Insulate the rafters - with what?

voodoo

Hi all,

We had the trusses for our roof built so that if we ever wanted to, we could develop the space into another room... Now that we see it, we are going to develop it now and I am struggling with what I should do with regards to insulation of the attic space? What should I put between the rafters? I was also going to put insulated boards on the rafters?

What do I put in the walls down the side also? I am not going to insulate the joists as I want the heat to rise from first floor into this space (and hopefully keep it there).

Please help

ei.sdraob

this is taken few days ago from my build



open cell foam, expands into all cracks everywhere
ill be putting in fibre insulation between the floors then

Poor Uncle Tom

How is the 50mm air gap, required by the Building Regulations, maintained between the felt and the insulation in this case?

voodoo

Yes, that's why I decided against pumped foam or anything similar... how do you keep the 50mm gap between the felt and the insulation... that's out unfortunately.

Am thinking of putting in 60 or 80mm Xtratherm (similar to that that is in the walls) but this means cutting the board and ensuring it's tight between the rafters. Would Rockwool give me agood U rating/heat retention?

sinnerboy

The rigid urethane boards are almost twice as effective as the fibrous boards "in the wrapping" .

BUT - workmanship is vital . Rule of thumb - 5% gaps = 20% reduction in performance .

If doing a DIY job I would use a semi rigid fibrous board to ensure a tight fit .

In either case - use also an insulated plasterboard below as well .

voodoo

Sinnerboy,

Can you name a semi fibrous board you would suggest (or PM me if you wish).

sinnerboy

this or this

gman2k

Poor Uncle Tom wrote: »

How is the 50mm air gap, required by the Building Regulations, maintained between the felt and the insulation in this case?

It's not, if breathable membrane is used, along with counterbattens for slates/tiles.

Poor Uncle Tom

gman2k wrote: »

It's not, if breathable membrane is used, along with counterbattens for slates/tiles.

If you read the OP you will see that this attic area was going to be a later conversion which is likely going ahead now. What is the likelyhood of having that roof designed with counterbattening just in case?

Poor Uncle Tom

This is another option and here is another.

gears

sinnerboy wrote: »

The rigid urethane boards are almost twice as effective as the fibrous boards "in the wrapping" .

BUT - workmanship is vital . Rule of thumb - 5% gaps = 20% reduction in performance .

If doing a DIY job I would use a semi rigid fibrous board to ensure a tight fit .

In either case - use also an insulated plasterboard below as well .

Myself I think this is the best idea but as SB has said workmanship is important. What I have seen when using this insulation is where the insulation is cut slightly short on both width and height and then fixed and sealed in place using a low expansion mounting foam, does anyone think this is a good or bad idea?

voodoo

Thanks for the advise and guidance guys. Am meeting a guy this evening hopefully to discuss further.

sinnerboy

Lets know how you get on

ei.sdraob

Poor Uncle Tom wrote: »

How is the 50mm air gap, required by the Building Regulations, maintained between the felt and the insulation in this case?

as i said its a open cell foam, fairly new stuff (pm me for name if you wish)
airtight but breathable, and good U values

its not like the old closed foam stuff based on urea, formaldehyde or other nasties like that causing all sorts of smells and mold issues in the past

sydthebeat

ei.sdraob wrote: »

this is taken few days ago from my build



open cell foam, expands into all cracks everywhere
ill be putting in fibre insulation between the floors then

what depth of rafter have you??

are you slabbing underneath with an insulated plasterboard?

what u value do you think you are achieving with that product, because by my calculations you are not achieving minimum regulations

sinnerboy

+1
And to pick up from Poor Uncle Tom -

ei.sdraob - be aware that "full filling" the void like this will only work in combination with
1. breather membrane ( looks like you have this )
2. Battens and counter battens to create a 50mm vent space over the breather membrane
3. Continuous ventilation strips at roof eaves (25mm wide ) and apex (5mm wide )

sydthebeat

sinnerboy wrote: »

