When you are building a new house insulation is an important tick within your checklist.
I am always getting asked to provide advice on how to insulate a concrete foundation and if it is worth the investment.
Before investigating the available options for insulating a concrete foundation it is worth saying that a concrete foundation is a better option than a conventional timber floor / ring beam solution since with a concrete foundation the exposed area along the perimeter of the floor is reduced.
The image below shows the ring beam thermal bridge. The deeper the ring beam is the higher is the heat flow and the energy consuption.
Image Source: Sam Hagerman, Thermal Bridge Free Construction
The best way to increase the resistance to the flow of heat through a concrete waffle slab is to intervene on the two main thermal bridges of this construction system: the perimeter and the ribs.
Perimeter and underfloor insulation systems may be used for this purpose.
Perimeter Insulation
A variety of proprietary systems (e.g. Firth Hotedge, MagRoc… ) to insulate the perimeter of a concrete slab are available and suitable for use with most of the foundation systems (RibRaft Xpod, RibRaft EPS, Superslab, Armadillo).
There are no particular drawbacks in installing a perimeter insulation system.
When a perimeter insulation is chosen the designer should:
- determine the appropriate width of the edge footings to transfer the loadings of the building onto the ground (taking care of the effective width reduction due to the insulation board)
- determine the appropriate width of the framing of the superstructure to ensure an adequate distance for the hold down bolts from the edge of the concrete slab.
I have noticed that demand for these products has increased in the last couple of years and (without considering the financial implications) I think they might be a good solution to increase the insulation of a concrete foundation.
Underfloor Insulation
The use of underfloor insulation requires more care, in fact, inappropriate underslab insulation products may be detrimental for the performance of the slab and it may cause structural issues.
Expanded polystyrene (EPS) insulating mats are affordable and easy to find and they are often used to insulate waffle slabs either with conventional polystyrene pods or with plastic pods (e.g. RibRaft Xpod).
I struggle to recommend these insulation products because they have not been designed for this use and they might fail in case of soil subsidence, ground shaking or they may creep under the weight of the building causing structural damage or issues to the floor levels.
There are products (PE or EVA foams, Aerogel mats…) with better performance but they are significantly more expensive than EPS (or XPS which is similar to EPS).
I always recommend my clients to require an adequate warranty from the supplier of the underfloor insulation system and I suggest to install underfloor insulating mats with a maximum thickness of 25 mm.
Alternative approaches are available to provide an underfloor insulation to a waffle slab.
In Europe, the compacted shingle used for building platforms beneath the foundations is often substituted by foam glass gravel. This method is safe and it offers very good performance but, unfortunately, it is not available in New Zealand yet.
In New Zealand a reflective DPM (Firth Thermox DPM) can be used in combination with the RibRaft Xpod. This product does not intervene on the thermal bridges but increases the insulation capacity of the air pockets generated by the plastic void formers.
The return for the investment
Indicative costs to insulate a standard 200 m2 home with 70 m perimeter are:
- Perimeter insulation: 50 NZD/m x 70 m = 3,500 NZD
- Underfloor insulation: 15 NZD/m2 x 200 m2 = 3,000 NZD
TOTAL 6,500 NZD
To evaluate the potential savings associated with the investment I referred to results of a case study presented by BRANZ and available online (1):
The average heat losses equated to approximately 5.7 W/m for the uninsulated sections of the slab perimeter. This figure dropped to 2.2 W/m where the slab had perimeter insulation installed. Integrating the difference of 3.5 W/m over the 70 m perimeter length gives an estimated heat loss of 245 W. When integrated over the 3,660 hours of the data, this gives an accumulated difference in heat loss of approximately 900 kWh for the 5-month winter period.
These thermal performance improvements can be factored by a further 100% or more with the addition of under-slab insulation
A 1,800 kWh saving for the 5-month winter period have been considered.
The yearly saving (calculated for the 5-month winter period) obtained with the combination of perimeter and underfloor insulation can be estimated in the order of 0.25 NZD/kWh x 1,800 kWh = 450 NZD/year
Assuming that the performance of the insulating products and the energy cost to be maintained unchanged over the time, the return for the investment associated with the use of perimeter and underfloor insulation is about 20 years (3% discount rate).
I notice that some insulation materials (e.g. EPS and XPS) are moisture sensitive and they may reduce their insulation performance over this time (causing an extension of the period to return from the investment). See 5 Good reasons for not using Polystyrene in your foundation
Someone might say that reducing the energy consumption of the house to be a sustainable choice regardless of the financial advantage of the investment.
In this article I am not investigating the environmental benefit of insulating the foundation but I would not be surprised to find out that the enviromental impact associated with the use of the insulation products (production, transportation, pollution/waste disposal during construction, pollution/waste disposal at the end of the product life) to be superior of the environmental benefit associated with a reduced energy consumption (especially in countries using energy from renewable sources).
Fabio Parodi
CPEng MIPENZ Dott.Ing(ITA) M.Eng(Hons)
CEO and Founding Director of Cresco
www.cresco-group.com
www.cresco.co.nz
(1) https://www.branz.co.nz/cms_show_download.php?id=d9c88ce18b93a0993e4bd59c4b3e7a299637b76b
(2) https://www.genesisenergy.co.nz/for-home/pricing-plans/electricity-pricing-plans/christchurch
lateral thinking. 