Hi,Fred Pearson wrote: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!)
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!
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