Dealing with a high heat load room: skylight load
I’ve recently performed a Manual J myself on coolcalc. (this is probably the 4th time I’ve sized the house over the years, using a pro, loadcalc.net, and coolcalc).
The goal of the sizing was to determine whether a hybrid heating system might address our poorly heated house that has a difficult forced air furnace installation space. But in the process, I managed to tighten up the parameters on the Manual J. I was appalled to find that our unducted ~140 sf sunroom that leakily shares air space with the house (there is a door but it is very leaky) has a heat and cooling load of 18k/15k btus by itself. For comparison, the remainder of the house has perhaps a 44k btu heat load (cooling load is lower).
Coolcalc reveals that the very large 1980s skylights are the majority of the load, contributing ~70% of the heating and cooling load. Some larger 1980s windows contribute 25%. Walls and the uninsulated slab are actually pretty modest in contribution.
I don’t really want to condition this room, especially not to design temperatures, but I need to get it more thermally stable if I’m not going to condition it.
While I’m considering options for windows (inner windows or replacement), it’s the skylights that represent the big problem for me.
I’m looking for some eclectic ideas for coping with these older skylights. I think there is about 60 sf of them!
I’m not prepared to tear them out as that likely precipitates a new standing seam roof. I’m also not prepared to install new skylights, both because I don’t know that high efficiency options have the needed sizes (zero ability to resize) or that I have the budget for those. I am looking into whether I’ve assumed incorrectly that the cost would be high and the performance gain low.
Ideas I’ve tossed around:
– Block the opening, install mineral wool wrapped in white membrane, drywall over. This option seems like it might precipitate early failure of the skylight, conceal leaking, and result in 100% loss of VT.
– Install an inner skylight: I can’t find such a thing.
– Install an inner conventional window. The skylights have a 45 degree pitch so the idea is not as crazy as installing a conventional window on the flat. I would guess I could get a pretty affordable fixed assembly or possibly find an old single hung. Would allow some VT still (likely getting towards 35% or less?), would insulate the room from the cold skylight, and prevent condensation I think.
– Make covers for them for summer to eliminate the cooling load.
Bonus points: all this uninsulated glass faces SE and SW.
I’d love to hear some suggestions. 4 Thermo skylights at $1600/each is just not in the cards.
So my questions:
– Is covering the skylights in the summer (with what?) a crazy idea?
– Is there a commodity priced skylight that warrants the labor to install it?
– Is an inner window a crazy idea? (FYI existing skylights are fixed). Thoughts on specifications?
As mentioned above, the plan is an air tight door to separate the heat and cooling load of this room but I’d like to get the uncondition load reduced and stabilized. I have 2 more of these elsewhere in the house that are in conditioned space.
I also need to figure out the long term thermal performance plan so I can place holder what this load might be on the whole house manual J.
Cheers and Thanks!
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