HVAC question: adding vents to un heated loft

Hey folks. I have an unheated/cooled loft on the second floor above the garage. Picture your typical pitched roof room but big enough to walk around the majority of it. The previous owners put Fiberglass insulation on the ceiling and sealed it with plastic so it's insulated on the top. I suspect the floor is not since it's just plywood over the garage but it does have carpet padding covering the floor.

Can I run vents into the room off of the trunk line without a return? It'd be a perfect place to grow if I had heat/cool in there and I figure with no return I'd minimize the chance of stinking up the whole house. Any thought? How much would this job cost for two vents?

ok it's above a garage. So if the roof is insulated, then are the floors and walls also insulated?if not the heat/ac added will be wasted. I would get a few cans of expanding foam sealant, go around to lights (make sure they aren't a fire hazard if you air seal them), any cracks where a wall plate is , seal both sides of the 2x4. Also seal the gables in between each 2x4 (gables look like triangles, where the two inclines of the roof meet the peak, on the bottom of the gable where the cieling is beneath you, there should be a seam. Seal that.
You may have Ridge venting, soffit venting, or gable venting. If you have any of these you will not want to condition that space.
after you airseal and make sure everything is insulated (weatherizing) you can now add in the supplies. You could also add a return with a carbon filter if you aren't getting the room dialed in enough. I recommend if you have a zoned system, and your zoning board allows it, to add in an extra zone damper just for the grow room and if you oversize the a/c for the room, you will have it perfectly conditioned.
there are CFM calculations you will have to do based on the footage of the room and the tool you use to do this is called a ductulator. 1 ton of a/c is about 400-500 CFM's of fan, so if you have a/c you can size it based on this. If it's just heat, i would size it based off of BTU's but I don't know the equivalency for that.

Thanks for your help.

Interesting. that's a little over my head. I will measure it tomorrow. I can tell you that it's all roof. The room is a triangle so everything is insulated with fiberglass except for the floor. There are vents sticking out of the roof but on the inside they are covered with the fiberglass and plastic over that so already sealed. I"m not sure if there are additional vents in the soffits I will check tomorrow. The house has a heater and AC for each floor with their own thermostat and the air handlers splitting them up. Surely I have enough power to do another room. The whole place is only 2700 sq ft without this room. thanks for your help. What does one charge for two pipes and vents added? They won't need to do anything fancy since it will just be hanging from the beams. Literally just cut into the trunk, cut a hole in the insulation and feed the pipes into the room.

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You have a ridge vent? Is there air gap trays above that insulation below the sheathing? There may be pre-existing issues that will contribute to mold problems on the underside of the sheathing. Attics are designed to breathe and be a temperature buffer zone to control roof temperature so as to prevent ice dams. Too much humidity in against the inner sheathing during cold weather condenses and causes black mold. If the roof is properly vented you're not sealing it for hvac....

SnapsProvolone said:

You have a ridge vent? Is there air gap trays above that insulation below the sheathing? There may be pre-existing issues that will contribute to mold problems on the underside of the sheathing. Attics are designed to breathe and be a temperature buffer zone to control roof temperature so as to prevent ice dams. Too much humidity in against the inner sheathing during cold weather condenses and causes black mold. If the roof is properly vented you're not sealing it for hvac....

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No, I have the vents that look like boxes on the roof. So a hole cut into the roof with the box on top. Sorry I'm so ignorant of the terms lol. Those are completely blocked by the insulation and then on top of that the plastic. So the room is sealed off to the outside.

texasjack said:

No, I have the vents that look like boxes on the roof. So a hole cut into the roof with the box on top. Sorry I'm so ignorant of the terms lol. Those are completely blocked by the insulation and then on top of that the plastic. So the room is sealed off to the outside.

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This is likely an issue a home inspector should catch. Some previous homeowner went rogue diy mode and tried to make living space out of an attic that was designed to vent in order to preserve the structural integrity.

I'd pass on growing up there if you care about health. I'd also get the roof vents opened back up and check soffit and gable vents for obstruction.

does it matter that it's not over a living space (garage) and is sealed off from the rest of the attic?

