New extension, pitched roof insulation required?

[xSuEx]

I have planning permission to build a single storey extension with a pitched roof, along the side and rear of my house.
However I would like to install a number of roof windows and have the ceilings at the rafter level which is not on the plan.
Can anyone tell me what insulation I would need, so I can get my plans ammended. The joists are 150mm x 50mm - 400mm centres.
Any help would be really appreciated.

 

[DevilDamo]

Hi,

I always specify Celotex insulation, which can be fitted either...

1. Over the rafters.
2. Between and over the rafters.
3. Between and under the rafters.

I usually specify insulation (ventilated) between and under the rafters and so if you wanted to go down this route, you'd be looking at a min. u-value of 0.25 and this can be achieved by...

- Outside surface resistance
- Tiling including batten space
- Sarking felt
- 50mm well ventilated cavity
- 100mm Celotex FR4100 between rafters
- 12mm Celotex TB3000 beneath rafters taped joints as VCL
- 12.5mm Knauff wallboard
- Inside surface resistance

So you'd be looking at an overall make-up (including rafters) of approx. 250mm.

As well as planning permission, do you also have building regulations approval and a current architect/technician who has put some building regulations together because if so... this is easy enough for them to work out?

And also... by not having a ceiling with ceiling joists, you may have to re-look at the rafter design (depending on the type of roof). With a gable roof, you should be ok but it may cause problems with a hip roof as you'd have to support the intersection. You'd be looking at a lot less insulation at rafter level than at ceiling level too

Hope this helps.

 

[xSuEx]

Thank you for your advice.

I do have building regulations approval as well as planning permmision. The planning permission runs out in May, so I'm trying to get the ammendments done quickly.
unfortuanetly I can't get in touch with my original architect. So I thought I would be able to do it myself.
I didn't even think about the rafter design needing to be changed.
My plans show 2 hips. Would it just be a case of changing the rafter sizes or is it more complicated than that?

 

[DevilDamo]

You don't "need" to amend the drawings before the planning permission expires. I assume you're talking about a condition on your approval that states you have to start within three years? If so, then just dig a hole in approx. the location of the extension, which is deemed as works commenced on site. Or... have all/part of the proposed foundations dug and have Building Control inspect them for them to confirm works have commenced. Or... are you talking about the actual five year expiry date?

Depending on how the roof has been designed, it'd be a little more complicated that. Do you have the facility to scan part of the proposed drawings so I/we can have a look? Or... if you have them as PDF's, by all means email them to me at [email protected] and I can take a look.

 

[xSuEx]

Thank you for your help.
I have emailed you some scans of my drawings for you to have a look at.
Sue

 

[DevilDamo]

And replied

You should be fine with that roof design and as said in my email, just start on the foundation excavations, call a BCO out and then you'll have confirmation that works have commenced on site so you'll be within your three year statement on your planning approval.

 

[xSuEx]

Thank you very much. Your a star.

 

[DevilDamo]

No problem If in doubt, speak to your BCO as they're there to help