Cold non-vented roofs - a discussion

tp25

BryanF wrote: »

that why i suggested a wood fibre board instead of osb maybe 100mm if the budget will stretch

best of luck

It's not easy to buy a wood fibre board here in Co. Wicklow. I will try to source elsewhere.

Another problem is a bunch of electrical cables loosely laid over the existing floor (above bedrooms), thinking to use battens (12cm above the floor, so installing 100mm boards would leave gap of 12cm for services, however this approach would leave the services (electrical cables) under the insulation, this is a problem, however let's focus on the roof for the moment.

BryanF wrote: »

on the air-tightness between joists etc there are no easy solutions other than lots of type and membrane, but its a tough retro-fit. and its why there is such a need for good drawings, details and spec at design stage


best of luck

yes I agree there are no easy solutions.

Would the following approach work on the roof?

- roof tiles
- counterbatten
- breathable felt
- 70mm ventilation gap
- 150mm breathable insulation e.g. Knauf Earthwool Metstud 34 (MetStud 34 roll allows water vapour to pass freely through the insulation, therefore allowing the construction element to breathe. ) - 0.034W/m2K or earthwool building slabs which are 0.035W/m2K - 0.037W/m2K
- proclima or similar VCL stapled onto the rafters
- 50mm service battens filled with 50mm rigid breathable insulation
- plasterboard finish over the battens

I will deal with the floor once the roof is complete

Sandman777

Hi guys..gonna jump into this thread to ask for some advice!

Basically i have insulated my new build (2 story...9 months in) across the ceiling leaving a cold attic. I have 225 ceiling joists with the attic floored out with ply above!

I have filled out the 225mm gap between the joists with:
- 25mm kingspan cut to fit in between joists and kept up tight to plywood.
- 200mm knauf earthwool between joists
- Airtight membrane under joists
- 62.5 k16 insulated board across the bottom of joists (incl. plasterboard)

Also have HP and MHRV.

My concern is should there be any gap left for ventilation in this detail?? Attic is ventilated at eaves as normal!

Any advice appreciated!
S

MicktheMan

Sandman777 wrote: »

Hi guys..gonna jump into this thread to ask for some advice!

Basically i have insulated my new build (2 story...9 months in) across the ceiling leaving a cold attic. I have 225 ceiling joists with the attic floored out with ply above!

I have filled out the 225mm gap between the joists with:
- 25mm kingspan cut to fit in between joists and kept up tight to plywood.
- 200mm knauf earthwool between joists
- Airtight membrane under joists
- 62.5 k16 insulated board across the bottom of joists (incl. plasterboard)

Also have HP and MHRV.

My concern is should there be any gap left for ventilation in this detail?? Attic is ventilated at eaves as normal!

Any advice appreciated!
S

I don't see a need for ventilation

Sandman777

MicktheMan wrote: »

I don't see a need for ventilation

Ok thats good! I came across this thread and was unsure if i had done the right thing!

Thanks

howman

Bitten & Hisses wrote: »

Syd, I'm not very well-informed on this, so please bear with me. A friend recently moved into his newly-built house, a dormer bungalow. He used a breathable "felt" (something like a TX membrane) and completely filled the space between his rafters with Icynene spray foam insulation. Does this make his method of construction a warm or cold roof? He didn't provide for any form of ventilation, didn't counterbatton and used an ordinary 2 x 2.5" slate lath and natural slate.

do you not have the issue here of the face to the joists being exposed to the cold/moisture and the rest of the joist in a hot environment?

sydthebeat

howman wrote: »

do you not have the issue here of the face to the joists being exposed to the cold/moisture and the rest of the joist in a hot environment?

not only that, but the spray foam insulation pushes the felt outward creating a drainage channel directly over the roof joist... which is the worst possible location...

howman

sydthebeat wrote: »

not only that, but the spray foam insulation pushes the felt outward creating a drainage channel directly over the roof joist... which is the worst possible location...

right now Im looking at a curved Barn style roof.
Curve is too tight for any pre-fabed type insulated panel.

