Options to cool attached sunroom

Question

I live in the northeast US and recently bought a home with an attached solarium facing south. Roof is composed of reflective double-pane Anderson windows. Sides are nearly floor to ceiling double-pane Anderson windows. All the floor to ceiling windows open. The room is completely connected to the rest of the house, though it does have its own AC system and zoned baseboard heat.

On days where it's cool enough to open the windows, the room is generally comfortable. It does get warm in the spring (maybe 80 degrees) but it feels fine with the windows open.

On hotter days where we seal the house up to turn the AC on, the room bakes. Until 2 o'clock or so where the sun starts to be on the other side of the house, the AC barely makes a dent. Yesterday it hit 94 and the AC was on all day and couldn't get the room below 85 until 4 o'clock or so. Since the room is attached to the rest of the house, the main AC obviously has to work harder too. The only saving grace there is the thermostat for the rest of the house is upstairs; it means downstairs will always be warmer but at least we aren't going to freeze in the bedrooms.

I've done a little research but I'm a bit overwhelmed by the options at reducing the solar heat gain:

• install all new tinted windows ($$$)
• install tinted window film (less costly but worried about how it will look)
• install shades on the interior (worried about how it will look and confused about how much heat that will block as the heat has already come in through the window; or perhaps that understanding is wrong)
• install exterior shades

My current leaning is installing exterior motorized shades. This is also a pretty expensive option but I prefer this option as I want the solar heat gain in the winter. If I tint the windows (either through film or new windows), I will solve my problem in summer, but won't I make the room colder in the winter?

Answer 1

Keeping what is effectively a greenhouse cool in the summer is a heavy workload for any AC system. Your best bet is to prevent solar heat gain by blocking the light. Doing this with an opaque shade on the exterior will be the most effective option, though window tints or interior shades would also help.

Of course anything that reduces heating in the summer will also reduce it in winter. You may be able to exploit the fact that the sun is higher in the sky during summer than during winter, and create a shade that blocks more of the summer sun and less of the winter sun. This is often done architecturally by using long eaves over windows. But it's hard to retrofit a solution like that because it depends on the orientation of your sun room, the building around it, etc.

Because of all that, a retractable outside shade sounds like a great solution. If you're looking for a cheaper option than exterior motorized shades, you may consider a removable shade sail or an awning that you can wind away. The idea there is that you wouldn't be adjusting it every day, just seasonally and whenever there's a possibility of severe weather.

Answer 2

I think you're overestimating the energy benefits you can get from window films. Those are primarily indicated to reject ultraviolet radiation that can increase fading of indoor materials.

You're also mistaken if you think that during the winter the heat gain from sunshine on poorly insulated windows can make up for the heat that is radiated back out of the windows 24 hours a day!

What you really need to check are the thermal characteristics of the existing windows. Here is a thermal image taken during the winter of a house with double-glazed Anderson windows:

All of the windows except the one circled in green are over 20 years old. The thermal image shows that when it's freezing outside they're radiating a decent amount of heat back out.

So what's the deal with that one window that is as well insulated as the walls of the house? That was broken and replaced two years ago. It looks exactly the same as the other windows, but it has a modern seal and is filled with heat-rejecting gas. So it vastly reduces the amount of heat that radiates through the window.

If your windows might be under warranty, it could be worth taking a thermal image to see if any are leaking. Otherwise, replacing them with low-E windows will be quite expensive.

Exterior sunshades or shutters will, of course, prevent heat gain due to direct radiation. (Deciduous plantings can be just as effective and much cheaper.)

Finally, highly-reflective interior blinds can reduce radiation heat gain by 45%: Windows work in both directions, so a good amount of whatever thermal radiation gets through a window can be bounced back out. (However, if you have well-insulated windows, then thermal radiation may not be as large a source of heat gain as you think.)

Answer 3

The entire purpose of a sunroom is to get warmed up by the sun. It's doing the right thing. You're supposed to enjoy it when you feel like it and seal it off when you don't. It doesn't make much sense to either cool or heat a room that's thermally well connected to the environment via extensive glazing.

Your actual problem is that the sunroom cannot be sealed off from the rest of the house. The best solution is to add this ability so you'll never have to worry about warming up the house in the summer or leaking heat out in the winter.

For an interim solution, you can procure a light-colored, possibly reflective (eg metallized) tarp on the glass roof. It doesn't have to be motorized, as you'll need it manipulated it only twice a year. At worst, it'll give you the idea how the house looks shaded, you may or may not like the new feel. Sunrooms rarely look good without the sun.

