300 sqft attic -- 1.5 to 2 tons???

[greenhall]

I'm finishing a space of ~300 sqft in the attic of my 1901 house with a 12/12 pitch slate roof in NE Kansas (Lawrence). I'll have expanding open cell foam blown in to insulate the entire roof, but with ~4 foot knee walls the finished space will be ~300 sqft including a 1/2 bath, 11 x 16 room and tower space.

I've asked for 2 bids and both are saying 1/5 to 2 ton system required. I think I'll use a Mitsubishi mini split. This seems like a big load calculation.

I know there is no substitute for a complete calculation, but I can tell that the people on this site have a lot of experience and I'd like to know if this sounds reasonable. Thanks for your help. The rest of the whole house is ~2300 sqft and 4 tons.

P.S. One more question: Should I insulate the knee walls? Space "behind" the knee walls and 3 gable ends will be unconditioned storage space.

[BaldLoonie]

I did a 300+ sq ft attic room for a friend. 1 ton with 5kw in the air handler. Did great. Bigger is not better. But the more you an insulated it the better!

[DanW13]

If the rest of your home is insulated like you have your new attic space I would tap off your existing system, or split your existing sytem with say a 2 ton and then 2-1/2 respectively and forget about doing a 3rd system when you currently have 4 ton. although you don't say if the 4 ton is one compelte system of if it is already split amongist the 2300 sq ft. but it's another option for oyu to consider.

[greenhall]

Main house has a 4 ton HP / furnace (American Standard variable speed gas furnace with 16 seer two stage heat pump) for ground and second floor. It is zoned. Unfortunately there is not a way to run a duct up to the attic that would be large enough to supply that space. I'm sure you are right that that would be the best option. As it is, adding a new unit seems to be the only way I can condition that attic space.

[DanW13]

Is this attic space not above a conditioned living space already? If so there shouldn't be too much trouble getting say a 6" oval duck up and over from an adjadcent room or enlarge a duct from the space below to accommodate the attic. I would sooner tear out alittle drywall than install another piece of equipment especially for a mere 300 sq ft. But that's just me.

[BURL-REF]

Did either do a Manual J load calc ? 1 1/2 to 2 sounds like a guess to me. 2 to 3 cfm per sq ft ? Thats alot in a residence.

Just a guess here but 9,000 to 12,000 mini split with the invertor compressor, providing you have on high wall to mount the evap on.