Installed performance of ceiling insulation

By - , Build 112

What gives the best performance – insulation installed between framing, over framing or a combination? This question was put to the test in a recent BRANZ research project.

Figure 3: Installing insulation between and over the top of framing gives the best R-values.
Figure 1: Polyester insulation fitted against framing in the way the R-value has traditionally been calculated.
Figure 2: Thermal resistance improves when insulation is cut to the width of the space between framing plus twice the width of the frame (2 × 45 mm).

BRANZ recently compared the in situ performance of ceiling insulation installed in the more traditional layout of friction-fitting between ceiling framing, with the same insulation installed over the top of framing (without any insulation between the framing).

Different options assessed

The project used heat flux transducers to take in situ measurements of thermal resistance. This was measured for the three ceilings in the study after the same insulation material had been installed in each of several different ways:

  • The test ceilings and existing insulation before a sufficient area (approximately 4 m2) of the insulation was removed.
  • Insulation installed over the framing.
  • Insulation cut and friction-fitted between the framing.
  • Insulation installed both between and over the framing.

Further laboratory-based measurements were made using the BRANZ heat flow meter, in conjunction with thermal modelling, to confirm the thermal effect of insulation friction-fitted around framing.

Size matters when insulation fitted

Figure 1 shows polyester insulation fitted between framing. R-values have traditionally been calculated for an installation like this using one-dimensional calculation methods, such as NZS 4214:2006 Methods of determining the total thermal resistance of parts of buildings. This incorrectly assumes that insulation is cut to the exact width of the spaces between framing, with no insulation over the top of the framing. It follows that increasing the thickness of the insulation will not reduce the thermal bridging from the framing.

In reality, however, insulation cut to friction-fit the frame spacing is the width of the space between the framing plus the width of the framing (usually 45 mm). This results in the insulation partially covering the framing, reducing the thermal bridging. If the insulation thickness is increased, the thermal bridging would be expected to reduce. However, there will still be some convective heat transfer into the ceiling space above the insulation because of the insulation gap above the framing.

Cutting the insulation slightly wider enables the insulation to close up over the top of the framing, minimising the convective heat transfer. Figure 2 shows insulation cut to the width of the space between the framing plus twice the width of the frame (2 × 45 mm) and installed with the segments end-on to the framing. In this case, the insulation closes up effectively over the top of the framing, and the thermal resistance expected would be close to that achieved when insulation was installed as two layers.

Over framing for simple framing

In principle, ceiling insulation installed over the top of framing creates a still air space between the ceiling lining and the underside of the insulation, providing additional insulation performance.

However, this study found extreme care was needed to fit the insulation with tight enough joins between sections to prevent severe convective heat losses through the insulation layer (see Table 1). These installation difficulties represent a significant risk to the thermal performance. The study recommended that the method should not be viewed as achieving better overall performance than the same material fitted between framing.

Figure 1: Polyester insulation fitted against framing in the way the R-value has traditionally been calculated.
Figure 2: Thermal resistance improves when insulation is cut to the width of the space between framing plus twice the width of the frame (2 × 45 mm).
Figure 3: Installing insulation between and over the top of framing gives the best R-values.

The level of care necessary when fitting insulation over the top of framing may limit its suitability to relatively simple framing profiles where truss members do not interfere with the ability to reliably butt insulation sections together. This need for care may also negate some of the assumed installation speed advantages of rolling out insulation over the top of framing.

Table 1: Summary of results from the three ceilings.
  Thermal resistance (m2K/W) Estimated measurement (uncertainty 10%)    
  Ceiling ACeiling BCeiling C
Original insulation R1.5 R1.7 R2.2
Single layer of insulation over the top of framing R1.5 (initial)    
  R3.5 (refitted to close gaps)   R1.5
Calculated from modelling R3.6    
Single layer of insulation friction-fitted between framing R3.5   R3.7
Calculated from modelling R3.6    
Layer of insulation between framing and second layer over framing >R5 R4.8  
Calculated from modelling R6.5 R5.2  

Better fit, better performance between framing

Where insulation was cut to a suitable width and fitted between the framing, it was possible to achieve a visibly continuous insulation layer with no framing showing. When this happened, the thermal performance measured was significantly better than is normally calculated for the common situation with visible framing. it was close to what is expected with the same insulation over the top of the framing, with no convective losses assumed.

Thermal modelling has demonstated that it would be possible to adjust NZS 4214-type calculations to account for the better thermal performance achieved when insulation is fitted to cover the framing. in contrast, it would be difficult to model the situation of convective bridging through insulation fitted over the top of framing.

Between framing has many pluses

Fitting insulation for good performance is easier between framing than over the top of framing. Convective losses from gaps are more likely in the latter case.

It is relatively easy to see that insulation fitted between framing is insulating the framing – the framing will not be visible when this has been done correctly. visually assessing that insulation installed over framing has been fitted well enough to minimise convective bridging is much more difficult.

Between and over the ultimate

As would be expected, installing a layer of insulation between the framing, and another on top and over the rafter, achieved the best results (see Figure 3 and Table 1). The downside is that it takes longer to install.

For more

For more information, see BRANZ Study Report SR211 on www.branz.co.nz.

Download the PDF

Articles are correct at the time of publication but may have since become outdated.

Figure 3: Installing insulation between and over the top of framing gives the best R-values.
Figure 1: Polyester insulation fitted against framing in the way the R-value has traditionally been calculated.
Figure 2: Thermal resistance improves when insulation is cut to the width of the space between framing plus twice the width of the frame (2 × 45 mm).

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