Need for making ceiling cut outs airtight with a warm roof?

[Madasahatter]

Wonder if anyone could advise.

 

Single story ICF build with warm roof - so an airtight 'box'.

 

MVHR will circulate both the ground floor living space and the roof space void.  I wish to do this in order to reduce the condensation risk of an infrequently visited space.

 

Most of the lighting will come from down lighters.  

 

So the question is do all the cut outs for the down lighters need to be taped up and made airtight?

 

Thanks in advance.

[JohnMo]

Easy enough.  VCL/airtight layer below insulation, tape to wall junction.  Then install 50mm counter battons, then your plasterboard.  Use low profile down-lights circa 15-20mm deep., They come with a small module that is about 50mm high that installs in the void, wiring can be run in the void also.

[Nickfromwales]

3 hours ago, Madasahatter said:

Wonder if anyone could advise.

 

Single story ICF build with warm roof - so an airtight 'box'.

 

MVHR will circulate both the ground floor living space and the roof space void.  I wish to do this in order to reduce the condensation risk of an infrequently visited space.

 

Most of the lighting will come from down lighters.  

 

So the question is do all the cut outs for the down lighters need to be taped up and made airtight?

 

Thanks in advance.

Do you mean the MVHR will service the rooms in the roof space, eg you will have habitable spaces up there? Infrequently visited spaces will all need full building regs compliance, eg the guarantee of airflow to manage CO2ppm / humidity etc, which is not a "choice". Not all spaces in an airtight house need to be serviced directly by MVHR duct placement, as a house with MVHR running 24/7/365 will already be inherently 'dry'.  

What is your target airtightness score, eg to know MVHR ( certainly the heat recovery element ) will be of any useful functionality. If the design airflow raters are less than the residual natural infiltration rates then MVHR becomes a questionable choice. You should be aiming at a score of <1ACH if wanting to maximise the return on the investment.

[Temp]

4 hours ago, Madasahatter said:

Single story ICF build with warm roof - so an airtight 'box'.

 

MVHR will circulate both the ground floor living space and the roof space void.  I wish to do this in order to reduce the condensation risk of an infrequently visited space.

 

Can I check if you mean..

 

a) A warm loft/room in roof

Or.

b) A warm roof construction that puts all the insulation above rather than between the rafters

 

 

[Madasahatter]

I don't think I explained myself to well, so here's a bit more detail.

 

The void warm roof space is a pitched roof with an internal ridge height of 2m.  This is not a habitable space as that could breach a covenant, and is designed to make conversion very difficult.  All that will be in the void space will be service runs, to include the MVHR ducting which will run over the rafters.  It is intended to ventilate the void space with the MVHR as well as the living quarters below.  So, I will have an airtight box <1ACH 

 

The void roof space will have no windows.

 

I'm going to end up with about 44 down lighters, that the fittings would be air sealed taped.

 

On reflection I'm thinking this may not be required as both the habitable and void spaces will be serviced by MVHR, so does it actually 'matter' if the down lighters aren't airtight?

 

What do the good people of Buildhub think?

 

Thank you in advance for your help.

[Temp]

OK so the downright aren't in the loft void, they are in rooms below? If that's the case then no need for them to be air tight.

 

If they are in the loft void then more info needed about how the roof is insulated. 

[Temp]

Just now, Temp said:

OK so the downright aren't in the loft void, they are in rooms below? If that's the case then no need for them to be air tight.  

OK so I should clarify...There should be no need to make them air tight if the ceiling void is reasonably well sealed to the outside world. However not all ceiling voids between floors are well sealed. Sometimes floor joists penetrate into cavity walls.

[JohnMo]

Thought I understood what you were doing, now just confused.