MVHR duct route issues (Glulams in the way)

[Smallholdertoo]

We are in the process of building our new house a 1.5 storey, well insulated house with MVHR. The timber framers are finishing the roof structure (400mm metal web rafters to be filled with cellulose fibre) and sarking is about to be fitted. MVHR ducts must be fitted next week per the framers !

We have a design for the MVHR which assumed that we could have ducts passing though the 400mm deep rafters, as suggested by the framing company early in the project. Unfortunately, the structural engineer added glulams that not only block ducts passing from the plant room to the south end of the house both in the joists and the roof joists . For most ducts we can bypass the glulams by moving from the joists up and over them within the boxed in section  adjacent to the FF eaves. This sorts the down stairs ducts and all but 3 of the upstairs ducts. These relate to beds 1 and 2 as well as the bathroom. They will have to pass from the joists, within the boxed in eaves and through the roof rafters sometimes more than once (see photo). I am concerned that from an airtightness POV this will be a nightmare, as the roof airtight layer is Intello which will attached to the rafters and held in place by 50x25battens (to be covered by plaster board later). I have nightmares of long cuts or even worse – rips and endless airtightness tape.

I can reduce the risk noted above by trying to persuade my wife that the vents could be on the sloping part of the ceiling or even the low wall that boxes in the eaves rather than the small flat area of ceiling in the bedrooms (-any thoughts). The use of the small wall would be easiest and would reduce the number of times the ducts will have to pass into the roof and therefore be sealed.

 

I am also thinking of using offcuts of smartply with holes cut for the duct  to be attached where the ducts will join and leave the roof to help with airtightness and to reduce the risks of rips.

 

Although the plan shows a floor vent in the FF gable at the front of the house, We will revert to the original  design of having supply 2 feeding the ground floor area within the front gable . My wife does not fancy a grill in the floor of her 'snug'

 

Any suggestions would be appreciated?

I will add photos of the roof shortly

[Nickfromwales]

Box off the bottoms of the eaves and have a service cavity there? 

[Conor]

Create a service void along your ridge and have all the ducting run there. You only need 150mm or so. The. You run a single supply and extract duct up from your mvhr to manifolds housed in the void. keep it all withing insulated and airtight envelope.

[Smallholdertoo]

Thanks you both.

 

I was coming to the same conclusion which was confirmed when I saw a friend is currently building a passivhaus. We will still take the three pipes, via the roof cavity, straight up  adjacent to the north gable. The pipes will exit near to the top of the roof into  a space between the flat ceiling of bed 1 and the roof, From there they will pass above the flat ceiling of the bathroom roof with one going above the ceiling in bed 2.  As the ceilings don't yet exist i will need to cut off the pipes at the required lengths and initially leave them dangling!

 

I still propose that the ducts to pass through 95mm holes within boards attached to the rafter when entering and leaving the airtight roof space,  to aid the taping of the ducts and ensuring that the membrane does not rip.

[SteamyTea]

Not read all of this.

But I was wondering why there are no 'pipe in pipe' systems.

A 0.15m diameter pipe, within a 0.22m pipe would give just about equivalent areas. This could be run as a main trunk route, then with suitable joints/adapters, individual rooms could be run as spurs.

As the surface area would be quite large, there would be extra efficiency gains as well.

 

A 10m length of 0.15m diameter pipe will have a surface area of 4.7m².  Depending on material and thickness of the inner pipe it should be possible to get around 200W/K through it.

That does not seem a lot at face value, but it is half my heat load during the winter.

 

Edited 1 hour ago by SteamyTea