+1
And to pick up from Poor Uncle Tom -

ei.sdraob - be aware that "full filling" the void like this will only work in combination with
1. breather membrane ( looks like you have this )
2. Battens and counter battens to create a 50mm vent space over the breather membrane
3. Continuous ventilation strips at roof eaves (25mm wide ) and apex (5mm wide )

also, a vapour control layer is required on the warm side of the installation, in accordance with its BBA certificate.
NB foil backed plasterboard is not a vapour control layer.

voodoo

Hey Sid,

Co-incidentally a guy from a foam insulation company and he was saying its the only solution that meets building regs. We have 9" rafters (went overboard) so he said that we could either fill with 7" and put a 20mm insulation board against it or pump the full 9" and not need the insulation board...

Am dubious about the solution however... Not sure of the cost either - it's priced on a per meter per inch basis...

sinnerboy

https://www.youtube.com/watch?v=-o6M4a-Fhqg

As you can see it is impractical to partially fill spaces with spray foam . That together with the claim "its the only solution that meets building regs" - completely false - should warn you off this chancer


.

ei.sdraob

thanks @sinnerboy and @sydthebeat

was meant to reply but got carried away i haven't been on site in week
so cant get exact measurements and it was still slowly expanding when the picture was taken, with bits having to be shaven off in places

top of head its ~8 inches (alot more in places close to wall at edge)
thats between 0.20 and 0.16U for the e500 spray according to spec (am i allowed to link to that? anyways there it is)
so that should meet the regulations last I checked

yes there is a membrane and space above it, and the tiles on top of that
there are also ventilation thingies (they werent foamed over of course and left alone) every so often along the top
but yes a membrane is planned to be added

the slabbing hasnt been done yet, waiting on few things to be done and the heat recovery and ventilation system to be installed before proceeding

im very happy with it since its airtight (0.1 ach) and very nicely sealed the complex dormer roof bits
the insulation (fibre) between the floors should be in very soon, and the walls were done with beads

soldsold

sinnerboy wrote: »

Continuous ventilation strips at roof eaves (25mm wide ) and apex (5mm wide )

Is this always the case with full filled rafters?

I have 9 inch rafters soon to be filled with cellulose.

Taped 15mm OSB inside
9 inch rafters soon to be filled with cellulose
60mm softboard
Cheap breathable membrane (Not strictly needed but as an added measure)
Battens and counter battens
Slates

I dont have any ventilation around the eaves or gables and the fascia and soffitt are on. My understanding was that the roof was continually less vapour diffuse from inside to out so any condensation will "wick" through the cellulose and out through the softboard.

Should I be concerned?

sinnerboy

soldsold wrote: »

Is this always the case with full filled rafters?

In almost all cases

soldsold wrote: »

I have 9 inch rafters soon to be filled with cellulose.

Taped 15mm OSB inside
9 inch rafters soon to be filled with cellulose
60mm softboard
Cheap breathable membrane (Not strictly needed but as an added measure)
Battens and counter battens
Slates

Great spec .
The taped OSB is a very good vapour check ( and air tightness measure ).Both cellulose and softboard are very hygroscopic - they will safely absorb interstitial condensation when it does occur - as it will . The entire "bodies" of the material will "share the load" of this moisture - untill the temperature rises to safely release it again - in a manner that protects the structural roof timbers . The battens and counter battens is also a very good measure to encourage a ventilation flow to disperse condensation

soldsold wrote: »

I dont have any ventilation around the eaves or gables and the fascia and soffitt are on. My understanding was that the roof was continually less vapour diffuse from inside to out so any condensation will "wick" through the cellulose and out through the softboard.

That's the idea alright .

soldsold wrote: »

Should I be concerned?

No need to panic at all .

HOWEVER - being a cautiuos divil - I would perhaps invest in in line vents located at 2m centres along each side of the apex and along each eaves . This may be considered overkill by many - I would not argue .

But I would still do it myself .

.