You'd be amazed at the number of times I'm asked why you need to insulate the exhaust duct of a Domestic heat recovery system(MVHR). People often forget that in warming the incoming air you are making the exhaust air cold. Historically, back when heat exchangers were only 70% efficient in optimum conditions, enough heat went down the exhaust duct to prevent condensation forming on the outside of it.
Heat recovery efficiency of MVHR has risen in recent years to around 90% and whilst this is great for energy efficiency, it means that air coming into and out of a property via ducting will typically be close to zero degrees in winter months.
To put this in context, if the internal temperature of a dwelling is 20°C, it's freezing outside, and you recover 90% of the heat then air is being exhausted at just 2°C. Just think how much moisture condenses on a cold can of drink or jar of food when you take it out of the fridge? Pretty soon there's a wet circle on your worktop. Then imagine how much moisture could condense on the outside of a duct at 0-2°C!
Until recently installers have insulated the intake duct with foil-backed bubble wrap and the last few metres of exhaust duct with the same to prevent cold-bridging. This practice seems to have continued in spite of new Duct Insulation Guidelines given in the Domestic Ventilation Compliance guide of 2010, which stated the need for a minimum of 25mm thickness of a material with a thermal conductivity of 0.04 W/(m.K) This equates to a thermal resistance of 0.625 m2 K/W. In contrast, the old foil backed bubble wrap is usually only 3.6mm thick and typically has a thermal resistance of only 0.033 W/(m.K), so has a thermal resistance of 0.108 m2 K/W which falls well short of the standard. The initial lack of a suitable product from major suppliers of plastic domestic ductwork probably contributed to a slowness to react by both designers and installers.
The New NHBC chapter 3.2 published January 2014 reinforces the need for higher levels of insulation. This gives guidance on the design, installation and commissioning of domestic MVHR and pays particular attention to the insulation of ductwork to avoid condensation. The document follows studies carried out by the BRE on existing MVHR systems and industry wide consultation.
Chapter 3.2 removes confusion over which ducts need to be insulated in houses and apartments to avoid condensation and to maximise the energy recovered by the system. To summarise the guidance notes, all ductwork passing through un-heated voids such as lofts in houses and all intake and exhaust ducts within apartment ceiling voids need to be insulated along their entire length. Insulation should be continuous and Vapour resistant. This would typically mean wrapping ductwork with a minimum of 25mm of foil faced mineral-wool or the use of a pre-insulated ducting system.
Foil faced bubble-wrap is no longer an option!