300mm timber rafters

[Rishard]

On 25/07/2024 at 14:23, Iceverge said:

390mm is the depth I'm adding up here from the plasterboard to the membrane on top. 

 

How about this. 

 

 

 

 

Or a hybrid warm roof as shown below with 150mm above the rafters and 100mm between. You'd need to tape the bottom layer of OSB as airtightness here.

 

 

 

 

Also. Is the osb below the pir needed or is this for racking strength? If I wasn’t using this osb as my vcl would it need to be there? Vcl would go below mineral wool in this option. 

Edited April 10 by Rishard

[Iceverge]

Okey dokey....

 

Got a chance to read back through the thread. 

 

QUESTIONS!

 

1. What U value do you NEED?

2. What U value do you WANT?

3. Can you source blown cellulose? 

4. Can you provide a cross section and plan of the attic. 

 

In an ideal world I would opt for completely pumped/blown insulation every time. Our precious humanity is to short to be wasted on chopping and breathing in insulation. Failing this complete boards of rigid insulation outside the timbers are my preference over inside.

 

I wouldn't build another roof without a layer of rigid sheathing. Sarking boards, OSB, or woodfiber. An unsupported breather membrane is too vulnerable to rodent, bird and wind damage. We didn't sheath our roof and it's something I'd change.

 

Airtighess is precisely one billion times more important than U value. This only works if it's planned from day Zero and easy to execute. 

 

I don't like exotica for building houses. Preferably everything should come from the local BMs. That said you may have I-joists avail locally?

 

Anyone who mentions the word "breathable" almost certainly doesn't know what they're talking about and definitely doesn't understand it's role Vs airtightness & windtightness.

 

 

 

 

 

 

 

 

 

[SBMS]

1 hour ago, Iceverge said:

Anyone who mentions the word "breathable" almost certainly doesn't know what they're talking about and definitely doesn't understand it's role Vs airtightness & windtightness.

Just to be clear @Iceverge because I massively respect your input and opinions here, and you’ve helped on so many aspects of my build.. are you saying that interstitial condensation is NOT a consideration and having vapour permeability is NOT a requirement if a near perfect level of interior airtightness is achieved? Not even as a belt and braces approach?

[Rishard]

Ok. My plans were submitted pre the last building regs change. Going from 0.13 to 0.11 ( from a quick google) In my phpp I have it at 0.104. Anywhere below 0.11 would feel satisfying. Although from some of your previous messages, the real terms difference of these uvalues may not have to drastic a cost implication from a heating perspective. 
 

I have had a quote for blown cellulose which is promising. It does come in as the most expensive option currently. Plus the additional cost of Ibeams or using local stock with extra additional timber screw on. 
 

My current favourite option is

170mm rafters

180mm mineral wool

100mm pir below rafter.

Vcl

25mm service void 

plasterboard. 
 

All locally available and altogether come in cheaper than the supply/install of cellulose. 
 

I could take my rafters down to 145mm minimum if needed. I’ve been in circles with this, as you may see. Flipping the above option presents a ‘condensation risk’ in my calc. Leaving it this way does have some small thermal bridges at the ridge and the wall - roof junction where the insulation thins a little around the birds mouth connection at the wall plate. Gauging which of the 2 small compromises is better or worse is where I’m at now. 
 

No doubt there are multiple other options. I’ve priced all the cellulose/wood fibre/ mineral wool/ pir / hybrid / warm options. The more layers I add seems to add either cost and or complexity. What’s the saying, keep it simple stupid? If only 😅

 

My original architects plans are fairly lose but generally it shows 200mm pir between with 25mm under. I think we both knew at the time it wasn’t going to be this but gave us enough to put it through planning. 
 

The roof will all be hand cut with a gluelam ridge beam. All vaulted. There is some adjustment on wall plate heights if I need to favour a warm roof. And 145mm rafters would tick the box for my spans.