you have 2 options
1- fix the room
2-oversize hvac system to overcome building failures.

option one...reduce heat gain & stop air infiltration into the room
so that a/c allocated to it can do its job

option two...oversize the hvac system, which may result in other
issues.
capping off the ducts to this room...1.5 ton system on 950 sq ft
would cause issues with the 1.5 ton system.

1-fix the room
spray foam insulation
or sealing methods in each kneewall as detailed top right
of page 3 of this pdf doc
http://www.southface.org/factsheets/AS-Airsealing 00-767.pdf

2- minisplit and dump the central air for this room in stairwell.

best of luck

 

Thank you for the reply. I'm actually leaning towards installing the mini split now and working on the insulation problem in bits and pieces when I can. I'm thinking that I need to install foam board on the back side of the knee walls, install foam board blocking between the floor I joists under the knee wall, and seal all gaps and seams with spray foam. The area where the knee wall meets the rafters is going to be tricky since the studs are nailed directly to the rafters.

I'm concerned about the difference in BTU's in the two systems quoted. One saying 1 ton would be enough and the next guy recommending 2 tons. Neither one of them did a manual J calculation. Would this calculation likely give an accurate BTU requirement, or since this room is fairly unique would that likely throw the calculations off?

 

Got a MSZGL15NA installed. It seems to be cooling well, but I'm still a little concerned if it's going to be enough. The 3rd contractor I spoke to, and the one I had do the install recommended a one ton too. I had to convince him to go up to a 15k btu unit.

 

what did you do with the two supplies from the central system?
left them in place?
dumped the air elsewhere?
capped them off?

as for the foam board.
measure, cut & number your pieces to take into the attic.
forget the spray foam & use caulk & a paint on mastic to seal
between floor joists to ceiling below.
with caulk & mastic...the sealant goes into the crack rather
than just covering large areas. foam will shrink in a couple of
summers...and you'll have leakage.

best of luck

 

Thanks for the reply.

The third contractor, the one that did the install, just recommended I close off the registers and leave the return open. He mentioned the return is more important as fas as not choking down the system, and also mentioned it would help cool the other rooms. I suppose I can follow his logic. I forgot to mention, I rarely use the other 2 rooms served by the upstairs system, so right now I have it set to a very high temperature. Is there a maximum recommended temp. to set the a/c to? I don't want to end up with humidity problems and such.

With that said, I am having humidity problems in the bonus room where I installed the unit. The unit has been running full blast all day, so I don't think it's oversized. This morning, the humidity level was 67%, and with the unit set on 70*, it's currently 73* and 55% humidity at 7:15 pm. 55% feels slightly uncomfortable, but 67% was very uncomfortable. I have emailed the contractor to see if he had any suggestions. I intend to call in the next day or so if I don't hear back.

This is the room I'm trying to cool.



This is how they insulated the knee wall areas. The insulation was just turned up 90* under the sub floor even with the bottom plate.







You can see the large gap between the insulation and the subfloor.

Thanks for the tips on the foam board! Is there a specific board you would recommend? I did see the wall insulation is only r-13. Apparently Georgia now requires a total of R-18 including sheathing.

 

Also, I was considering having a company come in and blow insulation in the garage ceiling. I'm wondering what the best plan of attack for this would be. I put down the flooring that is presently in the knee wall area. Would it be best to temporarily move the flooring I put down and block the area under the knee wall with foam and just fill the area under the actual room floor, or leave my flooring in place and fill the floor under the knee wall area too? I have no idea what this would cost, so I might not want to do the blown in insulation.
Also, since there is a lot of room under the flooring, could the insulation be blown in there on top of the current insulation, or would it better to go through the garage ceiling?

I've also read where some are recommending spray foaming the roof area and bring the knee wall into the conditioned space. I may be totally off, but it seems like this would add a large unused area to cool and heat.

I hope all this makes sense, it's kind of difficult to explain. Thanks

 

Check to see if the system your installing is an inverter type as most mini splits are. They have a working btu "range". If the system is slightly oversized, in this case, 3000btuh which equate to 1/4 ton of AC, not going to make much of a difference. Most working ranges for these small systems are between 30% - 130%. do the math. Or if you need concrete answers, have a heat load calc done for the area you are trying to condition, at least for piece of mind.

 

Most working ranges for these small systems are between 30% - 130%. do the math. .

Can someone explain this to me? I did find that the working range of my unit is 3100 to 18200 btu. So, if my unit is able to go down to 3100 btu, I'm pretty positive it's not oversized.
I'm pretty sure my contractor is not going to want to take the time to do a load calc. after the fact. I suppose I should have pressed more to have one done before the install.
Mitsubishi's site recommends in the absence of a load calc, to multiply 25 x square footage + 450 btu for each occupant. That puts me at almost 11000 btu, and that is not taking into account
the fact I'm in the south and the room is over a garage, open on 3 sides and gets full afternoon sun.

Just as a ballpark guess, does anyone think this unit is oversized?

I guess if all else fails I can run a dehumidifier, but that pretty much does away with energy savings I thought I would have.

 

Thanks, been looking on the net, but so far I have not been able to tell. I think it is. If nothing else, I'll ask the installer the next time I talk to him. The model #'s are MUZ-GL15NA and MSZ-GL15NA if anyone happens to know.

 

Also, I did try to call Mits. tech support. That is probably the worst experience I have had with a tech support rep. in a long time. Kind of makes me wish I had saved a bunch of $ and bought a cheap Chinese unit off Amazon. From what I can tell, their tech support is much better! At least they attempt to answer questions on Amazon.