Short answer: Most boat portlights, hatch lenses, and cabin windows are cast acrylic, usually gray or bronze tinted. Cabin windows are often 3/16 to 1/4 in. thick and hatch lenses about 3/8 to 1/2 in. Most RV side windows are tempered glass, but many RV vents, skylights, and some lightweight windows are acrylic. To replace one, trace the old window, have new acrylic cut from the template, and bed it with a sealant its maker approves for acrylic, such as Sikaflex-295 UV with its primer, or a neutral-cure silicone. Drill any screw holes oversize so the window can expand.
Key facts
- Material: cast acrylic is the usual choice for portlights, hatch lenses, and cabin windows. Polycarbonate is used where impact matters more than clarity.
- Sealants made for it: Sika makes Sikaflex-295 UV specifically for bonding acrylic and polycarbonate windows and hatches, used with Sika's cleaner and primer. Sikaflex-296 is for real glass.
- Sealants to avoid: BoatLIFE says its polysulfide sealants shouldn't be used on plastics and its polyurethane sealants shouldn't be used to bond plastics. Perspex says never to use plasticized PVC gasket strips, which cause stress crazing.
- Salt water: doesn't attack acrylic. 3A Composites lists Perspex as sea-water resistant, and Acrylite says its Premium FF sheet withstands saltwater spray.
- Expansion: a 24 in. window grows about 0.1 in. over a 100 °F swing, so screw holes must be oversize.
What are boat and RV windows made of?
On most boats, portlights, hatch lenses, and many cabin windows are cast acrylic, which stays clear in sun and salt and polishes easily. Windshields on fast boats are often tempered glass or polycarbonate.
RVs are different. A major RV window maker, Lippert, describes its side windows as tempered, tinted glass in aluminum frames, and an RV trade magazine reports that glass is still the main material in US RVs. Acrylic shows up in roof vent lids, skylight domes, and some premium lightweight windows made of thermoformed acrylic. Not sure? Tempered glass usually has a small etched stamp in a corner.
What thickness and color should you use?
| Window | Common thickness |
|---|---|
| Small RV and camper windows, vent covers | 1/8 to 3/16 in. |
| Opening portlight lenses | Often about 1/4 in. (6 mm) |
| Boat cabin and side windows | 3/16 to 1/4 in., more for large windows |
| Deck hatch lenses (may be stepped on) | Often 3/8 to 1/2 in. (10 to 12 mm); match the hatch maker's original |
These are common sizes, not rules. Measure the old window with calipers and replace it with the same thickness, or the next size up only if the frame allows. See actual thickness of acrylic sheet. For bigger windows, Acrylite's glazing guide sizes thickness by area. At a 30 to 40 lb/ft² design load (roughly 100 to 120 mph wind), it calls for about 3 mm up to 5 sq ft, 4.5 mm from 6 to 15 sq ft, and 6 mm from 15 to 32 sq ft.
Tint: gray and bronze cut glare and heat in a cabin, and we offer both in 1/8, 1/4, and 3/8 in. Tint strength varies a lot. Acrylite's bronze and gray range runs from about 11% to 48% light transmission depending on the shade, so new panels may not match old faded ones. Replace windows in pairs or sets for an even look.
How do you make a template from your old window?
The old window is usually the best template, even if it's cracked.
- Remove the old window carefully, keeping all the pieces. Note which side faced out.
- Trace it onto stiff paper, poster board, or thin plywood with a sharp pencil held vertical. If the window is broken, tape the pieces together on the paper first.
- Mark every hole by pushing a pencil or punch through each one, and measure hole diameters.
- Add key measurements: overall width and height, corner radii, and one diagonal, so we can check the scale.
- If there's no old window, template the opening or frame instead and decide how much overlap you need for bedding (often 1/2 to 1 in. on surface-mounted windows).
Then upload a photo or scan of the template lying flat with a ruler or tape measure in view, plus your measurements. A DXF or SVG file is even better. We'll draw it up and confirm the shape before cutting.
Which sealant is safe for acrylic boat windows?
This is where many replacements go wrong. The wrong sealant can craze new acrylic within weeks, starting at holes and cut edges (what is crazing in acrylic).
