"Double deep" rafters for more insulation.

sas

On last tuesdays "About the house" they featured one house where a pitched roof was constructed with what they termed "double deep" rafters. This was done in order to give them the necessary depth to include 325mm of fibre glass insulation between the rafters.

To all intents and purposes (from the 2 seconds they appeared on screen) they looked to me like a trussed roof which was created with 2 lengths of timber connected to each other using those machine applied nail pad (I don't know the actual name of these).

Apart from cost and the fact that it could add to the height of the build which could affect planning permission, what would be the downsides to this and is it something that is commonly done.

I want to get to a u-value in the region of 0.15 on the pitched roof and I would like alternatives to EPS,Polyiso or Phenolic boards. 320mm of rock wool (allowing for bridging) would appear to give me a u-value in the region of 0.15 (I've estimated the bridging by the timber at 10% and used the SEI u-value calculator).

muffler

I dont think an extra 200mm or so would be an issue in relation to planning.

+s - allows extra insulation and an increase in structural stability of roof

-'s Cost!

Mellor

sas wrote: »

To all intents and purposes (from the 2 seconds they appeared on screen) they looked to me like a trussed roof which was created with 2 lengths of timber connected to each other using those machine applied nail pad (I don't know the actual name of these).

Sounds like a gang nail plate. This would be a cheaper option that sourcing c.320mm deep rafters

Apart from cost and the fact that it could add to the height of the build which could affect planning permission, what would be the downsides to this and is it something that is commonly done.

I want to get to a u-value in the region of 0.15 on the pitched roof and I would like alternatives to EPS,Polyiso or Phenolic boards. 320mm of rock wool (allowing for bridging) would appear to give me a u-value in the region of 0.15 (I've estimated the bridging by the timber at 10% and used the SEI u-value calculator).

Cost is the only really downside.
An advantage that hasn't been mention is the fact the the deeper rafters will allow the rafter centres to be increased. So this reduces material (reducing some of the increased cost) and also reducing cold bridging.
44mm @ 600 c/c is 7% bridging.

sas

My roofer doesn't like the idea of 600mm centres, he maintains you're likely to get the ceilings sagging over time which probably means needing to counter batten behind the ceiling slabs which in turn negates the savings.

All in all though, this is nice positive feedback.

Cheers

SAS

Mellor

As long as use a suitable plasterboard it should be fine, various plasterboards have different flexibility (plasterboards for curved walls for example), increasing the thinkness also reduces flexibility.

ardara1

I saw the program my self - It was ordinary felt they were using I think? - There was definately no 50mm ventilation space below it! - there be condensation problems.

The bridging area 50mm rafter/400 cs is 12.5% (You should add for loft hatch - but take at 12.5)

U-value is 0.17 - not great - AND would not comply with Part L - Thermal bridging must be addressed by following set details - MUST be 2 layer system.


Roof Slope Vented 400cs Fibre Single layer to 0.17
Element type Exposed element
Area [m2] 1
U-value [W/m2 K] 0.17

Layers Thickness Conduct'y Resistance
[mm] [ W/m K] [m2 K/W]
Air layer ventilated 50 0.100
Glass fibre/wool quilt 320 0.044 5.845
Plasterboard 13 0.180 0.069
Internal surface resistance 0.100


Upper/lower resistance 6.181 / 6.114 W/m2 K
Correction for air gaps / mechanical fasteners 0.009 W/m2 K


Details of layers with thermal bridging or corrections for air gaps or mechanical fasteners are given below.


Glass fibre/wool quilt
Is there repeating thermal bridging? Yes
Bridging conductivity [W/m K] 0.13
Bridging material Timber joists
Bridging thickness [mm] 320
Fractional area of bridging [-] 0.125
Correction for air gaps Air gaps present but no air circulation
Correction for mechanical fasteners Not applicable

johnny_adidas

surely there must have been another reason for using 2 rafters other than wanting to fit 325 quilt insu? why didnt they just use rigid? were the spans excessive?

Slates

ircoha wrote: »

Rigid, hopefully, is going out of fashion for use in places where air-tightness on the warm side cannot be g/teed.

