Adding insulation: not always good In fact, doing it wrong can be very bad.

21 June 2019 by Jason Quinn

The deadline for insulating rental properties is imminent and anyone who can install insulation is going flat out. The legislative push to improve the horrible state of many cold New Zealand houses is a good thing. But: buildings are complicated. There’s a risk that adding insulation will make older buildings perform worse than before.

Without insulation, escaping heat helps dry out the building assembly — the walls, floor and roof. If you reduce the amount of heat leaking out by stuffing in some insulation, you can make it slightly less cold inside but also damper because you’ve reduced the capacity of the building to dry itself out.

I’ve been revisiting one of the first projects I consulted on, a retrofit of a run-down 70-year-old apartment block. At the point I was called up for some advice, the plan was to install fibreglass batts on the inside of the cast concrete walls and add more insulation under the membrane roof. I was very concerned that the added insulation would the building worse—not better, as was intended. After some work and computer simulation we selected durable assemblies that provided great results in terms of insulation values, improved building durability and increased comfort for the people living there.

I’m pulling together a detailed case study on that retrofit but it mostly focuses on the wall insulation solution. Here I want to discuss the roof retrofit.

Flat roofs are sexy. I grew up looking at pictures of Frank Lloyd Wright buildings and love the way his low roof lines just sit into the landscape. But flat roofs are not good from a physics point of view. And adding insulation to a flat roof can really be deadly. You had an uninsulated roof that was performing well, perhaps for many, many years. You add insulation to it and suddenly, you’ve got moisture problems. The roof now gets damp and mouldy or even potentially structural rot. You’ve destroyed the roof, and with what? With insulation.

Flat roofs are either a  cold roof or a warm roof. In a cold roof, the structural deck is above the insulation and therefore cold; a warm roof has insulation over the structural deck, keeping it warm.

A flat, timber deck cold roof has a layer of structural plywood directly underneath the roof membrane and insulation (if any) below the plywood. If the living space below is relatively well ventilated and heated, then an uninsulated cold roof stays dry because the heat and dry air can get all the way up to the plywood to dry it out. Plus, sunlight on the top of the roof drives any moisture that does get into that plywood back down into the space below where it can be ventilated away safely.

Figure 1: The warm roof is recommended but a carefully ventilated cold roof with a well selected air-vapour control layer can work as well.

An uninsulated cold roof can stay dry and lasts for a very, very long time. The main longevity issue is usually the membrane wears out and then it starts leaking from rain. Now, if you take that cold roof design and chuck insulation batts underneath it as a retrofit solution, you can easily add enough insulation that that roof deck is sufficiently cold where moisture will accumulate.   Whether the moisture level becomes high enough to cause mould or rot depends on many things including the colour of the roof, the air and vapour leakage at the ceiling level and how much insulation you added. But, in general, just chucking installation under a cold roof deck is not a good idea. You’ve taken a roof assembly that was durable (OK—it was also cold and nearly impossible to heat) and you’ve turned it into one that is not durable. I’ve seen examples of cold roofs with added insulation growing mould in Wellington and in Auckland. Some of these are new buildings, with insulation added with building consent, as recently as 2009.

Alright. What should you do, what is the best-practice solution? Make your cold roof a warm roof, where the insulation goes above the roof. You cover the structural roof deck with an air-vapour control layer, then rigid insulation and the roofing membrane. With a warm roof, as you add more and more insulation, the plywood (or other roof deck) gets warmer and warmer; as you add more insulation the roof gets drier. The only concern with a warm roof is making sure there is an air-vapour control layer below the rigid insulation. I’ve not aware of any failures in New Zealand (yet, at least), but overseas in climates like Wellington there have been issues with warm roofs that left out this layer.

Finally, to be fair, cold roofs with insulation can be made to work. The most common solution is to carefully leave a gap above the insulation and ventilate the gap between that cold timber deck and the insulation with an air-vapor control layer underneath the insulation at the ceiling. This works by preventing the warm, moist air from the interior of the building from getting into the roof; and any moisture that does leak into the roof (by diffusion or perhaps through a small hole) can then safely ventilate out above the insulation and below the timber deck. That keeps the timber deck dry and therefore safe from mould or rot. This is what’s implied in the Building Code as well as the Roofing Association’s handbooks. In practice this can be very difficult: with timber blocking and a low pitch roof, you need a way to make the air move through that roof space. Blue arrows on a drawing aren’t enough. You must have a functioning path that allows air to ventilate the moisture out.

My point is everybody involved with designing, building and maintaining buildings—and the people who make policy about those buildings—need to understand some basic physics about moisture and heat transfer and the ways that buildings dry out. We’ve got the computing power to accurately model how alternative approaches to insulation, for example, will affect the rest of the building. Let’s not create new problems when we try to fix existing ones.

I do much prefer to prototype in a computer model rather than in my clients’ buildings.

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