MVHR running between rafters??

[Stephen Aitch]

Hi,

We are building a chalet bungalow and are battling with MVHR ducting 75mm diameter.  This is supposed to run up internal partition walls, but for a number of reasons this has proven impossible for our bedrooms.

 

Our builder is suggesting running these up the roof between the rafters and then back into the room.

 

My worry with that is that we will need to decrease insulation where the pipe runs in the roof - from 150mm to 75mm.  This could cause our air supply to gain warmth in summer and cool in winter.

 

Has anyone else encountered this please?  Any better solutions or will this perform ok?

 

 

 

 

[john0wingnut]

Could you not look to use the rectangular style ducting which is flatter and wider?

 

Failing that, you have a 50mm cavity, then 150mm of insulation, so you would in theory only need to reduce insulation by 25mm to allow for ducting

[Russdl]

I'd keep it out of the insulation if it was me. Will there be any built in wardrobes or such like that it can be hiden behind? 

[Russell griffiths]

I’m not 100% sold on your roof build up. 

1.. I don’t believe you need to leave the 50mm void between insulation and your sarking board. As you have a ventilation gap above the boards.   I’m happy to be told I’m wrong. 

2. What is providing your vapour control layer ?

 

so why not use standard pir under the rafters instead of insulated plaster in the area of the duct and bring the duct closer to the surface so it has full insulation behind it and only a strip of pir missing where the duct is. 

[Russell griffiths]

Having thought a bit more, you wouldn’t need to cut out the pir to fit the duct, just push the duct into the other insulation. 

 

I would still up the other insulation to 200mm and check what you are doing for a vapour control layer. 

[joth]

Where is the airtightness layer in that roof build up? The MVHR room ducting is best kept entirely within that

[Russell griffiths]

Oh god, made a cock up read your post to quickly, 

i had assumed your insulation was a rockwool type fluffy bat

when it is actually pir board. 

 

In that case I would change that duct to the oval stuff and chop it into the second layer of pir under the plasterboard. 

Having removed 50mm of insulation in a small strip will not make any difference to heat loss. 

 

Its the lack of vapour control that is the issue. 

[MikeGrahamT21]

Agree with Russell, counter battening gets rid of the requirement to leave a gap within the insulation layer, you can full fill.

[oranjeboom]

13 hours ago, Russdl said:

I'd keep it out of the insulation if it was me. Will there be any built in wardrobes or such like that it can be hiden behind? 

 

That's what I am doing upstairs. Up through floor and into built-in w/r and then out through a top panel of the w/r. You could also just encase it in an exposed false beam (glulam-look rather than mock tudor!) or box the pipe in a corner of the room (if you have any). You could also just not hide it and have it exposed in some steel/aluminium pipe if the look is okay for you.

[Stephen Aitch]

thanks all for the inputs.....thiought I had acknowledged earlier but it didn't go through. Anyway...very much appreciated.

 

Taking on board the feedback we will be keeping the MVHR pipework inside the eaves cupboards rather than cutting into insulation and jeopardising the air tightness.

 

Many thanks all