Why is this ?

ardara1

ircoha wrote: »

Rigid, hopefully, is going out of fashion for use in places where air-tightness on the warm side cannot be g/teed.

What do you mean here Irocha - I thought a rigid to the warm side would help with air tightness?

sas

Aradara, you've lost me on that last large post!

Quick question though, the air gap I thought is required in cold roofs, but not warm constructions. Is this not correct?

Products such as the rockwool variants, cellulose fibre, woodfiber and hemp are what I'm looking at. Breathable products that unfortunately are a) pricey and b) not as good an insulator so more depth is required. Hence this thread.

I'm not trying to sound ungrateful for the advice to whoever suggested rigid insulation but I'm not interested in debating its use in a roof construction.

I've read some interesting articles on the effect insulation density makes to overall comfort levels so I'm leaning towards the above products.

Incidently, has anyone any experience with the Rockwool Rockfall product?

ardara1

sas wrote: »

Aradara, you've lost me on that last large post!

Quick question though, the air gap I thought is required in cold roofs, but not warm constructions. Is this not correct?

THE VENTILATION GAP OF CONTINUOUSE 50MM IS REQUIRED UNDER IMPERMEABLE (ORDINARY) FELT
IT CAN BE DISPENSED WITH ONLY IF THE FELT IS VAPOUR PERMEABLE (BREATHER MEMBRANE) - ALTHOUGH GOOD PRACTICE GUIDES BY BRE INSIST A 50MM VENTED GAP IS PROVIDED ABOVE ALL BREATHER TO PREVENT CONDENSING UNDER TILES/SLATE (TYVEK DISPUTE THIS)

Products such as the rockwool variants, cellulose fibre, woodfiber and hemp are what I'm looking at. Breathable products that unfortunately are a) pricey and b) not as good an insulator so more depth is required. Hence this thread.

SUITABLE BETWEEN TIMBERS OK - AS LONG AS THEY DON'T SLUMP - BUT ABOVE RULE STILL APPLIES.

I'm not trying to sound ungrateful for the advice to whoever suggested rigid insulation but I'm not interested in debating its use in a roof construction.

I've read some interesting articles on the effect insulation density makes to overall comfort levels so I'm leaning towards the above products.

I DON'T BELIEVE MOST OF THE RUMOURS YOU HEAR - USE A MIX OF THE TWO - FIBRE BETWEEN (EASE OF FITTING) RIGID ABOVE OR BELOW - SAVING IN SPACE.

Incidently, has anyone any experience with the Rockwool Rockfall product?

SORRY FOR THE CAPS - I CAN'T WORK THIS MULTI-QUOTE THING!:(

sas

ardara1 wrote: »

SORRY FOR THE CAPS - I CAN'T WORK THIS MULTI-QUOTE THING!:(

Thanks, I take it you weren't shouting at me then

smashey

ardara1 wrote: »

SORRY FOR THE CAPS - I CAN'T WORK THIS MULTI-QUOTE THING!:(

Click this yoke in every post you want to include in the multi-quote, and on the last one, click this one as well.

Mellor

ardara1 wrote: »

The bridging area 50mm rafter/400 cs is 12.5% (You should add for loft hatch - but take at 12.5)


Roof Slope Vented 400cs Fibre Single layer to 0.17

I am I missing something, where does the trapdoor come into the construction if it is insulated along the slope? This is probably really obvious.

44 is normally the finished dimension, so that 11%. Also if you used higher spec quilt insulation you could do alow better than 0.17
I believe the example you used had the insulation @ 0.044, it is possible to get products at 0.036

ardara1

Mellor wrote: »

I am I missing something, where does the trapdoor come into the construction if it is insulated along the slope? This is probably really obvious.

44 is normally the finished dimension, so that 11%. Also if you used higher spec quilt insulation you could do alow better than 0.17
I believe the example you used had the insulation @ 0.044, it is possible to get products at 0.036

H Mellor - under BR 443 - the actual bridge on the roof is increased by 1 to account for the hatch (it's most ignored - but that's the convention) - there will be a hatch somewhere - even to get access to the cold water tank.

you're right - a cut dormer roof will probably be 44 or 48 (Both are imported into Ireland) - but the nominal thickess is 50mm - I always air on the side of caution it's best to check.

