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Community and Q&A

Dealing with a high heat load room: skylight load

keithhoffman22 | Posted in Energy Efficiency and Durability on


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,, 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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  1. Expert Member
    Akos | | #1

    People make exterior covers for skylights (search for skylight exterior roll shutters).

    There are even louver bases systems, mostly for commercial ( ). You might be able to DIY something similar from a set of adjustable fence louver kits from the box stores. With your skylights at 45 deg, if you set the slats horizontal, it will keep most of the sun out during the summer but still allow light in.

  2. Expert Member
    Peter Yost | | #2

    Hi Keith -

    – Is covering the skylights in the summer (with what?) a crazy idea? Exterior shading works much better than any interior shading (once the heat gets through the glass, you pretty much "own" it). But exterior shading devices, especially operable ones, are quite expensive.

    – Is there a commodity priced skylight that warrants the labor to install it? Not that I am aware of. And if there was one that dealt well with winter and the heating season, you still need exterior shading to manage well solar heat gain in the summer.

    – Is an inner window a crazy idea? (FYI existing skylights are fixed). Thoughts on specifications? You should check out this website:


  3. charlie_sullivan | | #3

    You might consider going to a glass shop to order an insulated glazing unit (IGU) without any frame to install under each skylight. The hard part of skylights is making them waterproof, and since that's done, you can install an IGU with any makeshift support and airseal with caulk or tape.

    Or if you want to go cheap and still get some light in, 10 layers of bubble wrap.

  4. GBA Editor
    Martin Holladay | | #4

    In 2014, I wrote a blog about a homeowner who was so fed up with the disadvantages of the skylights in his sunroom that he covered them on the exterior with peel-and-stick membrane (Grace Ice & Water Shield). If you imitate this homeowner, you could fill in the skylight well with fiberglass batts, and install new drywall on the interior.

    I'm not recommending this approach -- just making a comment on how common it is for homeowners to wish that their skylights had never been installed in the first place.

    Here is a link to my blog: "South-Facing Skylights: Threat or Menace?"

  5. keithhoffman22 | | #5


    Yeah, with a standing seam roof, the skylight is a better weather layer than Grace. That's beyond my aesthetic tolerances as well.

    I've actually already pulled a skylight from a flat portion of roof. Easy decision because that was on low slope EPDM so the roof repair was trivial. We miss the skylight; we don't miss heat gain equivalent to the gain of the rest of the roof.

    Thanks for the blog link.

  6. keithhoffman22 | | #6


    That's something like I was thinking but I was thinking of spending a few bucks for a vinyl unit (I don't have wooden windows). The local glazing unit sounds good too.


    Thanks for the slatting suggestion. I like that. I'll have to look into whether I can fabricate something. We have a couple used building material yards but that's a lot of material to locate.

    Peter, that's a pretty interesting website. Thanks for the suggestion.

  7. keithhoffman22 | | #7

    I've tripped over a couple low-e double pane single hung fiberglass units at the local reuse yard that could work for innie windows on my conditioned space skylights (right size). However, one question that I'm not sure how to SWAG is how to Manual J such an assembly (1980s metal deck mounted exterior skylights with an inner double paned low-e window).

    - What's the U factor of such an assembly?
    - Do you have to trap the immediate air (presumably at least somewhat tight is needed)?
    - Is using the inner assembly U factor an adequate SWAG or does the trapped air become superheated etc and thus potentially reduces the efficiency of the insulated glass?
    - Or does such an assembly perform better than the U-factor of the inner window?
    - Are extra measures such as insulating the skylight 'chase' from the adjacent framing members necessary? Or unnecessary due to the depth of sloped and flat roof rafters (compared to walls)?

    Thanks for any more wisdom about sensible macgyvering.

    1. Expert Member
      Akos | | #9

      Keith I would be careful with low-e IGUs underneath the existing skylight. When I was toying around with DIY solar collectors, I looked at a similar idea, but standard IGU seals will have issues with heat. Your existing skylight might also not appreciate getting cooked as well.

      I've made these shutters as a privacy screen, making something similar to mount on-top of the skylights would be pretty straight forward build and would keep most of the heat out. Depending on the roof orientation, if you pick the correct slat angle and overlap between slats, you can even make it fixed and still work year around.

  8. bennettg | | #8

    Solar screening would be a relatively low cost alternative to exterior slats. It has worked wonders cutting the solar gain on my S&W facing clear glass glazing. Phifer is one mfr. Depending on your weather & wind, a roller shade or framed screen might work. "screen tight" is one framing system and is pretty easy to work with. It's what I used on large triangular windows and has survived the 70+ mph winds of Hurricane Florence.

    Obviously, they're no help with the loss in winter.

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