Loft Conversion Draught Issues and Roofing Query

[HarryN]

Hi All

We have recently had a dormer loft conversion completed and experiencing a draught from the dormer side of the room under the skirting board. Having looked through an inspection hole I've noted there is very little insulation in the floor at the front end of the dormer. When I've challenged the builder, he pushed back stating there needs to be air flow in the joists and so they don't fully fill the floor with insulation, and that the draught issues are only a problem because it's been extremely windy as of late. Does this make sense? Do the joists need such extreme ventilation?

On a different point, as we are experiencing issues with the loft retaining heat, so I've been looking through phots on how they put things together. The roof, which I previously shared concerns with them about, still doesn't make sense to me. Attached is a diagram of how the roof is constructed. It looks a hybrid of a warm and cold roof. I would really appreciate your view on whether this is accepted form of construction and that we are not in for issues further down the line.

Thank you in advance!

 

[Wayners]

I don't know the answer

Where is building control? You have planning permission and inspections?
Phone and ask.

I had a loft conversion and the 2020mm gap between ceiling and upstairs bedroom floor is full of rock wool insulation.
It acts as a fire barrier and insulation..

 

[HarryN]

First time getting works done. We went with a design and build loft "specialist". Planning permission not required as it was done under permitted development. Building Control is private firm engaged by the builder and have been useless to say the least. I'm trying to improve my understanding so I can deal with the builder more effectively.

What confuses me the most is that we gave them a loft with 300mm insulation. No idea why the floor isn't full to the brim, especially considering some of that 300mm would have had to be redistributed given the stairwell cut out!

An overall bad experience with many learnings of how not to engage contractors!

 

[freddiemercurystwin]

That roof construction is wrong no matter what.

A warm roof has the insulation on the outside, sat above the joists with the roof deck sat on top (and no ventilation.

A cold roof has the insulation sat on the plasterboard with a ventilated cavity above, between the insulation and the underside of the roof deck.

'Loft specialists' should be employed very carefully and using their own private Building Inspector is a big red flag but you know all that now.

The following are typically how the insulation should be placed for a dormer conversion though the insulation requirements have been increased for the walls, pitched and flat roof since these details were produced so ignore the insulation thicknesses in the text, but the principles remain the same.

 

[HarryN]

Thanks Freddie. Having looked through old photos it looks like the insulatiom is sitting on top of the rafters, so the construction is fine. I am just not clear on why there is a ventilation gap on a warm roof!

I'm usually very careful but have clearly misjudged on this occasion!

 

[freddiemercurystwin]

That void should not be vented, if it is it defeats the whole point of insulating above.