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DIY Tips For How To Build A Sunroom

By on Jan 18, 2016
DIY Tips For How To Build A Sunroom

Open Porches

Open porches are unquestionably a blessing, especially in warm weather. They provide some extra living space in the open air that just can’t be duplicated by decks or patios. They protect you from the sun and rain and if there’s a better place to hang a hammock, it’s hard to imagine. But when the weather turns cold an open porch loses nearly all its appeal. It becomes dormant real estate that doesn’t get much use until things warm up again. In some areas of the country this can take nearly six months, which is a long time for something that cost so much to be out of commission.

For die-hard porch enthusiasts, enclosing the space or a screened in porch may not be attractive. But for the rest of us, the tradeoff can be compelling, especially if you have a large porch that would add substantially to the year-round square footage of your house. And, if you enclose the space with large windows, like we did, you can end up with a dramatic sun room that has a special appeal of its own.

Because so much of the structure is already in place, enclosing a porch is a pretty straightforward project. All that’s really required are installing some windows, adding some insulation and providing some extra heat. Unfortunately, porches come in all shapes and sizes and each is bound to have its own idiosyncrasies that can complicate the job. But the methods we show here can be applied to most designs.


In many ways, enclosing a porch like this means nothing more than installing some windows, and it pays to use good ones. The tilt-wash feature makes cleaning the windows from inside much easier, and the high-end glazing improves the windows’ energy efficiency, which is crucial since the finished room will have so much exposed glass. Because we wanted the enclosed porch to have a period feel, we also opted for full divided light grilles, in this case one vertical divider per sash. This system features an exterior piece bonded to the glass and an interior one that snaps over the glass. We also specified extension jambs for the windows to fit our 2 x 6 walls and a factory-installed white painted finish on the inside with matching white hardware. The windows sell for about $325 each. The divided light option is about $200, and the other options cost about $75.

Installing these windows was a breeze. Preparing the structure to receive them was where all the work came into play. Your goal is two-fold: create square, level and plumb openings to receive the windows, and attach these openings to the existing porch structure in a stable weather-tight way.

Wall Framing

Begin by removing any porch railings and by cutting off any overhanging porch floorboards flush with the trim boards underneath. Then locate the positions of the knee wall soleplates that go between all the posts. Because we wanted the walls to be flush on the inside of the room, we lined up the inside edge of these plates with the inside surface of the posts. Cut a 2 x 6 to length for each opening and nail it in place.

Next, build a false post on both ends of the porch next to the house. To do this, just mark a plumb line on the siding from the soleplate to the porch headers. Cut a 2 x 6 to length and lag bolt it to the siding alongside the plumb line. The width of these false posts is based on the finished openings you need to accommodate your windows. Nail additional 2 x 6s to the first one until you achieve the correct opening.

When the false post framing is complete, cut an exterior trim board to size and tack nail it to the post. Scribe the profile of the siding onto this trim board and cut the notches with a sabre saw. Test fit the trim board, and then caulk the board-to-siding joint and nail the trim board in place.

Next, nail the end knee wall studs to the posts and the sill to these end studs. Typically these studs will be different lengths because most porches slope away from the house to allow water runoff. Just make sure that the sill board is absolutely level. Then fill in underneath with the rest of the studs spaced apart 16 in. on center.

Nail a 2 x 4 to the underside of the porch header so its bottom surface clears the bottom edge of the header trim. Then fabricate the jack posts that fall between the windows, as shown on the drawing, and toenail them in place. Once your wall framing is complete, nail sheathing on the outside. We used 3/4-in. marine plywood for this because it functions as the flat panel in our exterior trim scheme and would be exposed to the weather. Once the sheathing is on, install the windows. First, lift a window into the opening and slide shims under both lower corners. Then center the window from side to side in the opening. Level and plumb the window from the outside and, when you’re satisfied with the location, nail one top corner of the window flange to the wall. Recheck level and plumb, then nail the rest of the flange in place.

It’s very important from the standpoint of finished appearance to keep the windows lined up between all of the posts. Once you install the first window in each bay, use it as a reference point to start leveling the next window in place.

Exterior Trim

Start the exterior trim by nailing a trim board over the ends of the existing floorboards. Then nail the shim alongside the house as shown in the drawing. This shim holds the drip cap at a 5 angle to let water run off. Next, cut each drip cap to size and shape and test for fit. When you are satisfied, caulk all the surfaces that meet the drip cap with silicone caulk. Or, use the product we did, called Marine Adhesive Sealant 5200. It’s a polyurethane-based caulk made by 3M (3M Center, St. Paul, MN 55144) that bonds and seals above and below water. We used it to seal all the exterior joints on this job. It worked great and even took paint well. Once you’ve applied the caulk, nail each drip cap into place. Continue installing the remaining trim boards using the drawing as a guide. Make sure to caulk the entire perimeter of the back of each board before nailing it in place with galvanized finishing nails.

Interior Finish

Begin your interior work by running the wiring for the wall receptacles according to code. Now is the time to deal with your heat. You may be able to warm your new space using your existing central heating system. But we opted for adding auxiliary heat in the form of a direct vent gas stove. We chose the Pinnacle Model No. PDV20, with a sand-colored enamel finish, made by Vermont Castings (1000 E. Market St., Huntington, IN 46750). This unit is rated at 20,000 BTUs and can be vented directly through the outside wall with no vertical flue required. By adding an optional thermostat, the stove works on demand, just like a central furnace. This well-made unit can be installed by a homeowner. But the gas hookup should be left to a licensed plumber. This stove and vent system sells for about $1,500.

If you chose a gas stove, now is the time to frame in the rough opening and install the vent sleeve. Next, proceed with the insulation. The side walls are a breeze. Just cut and fit fiberglass batts between the studs and staple them in place. Then cover the batts with a polyethylene vapor barrier.

The floor and the ceiling insulation are another matter. Because our porch was elevated, we could easily add polystyrene insulation between the floor joists below. (We chose polystyrene because of its high R-value per inch of thickness.) But for those who have under 2 ft. of clearance between the joists and the ground, it’s probably a better idea to hire an insulation contractor. Insulating our ceiling was also relatively easy because we had installed a fold-down access door in the ceiling when the porch was originally built. If you don’t have access to the space above your ceiling, either install a fold-down door or, again, hire an insulation contractor.

Once the insulating is done, cut and nail the wainscoting boards in place and finish up the interior trim as shown in the drawing. Install the horizontal boards first, followed by the vertical boards. When the trim is installed, sand it with 180-grit sandpaper, then prime and paint inside and out. Finish up by installing a stove vent cap on the outside of the wall.

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