MVHR - would you keep the eaves space warm?

[SBMS]

Hi

 

On our build the top floor has attic trusses and is a fully habitable room.  The MVHR ducting runs within this eaves space.

 

There are two options from the MVHR supplier for the ducting in the eaves - make the eaves a 'warm space' (lagging just the intake and exhaust with a 50mm isosleeve), or keep the eaves cold (lagging all the supply and extract that are in the eaves):

 

Our current plan is to insulate between the rafters with 150mm PIR, with a 35mm cross batten of PIR. We were then going to insulate just the 'room' on the top floor with insulated plasterboard, and leave the eaves cold, but are now wondering whether we should not put the insulated plasterboard on and allow the eaves to be heated from the room? I've included a colour coded view of our options - the red and green bits (rafter insulation) is fixed:

 

 

So we have 3 options (plus a fourth option where we don't make the eaves warm):

 

1. Insulate the room on top floor with insulated plasterboard, but leave the floor space of the eaves uninsulated, so heat rising from floor below keeps it warm

2. Don't insulate the room on top floor with insulated plasterboard, but insulate the floor space of the eaves so heat from the top room keeps it warm

3. Don't insulate the room on top floor with insulated plasterboard OR the floor space of the eaves, so heat from the floor below and the top room keeps it warm

4. Insulate the room on top floor with insulated plasterboard and the floor space of the eaves, and lag the MVHR ducting with ISO sleeve (i.e. make the eaves cold)

 

We've pretty much been left to it ourselves (MVHR company won't make the call, SAP guys won't either).  Our roof UValue for SAP Calcs is provided by the 150mm PIR + 35mm cross batten so it's more a design choice for ourselves..

 

Any thoughts?

 

MVHR Plan (all ducting is in the eaves space):

 

Edited December 12, 2022 by SBMS

[BenP]

I think you're over thinking it personally. 

 

You are within you're proposed thermal envelope (thanks to between and over on the roof) so it's not going to be 'cold', just unheated. Maybe some token lagging may help in the depths of winter?

 

If it was me I'd ditch the expense of the insulated plasterboard and put the 100mm Rockwool across the whole floor as this is mainly for noise separation. 

[crispy_wafer]

For what it's worth, With our attic trussed room, I PIR'd from ridge to wall plate, seemed easier in my mind to detail insulation and airtightness running in straight lines from top to bottom.

[SBMS]

1 hour ago, BenP said:

I think you're over thinking it personally. 

 

You are within you're proposed thermal envelope (thanks to between and over on the roof) so it's not going to be 'cold', just unheated. Maybe some token lagging may help in the depths of winter?

 

If it was me I'd ditch the expense of the insulated plasterboard and put the 100mm Rockwool across the whole floor as this is mainly for noise separation. 

Would you put rockwall in the dwarf walls (sound insulation) again?

 

I might be missing the point here, so if we did insulate the floor and dwarf walls, would the eaves space not end up cold? Trying to work out what the actual difference between cold and unheated is... I guess eventually the heat from floor below and the attic room would end up migrating into the eaves space anyway?

[ProDave]

I would insulate the whole roof space as in all the insulation follows the line of the roof, easier to detail and you don't have to worry about a cold eaves space.  Even better still if it is not too late for you is make it a warm roof (insulation on outside of trusses)  really easy to detail and makes a  very nice cosy house.

 

This is one of the decisions I am best pleased with.

[Radian]

7 minutes ago, ProDave said:

This is one of the decisions I am best pleased with.

Oh how I wish I could wind back the clock and do this on our garage roof conversion. Ruled it out at the time due to the increase of ridge height it would entail. But the reality of having to fit PIR between rafters and get a tight fit turned out to be near impossible. Compriband is crazy expensive so didn't use it. I also worry that the overall 50m PIR beneath the rafters brings the dew-point too close for comfort on their underside. How much simpler to deck over and lay a continuous PIR raft on top. Dammit.

[SBMS]

1 hour ago, ProDave said:

I would insulate the whole roof space as in all the insulation follows the line of the roof, easier to detail and you don't have to worry about a cold eaves space.  Even better still if it is not too late for you is make it a warm roof (insulation on outside of trusses)  really easy to detail and makes a  very nice cosy house.

 

This is one of the decisions I am best pleased with.

Thanks @ProDave.  Unfortunately roof is battened and tiling has begun so can't switch to warm roof.  We were incredibly tight with ridge height due to planning requirements so would have been a push anyway.

 

Do you have a view on the internal insulation of the room (yellow and pink bits on my dreadful diagram) for the MVHR?

[ProDave]

I would still put ALL the insulation following your green line so all insulation is at the roof level.  but that only shows 35mm PIR which is way to little.  I assume you will be insulating between the rafters as well, leaving a ventilation gap above it?

[SBMS]

38 minutes ago, ProDave said:

I would still put ALL the insulation following your green line so all insulation is at the roof level.  but that only shows 35mm PIR which is way to little.  I assume you will be insulating between the rafters as well, leaving a ventilation gap above it?

Hi @ProDave yes - it was on the diagram as the Red line (150mm between rafters), then green line is 35mm over the rafters.

 

Our rafters are 175mm so we have a 25mm air gap/felt sag, but we are using a breathable membrane (and have MVHR) so building control are happy with that setup.