting U-value calculations. And once that point arrives, you can of course contact us to discuss any aspect of a ground floor or inverted roof build-up and the data used in a calculation.