Window Foil

I am curious about the effectiveness of putting foil over windows to try and keep the boat cooler in the summer. Specifically putting it on the inside of the window. I see it a lot and i wonder if it really does anything more than just a shade.

Obviously, the concept is to reflect radiant energy, but with the reflective surface on the inside of the boat, it seems to me that the heat energy just stays in the boat... bouncing around the foil and the glass window. It seems ti me that it almost makes it worse. Am I missing something? Would just a shade be better?

I always put it on the outside under the "see thru" window covers. I ordered the bubble foil from Amazon and cut it to the size of the window. On my front windshield, like yours, I cut one piece to cover all three windows.

Can be very effective if you use an insulating foam layer that holds the Mylar side right up against the inside of the glass, so no or very little air gap to heat up.

Some thin flexible steel strips on the inside and strong magnets on the outside, or snaps, or maybe velcro.

It's a challenge to make it look real nice though, maybe behind curtains.

Aluminum “bubble” insulation,about 1/4” thick from Lowe’s, or Amazon works well in summer Florida heat, Outside under the mesh sunshades works best. Inside is easier, both make the inside dark like a cave.

Anytime you can keep the heat out is better than trying to get rid of the heat after it is inside. I vote for putting the insulation on the outside of the glass.

Outside the glass might be slightly better but its not always practical. Honestly this is exactly like the countless people that swear by the effectiveness of those solar shades you place inside your car windshield; in my experience they make a huge difference even when inside the glass as long as they have a reflective surface to them. I've used them with success on many vehicles when living in Texas.
They work by reflecting a large percentage of the heat back, plus blocking the solar energy from being absorbed in everything inside the cabin.
For the boat at least as a temporary solution, we found huge rolls of that foam backed foil at an RV store very inexpensive, and cut it to size our helm windows for when the boat is stored for long periods.

I second the idea of putting the window cover on the outside. We had a cover made from Stamoid material that goes on the outside of all three windshields and snaps on the top, bottom and sides. It keeps the interior very cool even when the desert sun is beating down on the windshield.

Reflective window film ???
Seen and heard reports of great success on a sailing cat with it on

On the previous boat I had V-kook applied to the inside windows in the salon area, made quite a difference.

We have the roll of bubble wrap foil from Lowe’s. We used it on our last boat on the inside as well. We already have Phifertex window shades for the outside. I guess my question is a matter of physics. Does putting the foil on the inside really help beyond being shade?

I could never understand putting the foil inside the class as it allows the sun and UV to enter past the glass and remain inside. Block it on the outside or against the glass as a film works and I can understand the purpose.

Just My SSO

I had mylar roll-up shades made for all the windows in my wheelhouse on my last boat. They fit tight in between the window frames and close to the glass. My reasoning was not for heat reduction as that is of minor concern in the PNW but for glare reduction. I found that glare, even on overcast days was making me drowsy. Trying to stay at the wheel from sunup til sundown, on a commercial tuna troller, day in and day out was taking its toll. The shades made a huge difference for me and also for the screens on the electronics. Not having to wear polarized sunglasses all the time (and the blank screens that they caused) was a bonus. Another big bonus was reduced sun bleaching of the interior woodwork. I would have those shades down most of the day, except in heavy overcast. Especially helpful when the sun was below the brow.

"Sun?" What is this "sun" thing you're all talking about?

This is what I was talking about with reflective foil.
Actual film stuck to the glass like car window tint but reflective.

We had a local make up cloth window coverings , with lift a dot snaps.

The layout is such that they can be installed either inside or outside.

Squareish windows makes it possible.

If solar heat is a hassle , they go outside, some privacy in a raft up or at a dock, inside works.

Out of season, inside helps interior finishes and fabric last longer..

I had the 'heat' problem with my Nordhavn. I had the 'darkening, see though film put on the saloon windows. Reduced the heat and provided privacy during the day. At night with the lights on in the saloon, no privacy.....
Same stuff they use on cars. I used to remember the degree difference, with and without the film but.... that went away like the boat.
On this American Tug, I had port light covers made out of cloth for privacy. Works pretty good for a heat barrier too. For the saloon, I have a double shade system. A gaze pull down shade and then a privacy shade to pull down. I used to remember the brand. They aint cheap, 6 units of various sizes about $1500.

Tom.B said:

We have the roll of bubble wrap foil from Lowe’s. We used it on our last boat on the inside as well. We already have Phifertex window shades for the outside. I guess my question is a matter of physics. Does putting the foil on the inside really help beyond being shade?

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I think the answer is Yes. Heat occurs when the light strikes something and that something absorbs the energy and heats up. The reflective film reflects the light back out the way it came in with minimal absorption. With minimal absorption of light, there is minimal heating.

Yes absolutely inside help, less air gap the better.

But outside is better if your setup allows

I had the glass covered with 3M film. It's a special (and expensive) film that passes 95% of the visible light but blocks or reflects heat. It can even be used on the front windows because it doesn't block vision.

I would think foil or any solid covering on the windows would make for a very dark and unpleasant atmosphere.

twistedtree said:

I think the answer is Yes. Heat occurs when the light strikes something and that something absorbs the energy and heats up. The reflective film reflects the light back out the way it came in with minimal absorption. With minimal absorption of light, there is minimal heating.

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To understand this google the greenhouse effect in cars.

Basically it occurs when short wavelength visible light goes through a transparent surface the objects inside heat up and create longer infra red wavelengths that cannot easily pass back through the transparent surface.

So, two things: 1) put the reflective foil on the outside and 2) don't leave your dog inside a car with the windows rolled up.

See... Now that is what I was thinking too. I will maybe try putting the foil wrap right on the window from the inside and see if it changes. It isn't very practical to put it on the outside. I guess I will need to come up with another plan.

Where I am talking about is a window behind the fridge. In the summer the sun comes in and really heats up the space and, I assume, is causing it to not run and cool as it can.

I guess I will just need to come up with a better solution. I can easily make a solid cover out of Sunbrella, but just cutting out the foil seemed even easier. Primarily because pulling the fridge is a bit of a chore. Doing it twice to patter and construct is even MORE of a chore

Thank y'all.

aboatman said:

I had the glass covered with 3M film. It's a special (and expensive) film that passes 95% of the visible light but blocks or reflects heat. It can even be used on the front windows because it doesn't block vision.

I would think foil or any solid covering on the windows would make for a very dark and unpleasant atmosphere.

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I also used the 3M product I believe it is called crystalline
Much clearer than the original tint, but blocks much more heat. Just because tint is dark doesn’t mean it will keep out heat

"blocks or reflects heat."

Reflecting heat is a great idea

blocking heat just warms up the window , which easily passes the heat inboard.