Small Atmos Studio

[Paulus87]

Hello,

Sorry for the radio silence, I've been doing a lot of research and falling through deeper and deeper rabbit holes. My current area of research is the roof space in regards to cold and warm roofs and I'm struggling with some concepts. My studio will be in a timber clad garage, so naturally is a cold roof design. This is how I see the construction (sorry for mangling your image that I found elsewhere Paul!)

Cold Roof Thought 1.png

Does this look right?
Any reason why I shouldn't apply my outer layer under the outer rafters rather than creating a 50mm batten from the ceiling?
Can I do away with the ridge vent and just have the gap between the fascia board and the outer wall? (with some chicken wire or similar to stop pests getting in)
Do I have to place anything on the outside of the outer drywall layers in the roof space to prevent moisture?
I'd also love to know the theory of why we place the vapour barrier on the outside of the inner layer of plasterboard rather than anywhere else - just so I understand rather than questioning the position itself. Is it so it keeps moisture in the studio rather than stopping moisture penetrating from outside?

Apologies for the abundance of questions, just trying to get my head around this.

Cheers!

Hi,

You can put the outer leaf mass layers directly on the bottom of the rafters, no problem, it is a lot easier to do that. The only reason for using battens and cutting the mass layers in to fit between the rafters is if you have a height restriction and you want to maximise not only inner ceiling height but also maintain the required cavity depth to reach your isolation goal between the leaves.

If you do not have a height restriction, or if you do but are willing to compromise on height, then fixing directly to the bottom of the rafters is the way to go.

Vapour barrier needs to go on the warmest side of the insulation (not counting the acoustic treatment inside the envelope) so that any vapour that makes it through the drywall/OSB (which, is very unlikely anyway since they should be sealed air tight) will be retarded by the vapour barrier, which btw, also needs to be completely sealed and continuous - no point having a vapour barrier (or retarder) at all if there are any gaps or holes in it. Make sure you use the right type of barrier here as well.

Once you have a fresh air supply and return and air con installed then any moisture inside the building should be managed by the system, if you can install an HRV then all the better as this should also have a dehumidifier built in.

In the UK you can do away with a ridge vent IF the total width of the building is no more than 6 meters. You will either need a continuous 10mm gap all the way along the eaves on both sides of the building, or if using the small circular 70mm vents then you'll need one of those spaced every 160mm.

You must guarantee that there is a free flow of air on the under side of the roof deck from one side to the other, completely unobstructed between every rafter bay. If there is anything that obstructs the air flow then it will all be void and you'll very likely have problems.

Hope this helps,
Paul

[Fred Pearson]

Paul, thank you so much for your response. That answers all of my questions, makes perfect sense and has given me a much better understanding of the whole cold roof thing.

I'll definitely have 10mm gap all around the eaves, but might put a few small round vents in the gable end as close to the ridge line as I can, to ensure airflow.

[Fred Pearson]

Ok, slight issue. My rafters in the roof run perpendicular to the pitch, so they won't naturally create an air gap if I attach the outer layer of the MSM to the bottom of these. Can I space out the outer layer using 50mm battens from the bottom of the rafters as suggested in the previous picture. Will this create enough free-flowing air, or will the air get trapped in between the rafters seeing as they're perpendicular to the pitch?

Image below:

Rafters.png

[Paulus87]

Ok, slight issue. My rafters in the roof run perpendicular to the pitch, so they won't naturally create an air gap if I attach the outer layer of the MSM to the bottom of these. Can I space out the outer layer using 50mm battens from the bottom of the rafters as suggested in the previous picture. Will this create enough free-flowing air, or will the air get trapped in between the rafters seeing as they're perpendicular to the pitch?

Image below:

Rafters.png

Fred,

I'm sorry if you've already explained this in full detail but my time is limited and memory poor! What type of roof do you have? Tiles? Metal Sheets?

Is there a ply/osb roof deck on top of the rafters, under the roofing?

Outside, is there an overhang? How much? Any soffits?

Any existing insulation up by the rafters? Any existing ventilation anywhere?

Paul

[Fred Pearson]

Hi Paul,

Thanks for your reply and questions. I don't have the building yet, but can answer all of these with reasonable confidence.

It'll be felt tiles (I think laid in segments, so not really individual tiles).
On top of the rafters but under the felt will be OSB.
Outside there will be an overhang of around 20-30cm with no soffits (you'll be able to see the roof OSB from the outside).
There will be no existing insulation.
There will be no existing ventilation other than the gap between the rafters and the top of the wall.

I hope this helps as I'm still scratching my head a bit at this.

Cheers,

[Paulus87]

Hi Paul,

Thanks for your reply and questions. I don't have the building yet, but can answer all of these with reasonable confidence.

It'll be felt tiles (I think laid in segments, so not really individual tiles).
On top of the rafters but under the felt will be OSB.
Outside there will be an overhang of around 20-30cm with no soffits (you'll be able to see the roof OSB from the outside).
There will be no existing insulation.
There will be no existing ventilation other than the gap between the rafters and the top of the wall.

I hope this helps as I'm still scratching my head a bit at this.

Cheers,

If it's not built yet, then why not just build it with the rafters going the other way?

Paul

[Fred Pearson]

Hi Paul,

For time purposes, I've steered away from constructing the building myself and am going for a prefab that'll go up in 2 days rather than the months it would take me. I've found the company I want to use and this is how they do their rafters. If I can tweak my design so I can add adequate ventilation with this orientation of rafter that would be perfect, although I'm not entirely opposed to sourcing something else if this simply won't work.

