Air tightness strategy with trussed roof.

[Oz07]

Whats the options for dealing with airtightness to MVHR levels when using a fink trussed roof? Single storey property. The loft space wont be useable and will perhaps have 1 access point for any maintenance required. Building is blockwork so trusses will sit on top of a wall plate. MVHR unit to be in 'loft' area

 

My initial thoughts are from trusses down, Smart ply or some kind of airtight ply. Tape around perimeter over wall plate onto blockwork. Parge coat on blockwork to overlap tape. 2 layers of 2x2 conterbattened below ply on trusses to create a void for services so electrical/plumbing doesn't enter loft space and can achieve airtightness at first fix stage without having to worry about subbies penetrating. Tape around loft access point. Tape/seal around MVHR points.

 

Would airtighness board be recommended or is a membrane over the trusses/thin standard ply/osb a better way to go?

Are decent air tight loft access points readily available off the peg or is it a case of knocking own up with copies amount of draught excluder as per normal?

Would it be easy to seal around MVHR plenium penetrations through air tight?

 

Cheers

[joth]

> Are decent air tight loft access points readily available off the peg or is it a case of knocking own up with copies amount of draught excluder as per normal?

 

They are, Green Building store has one.

 

Note that generally it's advised for the MVHR unit to sit "inside" the airtightness layer, otherwise there's a lot more penetrations in the airtight layer for all the room ducting (can be minimized by having  a single pair of ducts come down into manifolds on the warm side). Also depending where the insulation layer is, the MVHR is likely more efficient if it's in warm area (total length of warm ducts in a cold loft generally higher than the  length of cold ducts line warm loft)

 

[Oz07]

The footprint is tight but am trying to do my best with it. Were talking 900sq ft here and ideally wanting 2 bed, study, 1 bath and separate wc so space at a premium. 

 

May be able to fit a tight ish plant cupboard in 1200x750 mm ish if can get boiler and mhvr in here?

 

I did Google and seems to be a bit of can of worms putting mhvr in cold roof but don't mind taking time with detailing if it can be done

[joth]

Even if you put the mvhr in a gnd floor cupboard, will you be able to get ducting from it to every room without going into and back out of the loft?

 

If space is tight seems an even stronger case for shooting for a warm loft, as you can put more plant or useful storage there. Is it a new build or retrofit? (I'm now expert on roofs so can't help either way, other than mention our retrofit we've been pursued to do a warm loft even though we're not converting it to living space)

[Oz07]

No chance of warm roof unfortunately. The wall plate is higher than usual so yes I have room to bring ceiling level up to around 200mm below trusses and still have 8ft ceilings