Yes there are denser fibrous materials on the market - frametherm and metsec - but they're about 4 times the price and rarely used. The best lambda is around 0.032 the common cheapy material 0.044 - iin a ceiling situation you'll save 20-30mm only and pay a hell of a bit more.

ardara1

ircoha wrote: »

My apologies for not being clear here;

If rigid id used , say between rafters in a roof, with the 1 or 2" airgap between roof felt and cold side of rigid.
Rigid is supposed to be flush with inner side of rafters and if you ignore the need for aanother layer acrsoss the timbers, if the rigid is not airtight against the plasterboard, ie the warm side, then it is no use.

The same applies to all insulation. including cavity walls

You can't ignore the layer to the underside - it's mandatory under the building regulations to provide a robust detail - all documents referred to insist on 2 layer system. It is also mandatory to avoid these gaps - PART L
Minimising the extent of cold bridging
Particular areas of potential cold bridging include
junctions with external walls at eaves and gables, and
junctions with solid party walls. Gaps in the
insulation should be avoided and the insulation
should fit tightly against joists, noggings, bracing etc.
Insulation joints should be closely butted and joints
in upper and lower layers of insulation should be
staggered.

Material will not perform if fitted incorrectly - any of them including the fibres.

ircoha

As i deemed myself not to be helpful here I have deleted my posts in this thread: perhaps you might edit yours to remove my ramblings.
your posts address the issue perfectly : i will delete this once you do the needful

muffler

ircoha wrote: »

As i deemed myself not to be helpful here I have deleted my posts in this thread: perhaps you might edit yours to remove my ramblings.
your posts address the issue perfectly : i will delete this once you do the needful

Woaahhh. What goes up stays up unless its deemed inappropriate by the mods and I would just point out that it is smashey and myself who will decide if a post needs to be edited or deleted or in this particular case if we consider ircoha's posts relevant to the discussion we can bring them back on the public board again.

Im not saying that this will be done in this instance. Users here and on all fora have the right to edit or delete their post within a fixed period of time just as they have the right to post a comment in the first place but at the end of the day it will up to the mods to decide whats best for the topic at hand.

A common sense approach is best and working together to ensure the smooth running of the forum is paramount.

sydthebeat

Im about to specify a double deep rafter to incorporate 250mm rigid PU insulation. I intend to batten over with 50 x 50 to create 50mm continuous air gap, with impermeable felt over and slate finish.
Internally im going to slab out with 47.5 CPB to reduce thermal bridging, with skim finish.
If i can get the trusses at 600 c/c all the better.

has anyone any issues with using this amount of PU insulation to basically surround a piece of structural timber?

I have followed an argument on another site with strong jargon being used against this method, whilst not being backed up with strong scientific data.

anyone any thoughts??

ircoha

STB: what follows is just an observation and not a critique:

One concern I would have is with the possible deterioration of the U value of the PU due to the possible heat build up in the roof, depending on the orientation of course.

There SEEMS to be some evidence that the higher temps in the roof space de-gassify the PU quicker than normal and that cutting them at 400 mm wide strips accelerates the issue as compared to full size sheets.

The other concern I have with PU in a roof is that it is perhaps not dense enough to limit solar gain: I accept 250mm plus the CPB is a fair wedge but...

And now for some questions::)
why the impermeable felt ?
where is the vapour barrier?

sydthebeat

im confident my design will allow for passive ventilation is summer to negate the over heating issue.

regarding the felt, with the 50mm air gap is there any need for a breathable felt?

the vapour barrier should be incorporated of course, my bad.... this will be under the trusses, then battened out with 50 x 25, then CPB

CRA analysis of this has shown no issues.

Mellor

ircoha wrote: »

The other concern I have with PU in a roof is that it is perhaps not dense enough to limit solar gain: I accept 250mm plus the CPB is a fair wedge but...

This idea of limiting solar gain gets thrown about alot,
Im my opinion its almost always misapplied, and their are plenty of people people and websites out there who "information" on the subject is biasd and wrong.