The roof is not exposed, therefore Im looking at insulating and sealing at ceiling level and going for a cold roof type design, with ventilation and no insulation
will have the roof structure, with moisture barrier, plywood and corrugated panel

what do you think?
is this a little stone age?

sydthebeat

what do you mean the roof isn't exposed?? can you describe the structure more fully?
where will the insulation layer be?

howman

auto correct kicked in there so not sure what point I was making!

insulating at ceiling (floor of attic space) and air sealing at ceiling level

the roof structure - beams, plywood, cross battens and moisture barrier with corrugated steel panels on top
this is in it's early stages

just going through design with builder

sydthebeat

howman wrote: »

auto correct kicked in there so not sure what point I was making!

insulating at ceiling (floor of attic space) and air sealing at ceiling level

the roof structure - beams, plywood, cross battens and moisture barrier with corrugated steel panels on top
this is in it's early stages

just going through design with builder

sounds fine, just ensure adequate cross ventilation

galwaytt

howman wrote: »

right now Im looking at a curved Barn style roof.
Curve is too tight for any pre-fabed type insulated panel.

The roof is not exposed, therefore Im looking at insulating and sealing at ceiling level and going for a cold roof type design, with ventilation and no insulation
will have the roof structure, with moisture barrier, plywood and corrugated panel

what do you think?
is this a little stone age?

Who told you it was to tight for a prefabricated roof panel - where did you check ??

newbuild__2014

Hi Folks,
I don't work in the industry and therefore don't know the details and would really appreciate a second opinion on some things. I am currently building a new bungalow with converted attic rooms. I have a contractor that is doing the build for me and they are planning to spray the roof with insulation soon but reading this post I am now slightly concerned.

The make-up of the main roof is: (it is a cut A frame roof)
-Slates
-2x1 baton,
-felt (I am not sure if it is breathable or not but will find out)
-150mm rafters filled with with 50mm gap, cavity trap, 100mm spray insulation.
There are eve ventilation trays and some vent slates used across one side of the roof. I am unsure if they have used any ventilation at all at the ridge.
-80mm Insulated Plasterboard across small ceiling section and down the slopes to the knee walls and then insulated sheets on the remaining part to the eves.
-Habitable room, ventilated by MHRV.

From reading the post I assume that this approach is not ideal but I am wondering if it is a high risk and really bad approach or is it a common mediocre risk approach. And is there anything I could change to help the situation at this stage.

I should mention one of the reasons for the spray is to aid in airtightness as it gets into all the awkward places.

I am sorry for the long post but hopefully someone has some spare time to help.
Thanks

hexosan

Looking at my roof insulation at the moment.

Currently I have
- slates
- 2x1.5 batons
- Solitex felt
- 225mm (9") rafters

Roofer had half the roof completed before I realized it wasn't counter battened. I've spoke with felt supplier and they say because it's not counter battened I can't use cellulose as it'll force the felt against the slates and may cause them to pop. They've told me I can full fill the rafters with Metac glass wool as the Solitex is breathable, but I've read on here that that's a bad idea as a 50mm vent gap should still be left. I will be fitting an airtightness membrane (intello) to the rafters.

Was talking to the Tegral rep and he stated that if the rafters are been full filled with insulation I MUST install a ridge vent roll which is basically a mesh that is rolled out under the ridge tiles as there been fitted, this then creates a 5-10mm vent along the ridge line which allows the warm air trapped beneath the slates to vent off keeping the back of the slates dry.

Anyone have any thoughts I what I can do in this situation.

BryanF

You didnt double batten so you your venting above the breather membrane is compromised. Leave 50mm gap between insulation & membrane and install the ridge vents. Also you need Over eaves vents

hexosan

BryanF wrote: »

You didnt double batten so you your venting above the breather membrane is compromised. Leave 50mm gap between insulation & membrane and install the ridge vents. Also you need Over eaves vents

Can these over eaves vents be installed after the roof have been slated

BryanF

Not ideal, what did your arch have to say? when the batten issue was spotted, what alternative eaves details were discussed?

fclauson

Do you have a picture of the cross section of your roof

this might be useful
https://www.facebook.com/Passive.House.Wexford/photos/pb.154344008104045.-2207520000.1417986907./165904310281348/?type=3&theater