This window installation method can be used to cut out and install windows in RV walls, Trailers, Vans, Teardrops, and more.  Though this example is geared toward a cargo trailer.

We used the type of window that has an interior trim ring. The interior panel trim ring comes with pre-drilled screw holes.   We made the decision to not cut out any existing factory vertical ribs and deteriorate the structural integrity of the wall frame.

Though we have seen builds that have welded in the necessary supports to install larger windows.   Some DIYers adding welded in horizontal metal supports, to stabilize the vertical ribs.  This is the only way I would do it myself.

We have even see installs that DIYer have cut a section of the verticals out and supported the opening with wood or a plywood backing.  I am sure others have tried bolting in metal supports rather than wood?

In our experience we would never recommend the last method.  (Wood or Bolting for a horizontal).  Skipping welded in horizontal supports.  In our opinion, to do so…. would degrade the structural strength of the wall both vertically and horizontally.

The outer frame of the window assembly we use.  Has a metal frame that contains the glass, and has an extruded slot near the  perimeter (the interior side) of the aluminum window  frame.  The interior rings come in various depths depending on your wall thickness.  Be sure to include the total thickness of the core, the outer skin, and the finished interior paneling. Order your ring accordingly.

Let me be first to say that this method is by no means the only way to accomplish a window installation. This is just one of many.  We decided to post this method because we happened to document some of our steps with photos.

This is a guide only. Be sure to read through all of this before starting. Identify and compensate for any differences and material. Have all material on hand before starting. It is also advisable to have a way to keep the cutout area out of the weather in case you encounter a problem.

Our first attempt on a single window took us 6 hours from start to finish. We did 5 total. After the first, with the learning curve out of the way, we were able to do them in 2 hours each.

Simply put, a portion of the outer frame with the glass, is inserted into the cutout opening, from the exterior. While the exterior window frame is held in place, the interior trim ring is placed on the inside.

The supplied screws are passed through the interior trim ring holes and threading into the exterior window frame’s extruded slot. The screws are then tightened to pull the frame up tight against the exterior skin and a rubber seal that is provided.  The interior trim ring is pulled toward the wall.  The exterior frame and the interior frame performing a clamping action with the wall in between the two.  If done properly no other sealant is necessary.

Window Parts used
14″x21 Trailer Windows (Vertical Sliders with screens) @ $70.95 each
2 1/4 1 7/16 Trim Rings (for 1 1/2 wall thickness which includes the interior finished paneling and the exterior metal skin) @$11.00 each
Shipping $45.45

Purchased from:
Vintage Technologies
Galesburg, MI 49053

Vintage Technologies Website (They Sell Teardrop Trailer Parts) We found them to be very accommodating and helpful during our selection and ordering process. Give them your wall thickness when ordering so they can send you the correct ring.

You must first determine your wall thickness. This includes the outer skin as well as the interior finished surface. When ordering your window the wall thickness determines which interior ring must be ordered.

The interior rings are sold in 1/16 inch depth increments for the various thicknesses encountered. You don’t need to know the ring size just your overall wall thickness. The unit should come with the exterior Frame with the Glass installed in it (Also make sure you get a screen if it is a type that opens), the interior finish trim ring, screws, and an outer rubber seal should all be included.

General knowledge of construction and the safe use of hand and power tools are required. Eye protection a must. For this article it is assumed you have selected a structurally sound location that is free of metal ribs, plumbing and electrical wiring.

If the interior panel or finished wall can be dismantled and removed, this will provide you with the best installation. It will also lessen your chance of error or cutting into something you may not have known was in the wall. Make no holes until you are absolutely sure you have the room to install the window. Any insulation in this area must be removed, then re-positioned during the installation.

From our photos you may pick up that we cut and removed a rectangular section of the factory interior plywood panel. If that is possible it sure makes things much easier. In our case that plywood in the immediate area around the window would eventually become our visible finished wall right at the window.. We didn’t want saw cuts to show so we went a little wide [See Finished window photos at the end].

In our final interior finish we furred out from the plywood 1 1/2 inches and installed 1 1/2 rigid insulating foam board between the furring strips. Interior side of the furring strips topped that with 1/8 Laun Plywood. The rigid foam directly against the Luan provided very good backing.  SEE INSULATING.

This installation provided an air gap between the factory metal exterior skin and the factory interior plywood. Then the 1 1/2 foam insulation. This proved to be a superior method of insulation. I have tried other methods in the past, included spun fiberglass insulation.  All other methods had condensation issues.

If this is your first Cargo Conversion you need to seriously consider condensation and insulation. I can’t say my current method is the right way to do it. I can say I have done it other ways…..which lead to building this latest CTC [Cargo Trailer Conversion].  My suggestion is to have an air gap between the exterior skin, and your internal insulation.  After trying several insulating materials on other rigs,  I believe rigid foam to be the best over many others.

We are discussing an installation where the interior finished panel can be removed. Be aware if the interior panel cannot be removed, it creates another set of problems concerning alignment you have to deal with. If you are drilling through a 1 ½ wall for instance, the drill angle becomes critical. It is easy to be a ½ to ¾ of an inch off, between the outer skin and the interior panel. Your interior finish ring will not cover and hide such an error. We are not discussing methods for installations where you cannot remove the interior panel.

