Exterior Door Construction Alternatives

Craftsmen discuss various ways of building wood exterior doors, including a foam-core panel method with interior and exterior wood elements isolated from each other. June 29, 2008

We're thinking of pressing veneer over an Extira [waterproof MDF] core for two fixed panels in a 2" white oak entry door. The panels would be glued in as opposed to floating. Alternatively, I'd consider a foam core, but have never made one. We're concerned about the weight of this very large door. Does anyone have any opinions?

Forum Responses
(Architectural Woodworking Forum)
From contributor J:
Sounds like that should work fine. I've used 3/4" pink eps foam sandwiched in between two panels with good results. How wide is the door? Did you do stave core or are the parts solid white oak? If you use good ball bearing hinges and your joinery is sized appropriately the weight shouldn't be a problem.

From the original questioner:
The door is 2" x 40" x 84". Do you glue your face panels to the foam? If so, what glue do you use?

From contributor C:
What about using hollow core for the substrate? It would be a lot lighter than MDF, I would just be curious about its moisture resistance. The "Resin Impregnated Core" at the link might be ok by itself; or you could edge it with the Exitra, that might solve that issue. I've never used it, but would be interested to hear how it works or from someone who knows it doesn't and why.

From contributor T:
Do it as Contributor J suggests. Make your panels of solid wood though and run the pink board through the widebelt. Make sure you remove the plastic film first to calibrate it to exact thickness. This allows you to get a nice tight fit in door. Do not glue panels into doors! They have to float. Leave space all around panel for expansion. Space Balls work real well. I always make exterior doors 2-1/4 inch though. I’ve never had a problem with stave core, solid wood panels, and foam thermal break between 3/4" panels.

From contributor T:
As for gluing foam to panels - no need. Just hold the three pieces together as you rotate them into the rail and stile groove, just good and snug. Don't hammer them in! Don't glue them either. Believe me when the door starts to expand and contract the glue will tear the sticking right off the rails and stiles!

From contributor D:
One can do panels either of two very basic ways:
1. solid, allowed to move in width;
2. stable, built up with cross-grained veneers or solids, or with man-made boards to eliminate movement.

Traditionally, panels are solid and left to move. Nowadays, some of the best builders of doors are using a stable core with a mitered perimeter of solid wood (see AWI specs). Of course, the stable panels can be glued in place if one likes, with no ill effects. Each method has it advantages and disadvantages, limitations and freedoms. How much solid wood panels will move is the tough question. Do you figure interior RH, exterior RH or both?

From contributor T:
That's the beauty of double panels in exterior doors. Outside panel can move independent of interior surface especially when the thermal break is in there in the form of say 1/2" blue board (or pink). I wouldn't trust "man-made" panels in an exterior application as it's properties are not compatible with solid wood. Some above posts relate to customer appeal.

Solid wood door makes a better impression on a customer who is asking for that "custom" door for their home. Otherwise they would have settled for a Home Depot or Lowe’s door. What is the questioner’s customer going to think when they find out he uses particle board in his door? MDF engineered in that customers mind is just particle board unless clearly defined and even then you have suddenly put a hurdle between that customer and ordering the door.

I always had a cross section (a 45 degree cutoff corner) of my door construction for them to take apart, feel, see the parts before and after construction and when they saw deep mortise, tenon, and all solid wood. Banding a raised panel made of MDF is ok for kitchen cab's but not an exterior door.

From contributor D:
I typically let the design of the door(s) drive the construction. While I'm no fan of engineered stiles and rails, I utilize every sound method I can to make the most appropriate panels. While back to back is sound and useful, there are also other ways to fill the hole.

Bookmatched veneers, very wide panels, inlaid panels, and two sided doors lend themselves to using stable panels. More often than not, these are still solid wood, 5-7 layers of solid and veneers to make them dimensionally stable. We don't typically glue them in place in the frame as a directive, but it does happen, and is fine.

Some of my customers are 2,000 miles away, so explaining the cross sections is typically not necessary or advantageous. 25 or 30 cross sections, plus drawings like crazy, are available if needed, but it is more important that they trust us with their work, and give us the freedom to make the best product we can. I don't tell Honda how to make their disc brakes, and I don't need to know anything about it to appreciate the fact that they work.