Cold unventilated loft - condensation risk?

[G and J]

2 hours ago, ETC said:

Read the BBA Certificate for the underlay. It will say if you need roof ventilation and under what circumstances you can omit eaves and or ridge ventilation. Some underlays can be installed without roof ventilation subject to a well sealed ceiling being installed.

 

On reflection I think Tyvec might be one of those that do allow an unvented roof but I think down lighters are not allowed. Check it out.

Noted re the BBA cert, have started that process.  
 

The few downlighters would be in the bathrooms, and we could have a boarded service void just in those rooms which would reduce the number of penetrations. 

[G and J]

10 hours ago, Iceverge said:

A quick sketch. Couple of things. 

 

I'Ve assumed a 35 deg roof pitch. 

 

PIR in the walls......................between timbers........................groan..............Anyhou.

 

 

Here's a sketch of my idea.  A fink truss with extended 300mm tails to allow for increased ceiling height. 

 

 

 

 

 

 

 

 

 

 

 

I was braced for the grimace re PIR in the frame!  I do note your concerns….  and I will price up blown cellulose for the loft…..   however as for the ceilings…..

 

I think I’ve understood, so I’ve drawn up what will be our version.

 

 

I know we could easily raise the ceiling more, but I hate chamfered rooms.

 

I did briefly consider raising the tie enough to accommodate everything, wires and ducts.  But it feels like more work and materials thus more time and money than plasterboarding straight onto the bottom chord of the trusses.

 

 

 

 

[G and J]

PS we’ve a 40degree roof.  And yes, we know that we could achieve the same headroom improvement much cheaper and easier by going to 39degrees…

[Iceverge]

How high is your ceilings as is? 

 

The "feel" of the height of a ceiling is more important than the actual height. 

 

Mathematically I think it's the ratio between wall length and ceiling height. A 2.4m ceiling in a 5m room will feel lower than a 3m wall. Vertical divisions like doors and windows on the long wall will make the ceiling "feel" higher. 

 

 

This will "raise the ceiling"

 

 

 

 

This will "lower the ceiling

 

 

This will make everyone sick. 

 

 

[Iceverge]

2 hours ago, G and J said:

But it feels like more work and materials thus more time and money than plasterboarding straight onto the bottom chord of the trusses.

 

Don't do this. 

 

Who ever is installing the plasterboard will tear the membrane. 

 

Use 70*20mm strapping to make a service cavity below. It might enable you to push the trusses to 600cc too rather than the standard 400mm needed for 12.5mm plasterboard 

 

 

[SteamyTea]

If you fit LED lighting in a ceiling, you need to make sure they can cool correctly. 

The light fitting manufactures will show minimum clearances, but they are just that, minimum dimensions.

[G and J]

5 hours ago, Iceverge said:

 

Don't do this. 

 

Who ever is installing the plasterboard will tear the membrane. 

 

Use 70*20mm strapping to make a service cavity below. It might enable you to push the trusses to 600cc too rather than the standard 400mm needed for 12.5mm plasterboard 

 

 

 

Plasterboard ceilings need to be supported on 400mm centres?  Good heavens, I’m so out of date.  I’m standing in my kitchen and both upstairs and downstairs ceilings are 12.5mm plasterboard straight onto 600 centres joists or trusses.  But then, this house was built a third of a century ago.  How times change.

[Iceverge]

I'm not an expert but I thought that was case. Maybe @nod or someone has chapter and verse 

 

I would double board them too. 

[nod]

I wouldn’t  omit either 

You need that airflow 

 

[G and J]

1 hour ago, nod said:

I wouldn’t  omit either 

You need that airflow 

 

Goodness, I’m so confused.  What are the things you wouldn’t omit?

 

And by airflow do you mean in the loft?

[nod]

7 hours ago, G and J said:

Goodness, I’m so confused.  What are the things you wouldn’t omit?

 

And by airflow do you mean in the loft?

Eaves and ridge ventilation 

[Iceverge]

I'm happy to stand my ground on having a sealed loft with no ventilation as per the tyvek detail. 

 

I had it out with the roofers on this one and they all were shaking their heads but that was 4 1/2 years ago and no issue so far. 

 

I was wondering @nod your thoughts though on plasterboard @400cc vs 600cc? 

 

 

 

 

[G and J]

1 hour ago, Iceverge said:

plasterboard @400cc vs 600cc?

Bear in mind that trusses at 400mm centres would mean I’d have to attend slimming world like forever!    :-0 

[G and J]

Just thinking through all this, and I can’t work out how the membranes would be done in practice.

Is there a way of sealing the lapped wall membrane to the roof membrane?   It could be just laid underneath it but I can’t see a way of that join being taped up.

[Iceverge]

Lay the wall membrane first. 

 

Then use double sided tape.