Texas, if that room is sealed up, you shouldnt have issues. You may want to pull the plastic back and properly seal up your vents. You're in a garage, and there is no health issue there, at least not that I'd be concerend of. Think about this though, if the room is sealed the ac will be pressurizing the room, that pressure in going to need a place to go, a vent will you. The AC will be acting as an active intake. If you're using air cooled lights you could attach a carbon filter to the fixture or a duct coming off of it, and on the other side an inline fan and duct to the out side, that will cool your lights, provide active exhaust for air exchangee and control odor.

(Filter)=>==(sealed hood)=>==fan=>outside

Sounds to me like you're a little in over your head. If you need someone to do your duct work, I take it you'll need someone to do your electricity as well......if you're using extension cords to get power up there, i suggest heavy duty cords if you're pulling any real amperage.

Joven Agricultor said:

Texas, if that room is sealed up, you shouldnt have issues. You may want to pull the plastic back and properly seal up your vents. You're in a garage, and there is no health issue there, at least not that I'd be concerend of. Think about this though, if the room is sealed the ac will be pressurizing the room, that pressure in going to need a place to go, a vent will you. The AC will be acting as an active intake. If you're using air cooled lights you could attach a carbon filter to the fixture or a duct coming off of it, and on the other side an inline fan and duct to the out side, that will cool your lights, provide active exhaust for air exchangee and control odor.

(Filter)=>==(sealed hood)=>==fan=>outside

Sounds to me like you're a little in over your head. If you need someone to do your duct work, I take it you'll need someone to do your electricity as well......if you're using extension cords to get power up there, i suggest heavy duty cords if you're pulling any real amperage.

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It's already wired so I don't have to worry about that. I would hire someone to do the ducting as I have to have someone come add insulation to my attic anyway. Thanks for the info

texasjack said:

does it matter that it's not over a living space (garage) and is sealed off from the rest of the attic?

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This is a firewall and should only be penetrated by a licensed contractor.

Mold remediation is not the cheapest thing, so beware of the exaggerated potential for black mold provided your biomass is significant and or air not exchanged often enough.

SnapsProvolone said:

This is a firewall and should only be penetrated by a licensed contractor.

Mold remediation is not the cheapest thing, so beware of the exaggerated potential for black mold provided your biomass is significant and or air not exchanged often enough.

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small time grower here. 1 or 2 plants at a time

ok so you're saying you have scoops installed on the slopes of your roof.
To really make your space efficient and not have to do a shit ton of work conditioning it, you have a few choices.
1) rip down the fiberglass and spray foam the slopes (uninstall or blank off roof scoops), also do the gables and then you can re-use the insulation from the ceiling on the floor joists and then cover it with plywood. Any soffit vents should now be covered. You CAN make it too tight so be careful, some people use a blower door test to get the BAS. Since the slopes are now insulated and air sealed, heat will not build up in the attic and cause condensation (temp difference) problems due to air exchange in the winter (black mold).
2) what i would do is build a grow cabinet or two out of foam board, it comes in several thicknesses (1 1/2 " should be durable and easy to work with, but for a higher R value, you can go with 2" )
build a box out of it and use the foil faced stuff. be careful with it and try not to puncture the foil. Tape the seams with silver tape and try and build it so its 100" air tight.
Now we can tie in the ventilation. Depending on the sq footage, you , may need a bigger or smaller supply and return hole size. I would use flexible ducting with band ties , mastic tape, and collars. The collars will act as a flange which you can attach and remove the flexible duct from.
you say you have 2 zones from 2 systems? If there is 3 zones you might have a zoning board with an extra zone to tie into and can buy a zone damper to dedicate to your grow room. You would tie this in with its own thermostat and into the system with the zoning panel and that way you can have the grow room controlled separate from the rest of your house/

i'd definitely go with building a structure inside the attic and insulating/air sealing it. If you're interested in this, let me know and i'll help you draw up a plan and get some dimensions.
also wanted to mention they sell collars that have dovetails spaced off for foamboard so that you can just cut the hole in the board and fold the tabs out to hold it in place.

thank you, I just saw this. I will follow up.

hi nizza.
i have probs getting air/smell out my attic. can i just run a ducting to the top of attic corner near gable end?
got 125 rvk ext fan.
f.