Page 78 - Build 153

When average wind velocities were below 1 m/s, 3.0
no signi cant impact on thermal resistance was detected, even for low-density brous products. Wind speeds above 1 m/s had a roughly linear impact on the still air R-value for
medium and low-density brous products: ● Dataforthelow-densityglasswoolreveals a 9% decrease in R-value for a 1 m/s
increase in average wind speed.
● The value for two low-density polyester products was similarly 12% for every 1 m/s
increase. 0.0
2.5
2.0
1.5
1.0
0.5
Solution for wind wash in exposed sub oors
This means wind generally has an insig- ni cant e ect on insulation where a closed sub oor has a Building Code level of ventila- tion openings (3,500 mm2/m2).
Where the sub oor insulation is exposed, good thermal design can be achieved by increasing the R-value of the insulation mate- rial by approximately 25% to counter the wind-wash impact. This assumes an average wind speed of 2 m/s under the oor.
It would mean that an R1.9 low-density 1.0
brous insulant could be assumed to perform as e ectively as an R1.5 medium or high- density insulant in an exposed sub oor.
0.0
3.0 2.5 2.0 1.5
0.0 0.5
1.0
1.5 2.0
2.5 3.0
3.5 4.0 4.5
24-hour average wind velocity (m/s) at ground level Figure 3: Thermal resistance of high-density polyester segment.
0.5
For more The BRANZ sub oors study report will contain more detail. It will be available from www.branz.co.nz in April.
0.0 0.5
1.0
1.5 2.0 2.5 3.0
3.5 4.0 4.5
24-hour average wind velocity (m/s) at ground level Figure 4: Thermal resistance of low-density polyester blanket.
76 — April/May 2016 — Build 153
24-hour average R-value 24-hour average R-value