Attics are the hardest place to get things squared away, although the most stealth.
For an attic box, first thing is to make sure you have it insulated really well. I would suggest lining the inside of the box with Dow insulation board. The aluminum surface of one side provides adequate reflective surface for the inside walls. No paint of mylar needed.
Also, where you get your air can be important in an attic where it gets brutal hot. If the attic is a full gable type, you could cut two openings in each end of the attic and place a gable fan at one of the openings. They sell gable vent covers at any HD or Lowes that match the siding of your house. I know this sounds daunting, but it would be the best thing to keep the attic temps in check. To save energy you could put a thermostat controller on it (not very expensive) to only kick it on when the temps get real hot.
If you don't have the ability to install any flow-through air movement with a gable fan, then you want to run a duct from the intake of your ac unit to where the attic takes in air. This will most likely be from the soffit area of the roof (lowest point) at the overhang outside the building. You can see little holes in the horizontal soffit, and that is where the roof breaths. That is the place to find the coolest air possible. Just have the end of the intake duct laying on top of the soffit from above (you may have to move insulation to find this soffit and the holes). I suggest using the largest duct you can find for this, at least 6" but better 8-10".
This duct would lead back to the box that surrounds the intake only.
For the exhaust...IF you do have good flow through ventilation in the attic, you could just let the back of the unit exhaust into the attic itself. You could fashion a partial box around the back just to direst the hot air where you want it..and not blowing hard on rafters and such.
The flow through air will not only allow you to keep the attic cooler, but it will help you shingle to live a much longer life than they would in a no air flow attic.
If no air flow in attic, then you need to enclose the back end of the unit and exhaust the heat one of two ways...
Run another duct from the exhaust back to the soffit on the other side of the attic. You could mount it onto a soffit piece if you have your carpenter on...but you could also just lay it over the holes as you did on the intake side.
OR...you could get real fancy and cut a roof hole for an exhaust stack. These are also cheap and easy to find with full instructions on how to....but again you need to have your carpenter on for that.
After you have your ac figured out and working, you will still need to exchange air in the box. For this I have intakes a couple feet above the floor on one side. Having ac it will take a small fan and very little movement of air to keep things fresh, but it needs to be there. Again, if you have your attic temps taken care of with a gable fan, then you can just suck air in and blow it out into the attic. If not, then you will need separate ducts for the air intake and exhaust set up like you did the ac ducts and getting air from the soffit area.
I used to also think about my ac and the cool air from the top, since cool air sinks....but, I have a different thought about it all now. The root zone is where the cool is most important. If you keep the roots nice and cool, the veg part of the plant can take very high temps with no problems..just like in nature where the ground has a natural cooling and constant temp, and the ambient above can get quite brutal in the mid-summer sun.
The last pic I posted shows the ac unit in the lower right of the shot. It sets on the floor of the cab in the next chamber. I would suggest you set your box up in a similar fashion. You will need a rectangular hole in one side of you box at the bottom just large enough to insert the front of the AC unit into the hole.
This is a lot to absorb, so if there is anything that you are unclear on, don't hesitate to ask.
For an attic box, first thing is to make sure you have it insulated really well. I would suggest lining the inside of the box with Dow insulation board. The aluminum surface of one side provides adequate reflective surface for the inside walls. No paint of mylar needed.
Also, where you get your air can be important in an attic where it gets brutal hot. If the attic is a full gable type, you could cut two openings in each end of the attic and place a gable fan at one of the openings. They sell gable vent covers at any HD or Lowes that match the siding of your house. I know this sounds daunting, but it would be the best thing to keep the attic temps in check. To save energy you could put a thermostat controller on it (not very expensive) to only kick it on when the temps get real hot.
If you don't have the ability to install any flow-through air movement with a gable fan, then you want to run a duct from the intake of your ac unit to where the attic takes in air. This will most likely be from the soffit area of the roof (lowest point) at the overhang outside the building. You can see little holes in the horizontal soffit, and that is where the roof breaths. That is the place to find the coolest air possible. Just have the end of the intake duct laying on top of the soffit from above (you may have to move insulation to find this soffit and the holes). I suggest using the largest duct you can find for this, at least 6" but better 8-10".
This duct would lead back to the box that surrounds the intake only.
For the exhaust...IF you do have good flow through ventilation in the attic, you could just let the back of the unit exhaust into the attic itself. You could fashion a partial box around the back just to direst the hot air where you want it..and not blowing hard on rafters and such.
The flow through air will not only allow you to keep the attic cooler, but it will help you shingle to live a much longer life than they would in a no air flow attic.
If no air flow in attic, then you need to enclose the back end of the unit and exhaust the heat one of two ways...
Run another duct from the exhaust back to the soffit on the other side of the attic. You could mount it onto a soffit piece if you have your carpenter on...but you could also just lay it over the holes as you did on the intake side.
OR...you could get real fancy and cut a roof hole for an exhaust stack. These are also cheap and easy to find with full instructions on how to....but again you need to have your carpenter on for that.
After you have your ac figured out and working, you will still need to exchange air in the box. For this I have intakes a couple feet above the floor on one side. Having ac it will take a small fan and very little movement of air to keep things fresh, but it needs to be there. Again, if you have your attic temps taken care of with a gable fan, then you can just suck air in and blow it out into the attic. If not, then you will need separate ducts for the air intake and exhaust set up like you did the ac ducts and getting air from the soffit area.
I used to also think about my ac and the cool air from the top, since cool air sinks....but, I have a different thought about it all now. The root zone is where the cool is most important. If you keep the roots nice and cool, the veg part of the plant can take very high temps with no problems..just like in nature where the ground has a natural cooling and constant temp, and the ambient above can get quite brutal in the mid-summer sun.
The last pic I posted shows the ac unit in the lower right of the shot. It sets on the floor of the cab in the next chamber. I would suggest you set your box up in a similar fashion. You will need a rectangular hole in one side of you box at the bottom just large enough to insert the front of the AC unit into the hole.
This is a lot to absorb, so if there is anything that you are unclear on, don't hesitate to ask.
