How to deal with the trapped air in a loft attic?

Question

I live in an apartment (not a house) with a loft attic. It has enough space and I use it as a study room. In summer, the air is extremely hot as I walk up the steps to the attic.

Initially I was thinking to get a solar powered roof fan installed on the roof with its opening inside the attic (of course this needs strata approval), however, there is a problem: My bathroom has a hole that leads to a dome shaped skylight window and at the end of the hole, it has access to the roof cavity (the gap between the roof and the ceiling). I guess they made it that way to vent the air in the bathroom.

Now, if I install this vent in my attic, the vent sucks the air inside the flat out and as the result the replacement air will be pulled from bathroom and since it's connected to the roof cavity, the air in there will be pulled inside the flat. I already have problem with dust and dead insects coming through that hole in my bathroom, if I install the vent, this gets even worst.

So then my next thought was that, perhaps I can install the roof vent without any opening inside the unit and the opening will be inside the roof cavity. However, I'm not sure if that will be effective and if it makes any different to the current state.

So I am kind of desperate and the air in my attic is so hot and humid that makes the attic useless. I can't even use it as storage because I'm worried molds will be grow so I keep it empty during the summer. I hope I can find a solution here.

I guess there are some misunderstanding, I draw something to explain what I mean. My attic is loft so from there you can see the living room. Also there is no access from toilet to attic, it’s venting into roof cavity

So here are my questions:

1. If I install the roof fan to vent the air from attic to outside, doesn’t that pull the air from the cavity via bathroom into the living area?
2. If I install the roof fan to vent the air from cavity and NOT the attic, does that even help? Considering the roof is shared with another unit.
3. Any other option?

P.S.: The wall on the other end of my attic is shared with the other unit, so I can't really open a hole to install an exhaust fan.

Answer 1

First off the bathroom should be sealed from the attic space, any venting from the bathroom should be to outside. Venting moist bathroom air into the attic space is a recipe for mold and should never happen, fix that first.

Next the attic is supposed to get hot, this keeps the heat away from the living space, if you are using the attic as living space it needs to be properly insulated and ventilated.

Don't skip the insulation step keeping the heat out is a great first step.

As for ventilation you can either as you said, add a vent fan to send the hot attic air outside, and setup other attic vents to either draw air in from outside (normal for an attic space) or draw air up from the living space (not normal, but might be what you want as you are using the attic as living space).

Another option would be to setup interior fans to increase the air exchange between the attic and the other living space, allowing the living space HVAC to cool the warm attic air.

A big thing to consider here is that cooling the attic space with living space air is likely to make the other living spaces warmer in the summer.

Answer 2

You could install a roof fan with a duct from the fan to the loft space that you want to vent. If there is a window you could install a small window A/C unit to help cool the loft.

Answer 3

If you can get approval from your landlord for it, you could install a Y on your bathroom vent. One branch goes back to the bathroom and one is for the attic, with the "leg" of the Y going outside.

On each branch of the Y, install an air duct gate so you can close off each branch selectively. Between the gate and the Y for the attic side, also install an inline duct fan.

When you use the attic, turn off the bathroom gate, turn on the attic gate, and turn on the duct fan. When you leave, do the reverse. If you are really handy, or have the money to do so, you can get gates with motors so all that could be done at the flick of a light switch.

Duct gate/damper example:

http://www.atlantasupply.com/nlhtml/images/zcrdn.jpg

Duct fan example:

http://www.espenergy.com/5_inli6.jpg