To address your questions:

install all new tinted windows ($$$)

It will only marginally help. To effectively reject radiation, the glass would have to be reflective, like on a skyscraper, not merely tinted.

install tinted window film (less costly but worried about how it will look)

Same as above. If you can find film cheap enough, you can give it a try. There is a chance you'll be satisfied enough, and you retain the option to tear it away in the autumn.

install shades on the interior (worried about how it will look and confused about how much heat that will block as the heat has already come in through the window; or perhaps that understanding is wrong)

Your understanding is not wrong. If shades are black, they pretty much do nothing to cool the room, but lightly colored or metallized shades reflect most of both visible and infrared radiation, so they do cool somewhat.

install exterior shades

These are the best, as the heat is rejected before it ever reaches the house. However, they still need to be as reflective as possible, otherwise the sun will heat them up to a very high temperature and they'll re-emit bit of it from the back side, into your room. Now we get into language barrier, because there are 2 basic kinds, some that merely roll in or out as a continuous stream of tarp or interconnected metal strips ("shades"?) and the kind that consist of stacked pieces that can be tilted ("blinds"?) thus allowing for adjusting amount of light. Exterior blinds are pretty expensive, but interior ones are fairly cheap, easy to install on your own, and still decent enough if in light color. There are also "roman shades" which look flimsy and are easy to install, but I could not find any other than super-expensive.

You also have to consider the strength of your all-window structure, because it may be not strong enough to support weight of some heavier options or the wind loading of some lighter ones.

If I tint the windows (either through film or new windows), I will solve my problem in summer, but won't I make the room colder in the winter?

Definitely it will. If you get heat rejection high enough to make difference in the summer, it would also stop heating in the winter. The solarium will lose its purpose.

Answer 4

So here are some note that may help you.

By far, the most heat comes from light reflected from a surface. So stopping that light from hitting, say the floor, is you best option to cool that room. Shades will do very well. The light won't hit the larger surfaces and will reflect back out from the smaller surfaces of the shades. Yes the AC will still have to work to more then if there were no windows (the shade will heat up) but not as much as if every surface in the room was reflecting light (and heating up).

Remember light doesn't generate the heat, the reflected light does (well not technically, but it works very well for trying to figure out how to cool a room).

As for tinting, it works very well. If you block 70% of the light comming in you will be blocking 70% of the heat coming in. As far as looks, it will look fine as long as you stay away from mirror tints or funny colors. Go with black or grey and you should be fine.

As to "smoked" windows (windows that have the "tint" built in) these are nice but won't work any better then the tint film. You have to decide if it's worth it.

External shading is the best option. As the reflected light happens outside the A/C doesn't have to work to replace that heat. But this has other issues like storms and snow (depending on region).

You also need to consider how much cool air is leaving through the windows. Are they air tight? If not then you could be loosing more "cool" though the air flow then you are through radiation (light reflection).

Finally, you may wish to consider installing a block off vent in that room, shutting it up, and not using it during the hot days. You may need some help form an A/C guy to do this because some A/C systems can't handle a vent or vents being totally closed off very well. But it's usually a very cheap install.

NOTE The light reflecting doesn't cause heat either. It's actually UV/IR absorption and light absorption that causes heat. So the "sun stuff" being absorbed, causes more heat. "Sun stuff" being reflected actually causes no heat, but it's very hard to think about it that way. Much easier (but not accurate) to think about surfaces that reflect light.

Answer 5

An additional option is greenhouse shading paint. It is usually whitish in color and can be removed with pressure washing or scrubbing. This also can ensure that windows are clean during the part of the year you want to let the most light in. The compound could be applied primarily on the upper portion, and use shades or shade cloths as appropriate for walls if they receive less direct light. Shade cloths come in various shading percentages and colors and can fit many styles. Depending on your neighborhood and requirements, an exterior and automated solution may be the option that provides the best long-term value for the home with minimal maintenance.

Another shading compound option is a product like Varishade. It is whitish when dry and can be diluted to a desired shade level. But, in wet weather, becomes more transparent and lets additional light in. Since it was found with a quick search, I have no idea as to the effectiveness.

One possible advantage of interior shades is the heat they can retain in the winter. Have them open to let in heat and light during the day, then close them to trap heat in the home from escaping through the windows at night. It may be a good alternative to shading compounds, exterior shades, and fixed tinting films.

Answer 6

Professionally installed reflective films can look perfectly fine. You'd have to get up very close to the edges of the windows to see that it's a film. Different reflectivities exist (the link is UK-based, but the products are made by 3M so will be available in the US as well), and by paying more you can get better blocking of IR with affecting visible light too much (how much more I don't know, I've only come across it in work).

DIY-fitting film is possible but I don't recommend it.

It will reduce solar gain in the winter, but it will also reduce radiative heat loss. If you have cold clear nights you can lose a lot of heat this way. It is quite likely to save you money taking heating and AC into account.

I don't know what's available where you live, but custom-made retractable interior blinds are also an option here if you've got the roof height so they're clear of your head. They keep heat in or out, and in motorised variants with a timer can be set to maximise solar gain in the winter, than close at night. In summer the operation can be reversed.