Triple leaf and ventilation

[dankelly]

Hey all.

My situation is similiar to those of others but I still haven't found a definitive answer (if there is one!)

Basically i'm going to buy a house with a garage that I will be converting into a studio by building a room within a room. As I don't own the house yet I cannot get any pics unfortunately but I will explain thoroughly what I am dealing with.

The garage is brick built and I am going to block up the window and big door leaving a normal door for access. I want to seal the whole construction and then build a room within a room but the ceiling seems to be my major weak point. It is a pitched roof with some sort of corrugated plastic or asbestos on top. Ideally, I would like to be able to insulate and drywall to the underside of the rafters, sealing everything to the existing walls so that the outer room is airtight. Then I would build the internal room, sealing everything.

My problem is that this would appear to create a triple leaf, consisting of 2 layers of drywall on my inner room ceiling, air space, drywall on the underside of rafters, air space, corrugated sheet roof. After spending time and money sealing the outer building construction, it seems a shame to then not be able to add drywall under the rafters to avoid this triple leaf as the ventilation from the eaves would render the construction non-airtight!

I understand that this is one of those things and that i just have to work with i have, but what is the trade off between the triple leaf and the leakage through ventilation?

My question basically, is am i better to create a triple leaf but have an airtight room within an airtight room, or should i avoid the triple leaf and have an airtight room within a non-airtight room?

Thanks for the replies peeps

[lakeshorephatty]

Hey,

I've tried to find similar answers to this roof thing to no avail. I'm also curious to find out how the ventilation as the 2nd air gap would affect your roof construction.

One thing I noted in your plan was to block up a window and door leaving one door. You will want to leave yourself a fire door/window for certain. I was reading about this in another thread and it clued me in to not blocking off all my 2ndary openings. You will want at least one other accessable opening either a door or window that people can fit through as a fire exit.

Russell

[dankelly]

Hi Russell. Yep, unfortunately you are correct regarding the fire escape. Which is a pain as its gonna cost money for another door which i don't need! Obviously safety first though.

It's definitely a tricky one to get right with the ceiling/ventilation issue. Hopefully some of the experts on here may have an idea.

[dankelly]

Does anybody have any advice regarding this?

[jhbrandt]

Yes, ventilation for the roof is only for thermal concerns.

For best STC rating don't drywall the underside of the rafters if you are building a room inside.

Be sure to fill the airspace with fluffy stuff. - And put a plastic moisture barrier toward the room (before you put up the drywall). If you can seal up the small cracks and holes in the exterior wall with a non-hardening caulk it would be a bit better.

I am currently designing a similar project studio in Australia.

It's your call on escape windows and/or doors, but they will reduce your transmission loss if not built correctly or sealed properly.

It is the internal room that must be sealed completely.
Cheers,
John

[dankelly]

Ok John, thanks.

I was under the impression though, that as well as sealing my inner room airtight, for maximum TL I should also be making my outer leaf as effective as possible? As an exaggerated example, if my roof were to not be there at all, I would not have a room within a room, but merely just a room. I could beef up walls and beef up the underside of the roof between the rafters but if i then have gaping holes for ventilation then it seems pretty pointless?
As everyone says on here, there is no point in putting 5 layers of drywall on the walls to then only put 2 on the ceiling as the ceiling will let the walls down. So surely by beefing up and making the best of my outer leaf, then the ventilation is going to let the whole isolation down?

Im sure this is one of those things that just has to be accepted, but of my options I am trying to make the right decision for the best possible results. So here is my question. Am I better to :

A) Leave the outer leaf (roof) as it is (corrugated sheet on top of plywood), then double drywall my new isolated, inner ceiling and insulate the void between.

B) Beef up the outer leaf between the rafters, then do the same as (A) with the internal ceiling.

C) Drywall to the existing trusses then do the same as (A) with the internal ceiling, insulating both voids. This creates a triple leaf but there will now be a perfectly sealed room, within a perfectly sealed room, underneath a ventilated loft space. I'm trying to understand the effects of the triple leaf. I know its bad, but is it worse than not having that extra layer of isolation?

Thanks a lot for the help guys.

[jhbrandt]

Originally Posted by dankelly

I was under the impression though, that as well as sealing my inner room airtight, for maximum TL I should also be making my outer leaf as effective as possible? As an exaggerated example, if my roof were to not be there at all, I would not have a room within a room, but merely just a room. I could beef up walls and beef up the underside of the roof between the rafters but if i then have gaping holes for ventilation then it seems pretty pointless?
As everyone says on here, there is no point in putting 5 layers of drywall on the walls to then only put 2 on the ceiling as the ceiling will let the walls down. So surely by beefing up and making the best of my outer leaf, then the ventilation is going to let the whole isolation down?

Actually adding mass is not pointless if you have ventilation holes in that layer. If you must have vent holes, make them with sharp corners and not too much larger than about 5" diameter. The impedance difference will reflect low frequency sound by up to about 17db. The length of the path (hole or channel to the ridge vent) also attenuates sound at a calculable rate per meter/foot.
The mass of your roof (asbestos or asphalt shingles?? are heavy) is probably adequate but could be beefed up by adding drywall to the underside of the sheeting - as long as there are no protruding roofing nails to prevent you.

Originally Posted by dankelly

Am I better to :

A) Leave the outer leaf (roof) as it is (corrugated sheet on top of plywood), then double drywall my new isolated, inner ceiling and insulate the void between.

B) Beef up the outer leaf between the rafters, then do the same as (A) with the internal ceiling.

C) Drywall to the existing trusses then do the same as (A) with the internal ceiling, insulating both voids. This creates a triple leaf but there will now be a perfectly sealed room, within a perfectly sealed room, underneath a ventilated loft space. I'm trying to understand the effects of the triple leaf. I know its bad, but is it worse than not having that extra layer of isolation?

(B) if you can. But (A) might be fine... I'm not there. Can you determine the mass of the roof?

Cheers,
John

[dankelly]

Hi John. Thanks for taking the time to advise me on this.

I would say that the roof is most likely asbestos. Is option C a definite no then? It's less work for me if I go to option A so that's a bonus. I'm assuming that you think the triple leaf would have more of a negative effect on the isolation than the vents?

[jhbrandt]

Most likely, yes.

Can you get an idea of the mass of the roof layer??

[dankelly]

I'm not sure how to work that out. It is plywood, probably 18mm with a heavy corrugated sheet fixed on top which I suspect to be asbestos. Sorry if this is not helpful!