Insulate supply and exhaust in an unheated loft space?

[Tommy8884]

Hi All, 

I have finally got around to installing my MVHR. Its in a VERY tight unheated loft. I have built an insulated box for the system itself (a box lined with some spare multi layer foil insulation). I'm in the process of looking for ducting options, its going to be challenging as the loft is so tight (very low pitched roof)

Anyway, all advice seems to say insulate the supply and exhaust. I'm struggling to understand why I would in my case, with the loft space being unheated, the temps should be close to outside in winter, so there is certainly no point insulating the supply, and probably not the exhaust? The exhaust might be a contender but will only have about 1.5m to run to the outside. 

I of course intend to insulate the supply and exhaust to the house. Mostly by placing between the 2x150mm layers of insulation in the loft. 

Thanks in advance

[Marko]

Not sure if you're on about the ducting supply and exhaust air (house side) or the supply and exhaust (external). Regardless, there's always the risk of condensation in the winter. In the summer my loft space is gets extremely hot so I have individually insulated all ducting (house side) with ISO sleeves as well as 300mm loft insulation on top. Just belt and braces to avoid the house overheating (even with summer bypass). I don't know if that answers your question. 

[Mike]

+1 for @Marko's advice. Also, make sure that the exhaust (to the external air) is either falling towards the MVHR unit, or towards the outside, so any condensation runs out of the duct.

 

If you can find the space, I'd also upgrade your multifoil-lined housing with something thicker.

[Tommy8884]

Thanks both. I was referring to the to house and from house side of the ducting. I think as it's only a very short run I will leave the from outside duct, but the point around overheating is well made and I hadn't considered that. Thanks 

[gregcope]

+1 to comments above.

In winter you want to insulate the exhaust to ensure no condensate freezes inside it, as there will be some residual vapour as the HE is not 100% efficient and on the house side it will be full of damp (very damp if showering) warm air.

 

In summer you want to insulate the supply as your loft could get very hot (50C) and you will be hence heating your house supply on days when you want it colder.

I have a temperature sensor in my loft.   It varies from -2C to 50C+ South facing pitch roof, in sussex.