Cheers,
Fred

[gullfo]

presumably the need to provide roofing ventilation as part of build code so, does that mean the venting it something they're doing on top of the rafters? maybe if you can share their detailed build specs/plans for the roofing we could figure this out.

[Fred Pearson]

Hey gullfo,

Thanks for your response. In terms of ventilation they do have small round vents in the gable ends (which I will opt to not have as they'll be right in the middle of the air gap) and a gap between the eaves and the rafters. Lovely promo picture of the vents in the gable ends below.

Screen Shot 2021-01-19 at 18.14.13.png

Could the ventilation work like this?

Screen Shot 2021-01-19 at 18.27.29.png

Cheers,
Fred

[Paulus87]

Hey gullfo,

Thanks for your response. In terms of ventilation they do have small round vents in the gable ends (which I will opt to not have as they'll be right in the middle of the air gap) and a gap between the eaves and the rafters. Lovely promo picture of the vents in the gable ends below.

Screen Shot 2021-01-19 at 18.14.13.png

Could the ventilation work like this?

Screen Shot 2021-01-19 at 18.27.29.png

Cheers,
Fred

The proposed prefab building is

1. not intended to be used as part of a completely air tight, double leaf assembly
2. the normal way of venting, which would apply for a residential or commercial building does not apply because of it's intended purpose is as a garden shed where there are usually no constant sources of moisture for extended periods of time i.e. no humans breathing out hot air/sweating/boiling the kettle, therefore
3. the design of these shed by default are very leaky anyway; cracks between the doors/windows/framing etc.

The problem you have with your proposed venting idea (along the gable end fascias) is that you will not have a constant, unobstructed airflow from one end to the other between EVERY pair of rafters. In your proposal you are suggesting bringing air in from the eaves and venting it out of the gable end vents, the problem I see with that is potentially only the lowest rafter bay (the one nearest the eaves) will actually be fully vented, all the others will have progressively less air flow since air will travel the path of least resistance. It might work, but you would need to speak to someone in building control or a cold roof expert.

Now, you could vent from one gable end to the other but there's a couple of problems with that....

Rafters.png

Now, what is that? You've drawn it as a solid white object but I'm assuming it is supposed to be framing?

If it is framing, then yes, technically you could have vents from one gable end to the other BUT that would be longer than 6m I am assuming?

In actuality, it would *probably* work fine, but it's an unknown. By deviating from the usual proven way of doing things you run the risk of it not working and therefore having problems later on down the line. if your building is going to be subject to building control then I do not think it would pass. If it is not subject to building control (or you build this on the down low) then you also take the risk upon yourself.

You could do either of the following:

build it like your proposal, but instead of the vents on the gable ends, just install a ridge vent. This way the air gets drawn in from the bottom at the eaves and then vents out of the top at the ridge. The only unusual thing in this instance is that usually the air flow would flow directly under the roof deck without any resistance from rafters, whereas in your roof the air would have to flow UNDER the rafters first, which means the air will not "contact" every part of the roof deck... it will skim along the bottom of the rafters.

Have you considered getting rid of the vents altogether and just installing a warm roof? By doing a warm roof it doesn't matter which way the rafters run since you do not need any venting or air flow below the roof deck at all.

Paul

[gullfo]

it might be better (for more interior ceiling space, to put the battens on the top of the sheathing that you apply on the rafters. this creates a 50mm air space which can run under the sheathing for the roofing and out the top of the ridge vent. then you'll have the interior rafter space for insulation, lighting, etc.

if the exterior sheathing needs to breath fully, then you'll need it perforated to in strips to accommodate (so large temperate differences) and the vapour barrier would be on the inside of the interior ply/drywall. if it doesn't need perforation, then the vapour barrier can be on the top of the rafters before the exterior sheathing. check with you builder on this.

[gullfo]

also note: the insulation type - if using the sprayed on closed cell type (SPF) it forms a vapor barrier so you could just add the exterior sheath on the rafters, add mass augmentation if desired, and seal it with the spray on insulation.

[Paulus87]

Glenn: That's a nice idea as long as a decoupled double leaf assembly is not required. But, even if it is required then your idea can be adapted to accommodate that. Essentially, it's just building another roof on top of the existing roof so that the required air gap can be achieved.

Spray foam on the underside of the roof deck and rafters would also do the trick and no longer require any venting, but it is quite an uncommon procedure in the UK. Also, in order for it to work (and not make things a lot worse) whoever sprays it in must ensure that there are no gaps in the insulation at all, the rafters and underside of the roof deck must be completely covered.

Paul

[Fred Pearson]

Paul, Glenn,

Thank you for your help on this and the time you've spent trying to solve my problem - I really appreciate it.

Would love to just check my understanding of your proposal Glenn. So top layer would be outer sheathing and roofing - followed by 50mm battens running from eaves to crest. Then where it mentions a vapour barrier - is there, or can there be, a layer of OSB there too, followed by however many layers of plasterboard (picture below). Then I can continue with my MSM system and build a frame within this roof space.

OSB.png

Also, my understanding is that out of the two vapour barriers notated, only one is needed dependent on whether the sheathing is perforated or not. What do you mean by perforated - perforated in makeup of the material or perforated as in affixed by punching nails/screws through?

Thanks!

[gullfo]

perforated in terms of open space for air access but this was based on some drawings i found - so ignore. i think the approach - add the exterior sheathing to the top of the rafters, add mass (or not)_ fill with closed cell spray in insulation, then add drywall layers inside. the insulation will do the necessary vapor barrier. on the outside - if lots of temperature swings - use the batten approach so you have air flow from edge to ridge vent.