- Use a product whose maker approves it for acrylic. Sikaflex-295 UV is a one-part polyurethane that Sika formulated for bonding acrylic and polycarbonate windows and hatches. Follow Sika's pretreatment chart: the black version is used with Sika's cleaner and primer. Neutral-cure silicones such as DOWSIL 795 are another common choice for bedding; Perspex specifies neutral-cure, acrylic-compatible silicone, so skip vinegar-smelling acetoxy silicone.
- Don't assume a marine sealant is plastic-safe. BoatLIFE, which makes Life-Calk polysulfide, says polysulfide sealants shouldn't be used on plastics and polyurethanes shouldn't be used to bond plastics. A general-purpose marine polyurethane on a portlight risks crazing and makes the window very hard to remove later. Read the label and the data sheet, not the aisle sign.
- Gaskets and tapes: Acrylite lists EPDM, Santoprene, and closed-cell neoprene gaskets and vulcanized butyl tape as acceptable glazing materials. It warns that plasticizers in some gaskets can attack acrylic, and Perspex says never to use plasticized PVC strips.
- Prep without solvents that attack it. Clean the frame of old sealant, and wipe the acrylic only with mild soap and water, or with what the sealant maker specifies. Acrylite uses a kerosene-dampened cloth to wipe soft sealant off its sheet. Never use acetone, lacquer thinner, or MEK on the acrylic.
How big should screw holes be?
Acrylic expands 3 to 8 times as much as glass or metal, Acrylite says, and boat and RV windows see big temperature swings in direct sun. A 24 in. window grows about 0.1 in. over 100 °F. Acrylite's rule for bolt holes is a hole at least twice the bolt diameter, centered at least 1.5 hole diameters from the edge, with a rubber grommet or soft washer. Tighten to snug, not hard; overtightening is the classic cause of cracks radiating from screw holes. Acrylite also says panels 4 × 4 ft or larger should go in frames rather than be bolted through. We can drill and countersink holes as part of the cut, or see how to drill acrylic without cracking.
What edge finish and curves work?
Where the edge sits in sealant or a gasket, a machined edge holds less stress than a laser edge and is less likely to craze. For a visible edge on a surface-mounted window, flame-polished looks glossy. Curved windows can be flexed into a frame cold if the radius is at least about 330 times the thickness (Acrylite, Plexiglas), or about 82 in. for 1/4 in. sheet. Tighter curves need heat forming. We do straight-line bends up to 1/4 in. (acrylic bending).
How do you clean acrylic boat windows?
Rinse salt off with fresh water first, then wash with mild soap and a soft cloth. Plexus, developed for aircraft windshields and suggested by its maker for boat hatches, leaves a thin dust-resisting wax layer; Novus 1 is another common choice. For light scratches, see how to remove scratches from acrylic.
Acrylic or polycarbonate?
Acrylic stays clearer, resists scratching and UV better, and polishes easily. Polycarbonate is far tougher but scratches more easily and needs a UV-protective surface. Use acrylic for most cabin and side windows, and consider polycarbonate where a wave, fender, or branch might hit (acrylic vs polycarbonate). Vehicle and vessel glazing can be regulated, so check the applicable rules and the boat or RV maker's specifications.
See acrylic replacement windows for more, or start with your template in the instant quote.
Questions
Can crazed boat windows be polished clear?
Surface haze can, but crazing goes into the plastic and won't polish out. Most crazed boat windows need replacing.
Does saltwater harm acrylic?
No. Sheet makers rate acrylic as sea-water resistant. Rinse salt off with fresh water before wiping so the crystals don't scratch it.
Can I use 3M 5200 on acrylic windows?
It's not the right product. 5200 is a permanent polyurethane adhesive-sealant, and BoatLIFE, for one, advises against polyurethane sealants for bonding plastics. A permanent bond also makes the window nearly impossible to replace later. Use a product its maker approves for acrylic, such as Sikaflex-295 UV with its primer, or an acrylic-compatible neutral-cure silicone.
Can I replace a glass RV window with acrylic?
Often, for side windows and vents, since acrylic is half the weight of glass and much harder to break. It scratches more easily than glass and expands more, so leave clearance in the frame. Check the RV maker's guidance for windows near exits or ones that serve as emergency egress.