Roof Insulation Spec in BuildDesk

bauderline

Folks,

Attached is my attempt at roof insulation specification using builddesk. I believe I have the basic idea, but I am not sure if I have specified the "format" of the roof correctly, especially the outer layers....

Any guidance very welcome !

kind rgs. baud.

bauderline

Sinnerboy and Sydthebeat,

Can I ask for your 2 cents worth on this one ? I am not 100% certain on how to spec a pitched roof in BuildDesk....

I value your input immensely on this sort of stuff !

b.

sinnerboy

Ok , assuming this is pitched , roof , ceiling slopes with roof

for rafters at 400 c/c % timber is 12.5%
at 600 c/c % is 8.3 %
However

hips , valleys , veluxes etc will increase these %'s

SO - for 400 c/c I would enter 16% for 600 c/c 12%

the 6.3% you have entered is under calculating the thermal bridging

IF €'s will stretch to it

Use 225 deep rafters . Depending on spans you may then increase c/c to 600 and reduce thermal bridge % to 12%

Then insert 2 layers of 100 thick PU between - 50 below

Ventilate OVER the breathing membrane -
use 25 x 50 counterbattens , placed directly over rafters and
38 x 50 tiling battens - this creates a 63 vent space over

use a PVC roof over hang vent strip to admit a continous 25mm vent strip here AND fix your ridge with a PVC dry ridge system to give a continous 5mm vent strip there

WARNING

1. this will cost more

2.if not installed to the highest standards - don't bother trying to fit PU between rafters - a badly fitting "gappy" job won't work. If you cant guarantee a good job - fill between the rafters with rockwool instead

3. don't mess all this work up with recessed lighting

4. if running any cables in this roof , keep them on the cold side of the insulation

AND

you are only half way there now ..... you have to consider IN DETAIL the junctions with other parts of the building

to finish off go to

http://www.planningportal.gov.uk/england/professionals/en/1115314255826.html

and download the details for your build type

bauderline

sinnerboy,

thanks for that, I understand what you are driving at, but don't understand the jargon in the first part. Can you explain the difference between 400 c/c and 600 c/c and what this actually means ?

I do understand that you are suggesting 9" rafters rather than 7" and I know the percentage value you are referring to in builddesk, just not sure how that ties in with 400 c/c and 600 c/c.

baud.

sinnerboy

space the rafters 400 mm apart - they take up 12.5 % space leaving 82.5 % of space for insulation

space them 600 mm apart - they take up 8.3 % leaving 91.7 % space for insulation

look again at the pdf you posted - 6.3% space is entered for % space occupied by timber

bauderline

sinnerboy,

thanks again ! that's crystal now !

best rgs. baud.

bauderline

sinnerboy,

if you have time, here are the specs as described above, i think.

The 25x50 battons had me scratching my head, I can only guess they run vertical with the rafters. The 35x44 battons run horizontal across the rafters as per normal.....

baud.

ardara1

All breathers need vented ABOVE the membrane - can't count slates/tiles
Equalize int and ext rsistance to RSI value of R0.10
Air space - unvented below breather.
thermal conductivity of Kingspan - is 0.023 not 0.025
DON'T allow airgaps.
Timber bridge divide thickness 50mm by centre 400mm + 12.5%
Changed to Celotex?
DON'T allow air gaps.

sinnerboy

25 x 50 COUNTER battens run in line with and over the rafters ( after the membrane is installed

you don't need to include the slates , the vent space , the battens or counterbattens in this assessment . the pdf shows you have them deselcted from the calculation - but you did not need to put them in to begin with

start from breather membrane down

timber % still wrong

change items 5b and 6b to either 12% or 16% ( for 600 or 400 rafters spacings )

and run the condensation risk check

sinnerboy

ardara1 wrote: »

Timber bridge divide thickness 50mm by centre 400mm + 12.5%
DON'T allow air gaps.

I prefer to enter 16% to provide for hips / valleys / extra trimming around veluxes etc

bauderline

adara,

thanks for your input as well ! I think you were looking at the first pdf rather than the second one. I had cleaned it up a little following sb's initial comments, but all valid points in your comments to.

Note the plans say rafters at 600mm spacing, so that is what I will go with, I have bumped up the timber bridge to 16% as per sb suggestion.

Air gaps gone and RSI value equalized... I have left the other stuff in there for my own reference it doesn't change the calcs anyway......

Thanks for the input guys ! Appreciated !!

Baud.

sinnerboy

12 % - 600
16% - 400

bauderline

ahhh, sorry to much haste, I was looking at the comment you made regarding adara's post, which was actually based on 400mm... sorry didn't read that bit.... doh !

noted, will adjust to 12% ...

thanks.
P.

Sparky78

Would it not be better to reduce the size of the rafters to as small as possible and put more insulation under the rafters?

This would reduce the cold bridge from the rafters.

Not an expert on this at all,just speculating and would love to hear feed back.

ircoha

outside would work even better: a practical issue is how u fix say 200mm of rockwool or PU or what ever to the rafter and then fix on your roofing
http://www.rockwool.co.uk/sw47799.asp

ardara1

Warm roof animation. - I agree with Irocha - it's the only way to get really good u-values in a roof.

http://www.xtratherm.com/interactive/vaultedroof.php

bauderline

Ok... so warm roof would look like this in BuildDesk...