Tools for the opening cutout depend on the material both inside and out. We use a grinder and a Dremel Saw Max fitted with a thin abrasive cutting blade on the outer metal skin.

We use a jig saw on the interior wood panels. (We use down plunge cutting blades in the jig saw for cutting the wood panels). If your outer skin is some type of steel that will eventually rust, we suggest cleaning up the rough cut edges, masking off, and applying primer and paint to seal the raw metal. Let that completely dry before proceeding. (Mask so new paint does not show after window frame is installed)

Drilling exterior skin reference pilot holes from the inside
We mark the outer skin from the inside first. Create 2 paper templates of your exact cutout size. Working from the inside, position one of your patterns and tape it in place. Trace around it with a fine tip “Sharpie” If need be, drill two 1/8 holes through the skin (somewhere in the center of your eventual cutout.

These 2 holes can be used as a more finite measuring point if tolerances to metal ribs and such are critical. Use these reference holes to line up your pattern on the inside with a outside visual reference line. We have encountered ribs that we actually had to be exact with no fudge factor. Be sure you have positioned it plumb with something you can measure off of.

Levels and plumb-bobs are not useful unless you have completely leveled your vehicle…which is nearly impossible to do for this purpose. Many aren’t built square or plumb to begin with. Use an existing exterior door frame, siding seam, or gutter edge, as your line of sight reference on the outside. Measure off that and double check everything.

Ultimately you want it to visually look good when complete. Double and triple check, to make sure you are satisfied everything is lined up where you want it.   Above all, take your time!

Once you are sure your interior template is aligned and taped to the inside surface of the exterior skin, drill 4 to 8, 1/8 inch holes. Most RV windows are not square, most use radius corners. Start drilling from the inside 1/8 inch holes just inside the traced straight template line. We use a total of 8 pilot holes. One at each end of the straight line making up the 2 sides, the top, and the bottom….and just before the radius curve starts. This will line up your top, bottom, and both sides. The radius is drawn in while tracing the template.

These 8 holes become your reference points to tape your temple to the outside and mark your cutout line. In most cases you will want to make your cuts from the outside.

Notice the row of exterior skin screws in the photo to the left. These indicate the structural ribs of the trailer the exterior skin is attached to. We did not want to cut these ribs and create more work in terms of welding an internal frame to maintain the structural frame strength.

We chose to use two small windows side by side to straddle the rib. In our case the ribs were steel metal “Hat Channel” or also know as “Furring Channel”

First cutout complete. Ready for second
Once the exterior skin is cutout, the interior panel is temporarily positioned back in final location. This next step is a little tricky since you are working with the hollow wall thickness. It is difficult to accurately trace the outer skin cutout onto the backside of the interior panel just using a pencil through the hole. Being off a ¼ inch here or there will result in a gap showing on your finished work.

Cut a small block of wood the thickness of the gap between the exterior skin and the interior paneling. Also right down this exact gap thickness since you will need it in a later step. Measure in several locations and use the smallest dimension you measure. Use this wood block held between the exterior skin and inner panel to guide a pencil tip around, the edge held perpendicular to the exterior skin cutout, tracing the outline of the hole onto the backside of the interior panel. Measure to double check those lines with the interior ring dimensions. You don’t want an error that can’t be covered with the interior finished trim.

Once again remove your interior panel, now mark for the cut-out. Using your saved template align it over the tracing that is on the backside surface of the interior panel. The tracing should not fall outside the template. If any line does fall outside the tracing, align and tape the temple over the tracing. Center the best you can, then re-mark with a “Sharpie”. Using a 1/16 bit drill pilot holes through from the backside of the paneling, just inside the perimeter of the cutout.

Use the same technique used on the exterior skin to transfer the cut out to the inside of the panel by using the holes and template. Once the holes are drilled through the panel, position your template on the inside surface using the pilot holes as reference. Use a fine point “Sharpie” to trace the outline on the inside surface of the interior panel once you are satisfied with the positioning.

We use a sabre/jig saw fitted with downward cutting tooth blade. Use as Fine tooth as possible. We cut from the inside surface so any splintering happens on the back, Taping the opposite cut side reduces splintering too. Make this cut on the inside of your tracing. Double check your measurements before making any cuts. You can always sand or trim more if the fit it too tight. The inner trim ring doesn’t allow much fudge factor so don’t cut on the wrong side of your line or the hole will end up being too big.

Creating an inner wall wood support frame
Once your interior panel cutout is complete, re-insulate it if needed.  Position the interior panel in place and temporarily attach it once again. For a really good installation you now have to create a wood inner frame just inside the cutout,  that is actually inside the wall, the thickness of the gap. We use hardwood verses pine.  Poplar, Maple or Oak is normally available.

