Ground rules: Choosing and installing underfloor insulation and moisture barriers

 

 

Insulating under the floor and installing a moisture barrier is about more than just keeping homeowners’ feet warm and dry in winter.

A poorly insulated subfloor can account for up to 20% heat loss in a home and raise the risk of moisture and mould. Under H1 rules, all unheated slab floors must now have a minimum R-value of at least 1.5 (up to 1.7 in cooler parts of the country), and suspended timber floors need to have an R-value of between 2.5 and 3.  

Choosing and installing the right under-floor insulation 

While fibreglass, wool or polyester sheets can be used to insulate under timber floors, these days the standard is increasingly rigid board systems (typically Extruded Polystyrene, otherwise known as XPS, and Expanded Polystyrene, or EPS panels). These can be installed under suspended timber floors or placed beneath concrete slabs on the ground.  

XPS is the gold standard, made of compressed foam. As well as being incredibly durable, superior in warmth, and highly moisture resistant, it doesn’t make a mess when it’s cut. Plus, it can hold up to higher loads under a concrete slab. Load-bearing capability is an important consideration when choosing which insulation to use. If in doubt, consult your supplier. 

Expanded Polystyrene (EPS) is made of beads of polystyrene. It doesn’t have quite the same thermal performance or moisture resistance of XPS, but it’s suitable for suspended timber floors and under concrete slabs. Rigid EPS panels like EXPOL’s R1.4, R2.5 or R3.1 come in different widths for different climates and are ideal for accessible subfloors, being easy to cut to joist widths and simple to fit between timber joists.  

You can also get black graphite-infused versions, which contribute to greater thermal efficiency (some graphite products are up to 30% more effective than standard EPS panels). If space is limited but higher insulation values are required, this could be a good option.  

When installing beneath timber floors, make sure to cut any of these products precisely, as gaps reduce R-values. For concrete slabs, it’s essential to insulate around the vertical edges of the floor as well, where a lot of the heat can escape. BRANZ has a useful guide to designing insulation for concrete floors. 

Installing moisture barriers 

Before you install your insulation, BRANZ and MBIE recommend laying a polythene vapour barrier over the ground to control moisture rising from the soil. Too much moisture can end up damaging your insulation and seep into the home. A moisture barrier is also a requirement for all rentals under Healthy Homes Standards. The barrier must either meet the specifications in NZS 4246:2016 or achieve a vapour flow resistance of at least 50 MNs/g.  

The essential ingredients to laying a moisture (or vapour) barrier right are:  

• Clear and level the ground - remove debris, and level small hollows. No surface water should be able to accumulate on top of the vapour barrier, so ground should slope away.  
• Lay heavy polythene sheeting (minimum 0.25mm thick), overlapping seams by at least 150 mm. 
• Seal the edges: tape overlaps and tape barrier up walls or piers. 
• Maintain subfloor ventilation: don’t block vents. 
• Install insulation over the barrier, ensuring firm fit between joists or below slab. 

For more about under-floor insulation, visit the Level website.