How you fix insulation to the building substrate can have implications for your insulation values and thus the overall heating load. Let’s take a close look at the implications in different scenarios and how to calculate their impact.
If your project uses metallic fasteners—nails, screws or reinforcing—that penetrate the insulation layers, a thermal bridge is created. The impact needs to be taken into account in some but not all situations.
Rule of thumb
If the majority of the insulation layer is bridged with metallic fasteners, especially if the insulation is between building materials that are thermally conductive, the impact of the fasteners can potentially be large, over 10%. In such cases, the thermal impact should be estimated.
This is particularly important when your design calls for high-performance insulation with few or no other thermal bridges. Warm roof insulation, external wall insulation and slab edge insulation are all examples of this. Because it is continuous insulation in these cases, with very low thermal conductivity, the impact of the metallic fasteners can be large. This is magnified even further if the layers on either side of the insulation are conductive, eg concrete, stucco or metal cladding or roofing. These will act to channel the heat flow into the fasteners and through the insulation.
When to overlook fasteners’ impact
You can overlook the impact of metallic fasteners if they only penetrate through a small fraction of the insulation and do not connect to conductive layers. For example, timber framed walls and roofs with screws/nails fixing cladding that penetrate the battens, rigid plywood air barrier and into the structural studs. Even with a metal roofing or wall cladding, the additional heat flow is small enough that it doesn’t need to be calculated.
Let’s work through an example. Take a wall with a high-performance continuous external insulation layer that is penetrated by metallic fasteners; the thermal impact of the fasteners is reasonably high. The illustration below is a detailed 3D finite element model from background work Sustainable Engineering did for the Nuralite Outsulation Guide. All of the insulation value is in the PIR foam insulation layer so even a stainless steel fastener’s impact is important. It increases the U-value of the wall element from 0.239 W/(m2K) to 0.248 W/(m2K). That’s an increase in heat loss of 4%. (If you think better in R-values, the heat loss resistance drops from R4.18 to R4.04 m2K/W.)
Graphic: © 2022 Sustainable Engineering Ltd. 3D input model showing one-quarter of the fastener. Stainless steel is depicted in red. The diagram on the right shows the temperature implications. The conductivity of the fastener acts to reduce the temperature along its whole length.
Estimating heat loss
Here’s how to estimate the additional heat flow of the metallic fasteners, should you have realised this is necessary. There are three options.
- 3D-model the construction in a finite element package and calculate the additional heat flow.(note Flixo cannot do this, it can only look at 2D situations). This is the most accurate method but also the most time-consuming and requires specialized software.
- Look up the additional heat flow in a handbook or as provided by a manufacturer (eg, a warm roof manufacturer can supply this detail).
- The most practical method is the hand calculation methodology in ISO6946 Annex D Corrections for metallic fasteners.
However, the ISO6946 Annex D method does have several limitations.
- It assumes that the insulation layer bridged by the metallic fasteners has conductive layers on both sides (such as concrete or stucco).
- It cannot determine the temperature distribution through the construction.
Not calculating local temperatures means the fRSI index can’t be assessed and thus the risk of surface mould or interstitial moisture. This risk then needs to be assessed by another method.
Even with these limitations, the ISO6946 Annex D method is very useful and acceptable for use in most situations; however it sometimes can yield slightly conservative results. The slide below is from a thermal bridging webinar we delivered, and it succinctly explains the methodology. The equation is straightforward, with the following three provisos.
- Be careful to use consistent units.
- Remember that nf is the number of fasteners per square meter.
- The cross-sectional area of the screws (per screw) is also in meters squared, so be prepared for quite a small number.
Graphic: © 2022 Sustainable Engineering Ltd
Sustainable Engineering most commonly uses the ISO6946 Annex D method for warm roofs, wall outsulation and slab edge insulation that has been screw fastened to slab edges.