You need stock that is slightly thicker than your gap. This might require gluing and clamping to create what you need. And at least 1 inch the other way. We try to make the final wood a square dimension so you don’t have to worry which way it is turned during installation. In our case the gap was just over 1 inch so we had to glue up 2 pieces of 3/4 inch stock ahead of time. We rip the over sized stock down on a table saw to the 1+x1+ dimension we needed.

Cut the length of your top and bottom header pieces first. Make them at least 2 1/4 inches wider than the actual hole cutout to extend into the wall, and the additional width of both your vertical wood support pieces. Start with the bottom header piece first. Screw a couple screws part way into the side of the piece you will be inserting. The screws give you something to hold on to and not let it drop down in the wall.

Dry fit first to make sure your cut dimension is correct. Snug is good. Up to a 1/16 gap between the panel or skin is OK. This gap might vary slightly anyway. Once you are satisfied with the dry fit, glue the surface that faces the interior panel, slip your hardwood piece into the gap, holding onto the screws. Slip the wood in so it is slightly inside the cutout edge by about 1/32 of an inch. (You don’t want to have to trim these four pieces during on the final window installation)

Once the wood is positioned, clamp through the cutout opening to hold it in place, then tack with a brad or pin-nailer gun through the interior panel. Once it is positioned and secured, remove the clamp and the 2 screws that were used as holding handles.

Repeat this process for the upper wood header gap piece. Be sure it is also long enough to extend past the width of the wooden side pieces that will be inserted. Once this piece is inserted, glued, and secured with brads, remove the tempy screw holding handles.

Next cut and dry fit the side pieces. They should be snug to the top and bottom pieces already in place.  On these two vertical hardwood supports, glue the surface facing the interior panel as well as both ends. Again setting inside the cutout about 1/32 of an inch. Insert, tap in place, and secure with brads. Remove the tempy screw handles.

Clean up any excess glue and let the glue dry. This creates an interior wooden frame around the entire window cutout. It provides a solid wall at the points the interior and exterior window frame clamp together holding everything together with a waterproof seal.

Dry fit the exterior window frame as well as the interior trim ring. Make any final adjustments or trimming of any wood, exterior skin, or interior panel. File off any sharp points. Gently hammer out any bends or puckers around the cut line. Touch up any raw metal with paint if you care too.

You should be ready to permanently install the window.
The window installation is a two person job. Don’t attempt it without help. A dropped window is a costly error not to mention the delay it will cost you waiting on a replacement.

Install the provided rubber seal on the back of the window frame/glass unit per the manufacturers directions. It usually has a matched sized channel extruded in the back side of the window frame. The seal fits snug in the channel to hold it in place.

Trim off the starting end of the length of seal, so it is a clean squared off end. Don’t stretch it during the process of fitting it. You don’t want it to creep or contract over time. Start installing the strip in the center of what is the bottom of the frame. Work it around the entire frame back to the starting point at the center of the bottom.

Remember, do not stretch the seal during this installation. You do not want it to relax and creep back after you make your final cutoff.

Trim off excess seal once you are satisfied with a clean squared off cut with about 1/4 inch excess. Work this in so there is no gap showing at that bottom seam. DON”T HAVE THAT SEAL SEAM END UP ON THE TOP!

With the rubber seal in place start the window installation. One person holds and presses this window/exterior frame and rubber gasket up against the exterior skin. The other person works from the interior placing the finish trim ring in place.

Most screws supplied are Phillips head. It takes a proper fitting Phillips in good condition to drive these screws. An option is to purchase same length and diameter stainless steel 1/4 inch hex head screws, then turn them in with a 1/4 inch nut driver. But the hex heads do have a tendency to twist off so gently tighten and even reverse a 1/2 turn once in awhile to help create the needed threads. Lubing the screw threads assists in making this easier.

Start each screw through the pre-drilled holes of the interior trim ring and into the extruded slot of the exterior frame. Lubricant on the screw threads makes this process go easier. Soap works in a pinch. This is the point the person on the exterior needs to apply lots of pressure to the frame…But do not push on the glass and break it. The second person works around the frame on the interior. Starting all screws until they are grabbing well in the slot of the exterior section of the window frame

The slot in the exterior portion is not threaded and it requires lots of pressure on your tool handle when starting the screws. This is all self threading. At the same time the outside person needs to be pushing in to back up the pressure from your tool. Just watch you don’t slip off the screw with your driver, and punch through the window and into your helper on the outside. [The other reason I prefer hex head and a nut driver.]

Once all the screws are started circle around the window tightening each screw 5 or 6 times, moving to the next screw, without completely tightening any at this point. Check positioning of the trim ring, window frame, as well as the gasket before completely tightening.

Continue around the interior frame tightening each screw a few more turns. The frame should be drawing up uniformly tight and compressing the rubber seal. Circle again tightening each screw a turn or two. Repeat as required until the exterior frame is tight against the skin creating a waterproof seal. No caulking or sealant is required. You may have to pull one of these out someday to replace a broken or damaged unit.

Once the window is completely installed, we do mask off the window frame and exterior skin to create a ¼ caulk line all around where the frame meets the exterior skin. Then caulk using clear exterior silicone to finish off the exterior with a professional looking final touch. But this additional step is not required to make it water tight.