Understanding R values
The term R-value everyone uses is widely mis-understood. R-value measures the resistance to heat flow of a building component. In your home multiple components are used (timber framing, air gaps, lining and insulation), which all have different R-values. Most insulation manufacturers, suppliers and bureaucrats talk only about the theoretical R-value written on the bale. The Total (or installed) long term R-value is much more important to you than the theoretical bale R-value, as this takes into account the heat flow occurring through timber, gaps, etc. In many cases these factors will reduce the bale R- value by 40 – 50 %, meaning you aren’t getting value for money.
Even if the correct product is selected for each section of your home, the MOST critical effect on Total R-value (long term) is correct installation, that needs accurate system design to counter typical installation errors.
Don’t leave any cavity between the insulation and internal lining. In theory unvented cavities increase insulation, but in practice most cavities end up vented – which halves the insulation value.
Thermal bridging through uncovered framing timber or any edge gaps (even 2-3mm) reduces the bale R value by 40-50%.
Prevent it by always specifying at least 50mm of insulation over any framing timber less than 150mm and double layer any blanket, batting or boards.
Select materials that don’t absorb moisture (less than 2% over 96 hours) if your location is anywhere near the coast (most of N.Z.), to prevent performance reduction and product deterioration in our humid coastal climates. Be wary of products made from waste or ‘recycled’ material, as these are usually inconsistent. We use only first grade materials from significant manufacturers.
Select only non combustible materials if fire is of concern in your situation. For blanket/batting materials check the weight of every bale and that the product recovers to the claimed nominal thickness within 10 minutes of opening (or double them up)
For loosefill/sprayed materials order only by weight. If a supplier’s quote doesn’t state the number of bales or weight, don’t deal with them, as they are likely to be cheating by putting in less material for a given thickness.
For Recessed lights specify all insulation is cut back at least 50mm and for loosefill/sprayed materials specify Insultech XLP protectors around every light.