Insulating Door Sills
The home I’m designing has continuous exterior foam insulation with additional Rockwool between the studs. I’m bumping my framing plan around to make room for Thermal Buck around the windows and doors and I just realized… Is there not a way to insulate the sill of the door frame?
It seems obvious to me that the door has to sit on something more structural than a foam ledge. And if I have the door flush with the framing, resting on the sill, the thickness of the exterior foam would create a “doorway” almost, which would only be as wide as the door which feels… weird and actually kind of dangerous in the case of outswing (I know most exterior doors are inswing).
The home is built on the slab, and the floor is the slab, so I’m likely not going to have thermal buck around the bottom. Does that mean my doors can’t be “outtie” doors?
My current wall design is 2×6 framing, 1/2″ sheathing, WRB, 2 layers of 0.5″ comfortboard, 1x furring strips, and then… probably fiber cement. I will be trying to hit 2″ of exterior insulation on the slab as well if my inspector will allow it. What if I have to have a gap on the slab for the termite gap? There would be no insulation under the door sill at all! I’d expect that spot of floor to stay cold and damp.
P.S. the search function of the detail library is real bad! You get hits for any one of your search terms; Adding more search terms gives you more hits. There doesn’t seem to be a good way to narrow the search or use “AND”.
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Replies
We have a similar wall assembly. In our case it's 2x6 framing with 4" of exterior rockwool.
I installed plywood window and door bucks rather than using the Thermal Bucks. We installed 'innie' windows and doors --- almost exactly in the middle of our wall assembly.
I also struggled to find details on how to insulate around doorways in thick walls without having a door supported entirely by a buck. I ended up using what amounts to an insulated header directly below the plywood door buck in order to give it support. I attached it with the same structural screws, FastenMaster Headloks, we used to attach our furring strips over the 4" of Rockwool.
Towards the end of my build I came across The New Net Zero by William Maclay. The book has a lot of interesting details for thicker, more energy efficient assemblies of all kinds. On page 343 there's a detail for how to insulate and support the bottom of a door buck with fiberglass angle.
If I had it to do over, I'd use the fiberglass angle. It offers rigid support for the door and anyone stepping on the threshold, while taking up very little physical space, allowing the maximum amount of insulation to be used in this area.
I have links and photos towards the end of this blog post:
https://kimchiandkraut.net/2018/11/12/passive-house-windows-and-doors/
Even so, I'd consult an engineer to ensure the right size angle is used to fully support the door over the long term --- including when movers are stepping over the threshold with heavy furniture :)
Tyler, I wrote about a few approaches I've used here: https://www.finehomebuilding.com/2019/09/10/doors-in-thick-walls. They aren't super detailed but maybe they will give you an idea.
Great Article,
Struggling to find the high density EPS. I don't suppose you could suggest a brand/seller?
Thanks
Tyler 2,
Why not cast in threshold extensions? Think brick ledge flush to floor height and of a beveled profile. If your floor is the slab you are already facing some amount of heat losses at the edge which, while maybe a bit greater at door openings is probably very little in the grand scheme. Very few doors offer high R values so keep in mind that you are already poking a low R hole in the wall.
Your wall schedule appears to be for a mild climate if the "two layers of 1/2" comfortboard" is not a typo. If instead you mean two 2" layers, then you will likely need to review attachment of the siding and trim choices as much as how to support the door. Doors can be ordered with jamb extension to accommodate thick walls. The in swing-out swing choice will determine how far the door will open without interference.
Framing the door opening with 2x8 or 2x10 (ripped if needed) will ensure solid material to anchor the door and trim if you set outward on the little concrete balcony I am suggesting. The foundation foam might need some fussy carving to remain flush with the finished siding plane, but nothing too onerous.
Having the door opening extend past your sheathing may help how you terminate the insulation and furring in a sold way. Again the heat loss increase for a 1 1/2 inch band around a door is not going to be much overall. You will want to be rigorous with taping the WRB.
If you can get the door protected by an entry porch roof you will be happier long term. My doors are a mix and the one set without the overhang protection is taking a beating. If you are planning a main/formal entrance door it is just nicer to have a place to stand out of the elements while being asked in.
Good morning - Tyler!
You can use ThermalBuck on all 4 sides of the rough opening and insulate the sill. It will raise the sill piece 1/2", because of the tongue that extends back over the sill 2.5". The rest of the width is filled in with scrap wood. Attached is our installation instructions for all 4 sides. Feel free to reach out with any questions - we'd be happy to discuss the specific installation with you. [email protected]
ThermalBuck BRINC BP: Can the sill piece of Thermalbuck be attached directly to the concrete? Since the concrete slab will be the floor, I'm concerned with the trip hazard presented by 1.5" material + 1/2" Thermal Buck + door sill. Is ThermalBuck suitable for sill support only up to 1" thickness foam? See attached image and confirm that this is what you're suggesting.
Roger Berry: Yes, 1" total foam. This is in addition to the insulation in the 2x6 studs. As this is my first build I didn't want to go too off the wall, pun maybe intended. My concern isn't so much energy loss, but... the door sill area being too cold in the winter time, and the formation of condensation.
Eric Whetzel: The use of fiberglass angles is very interesting and I like it. I'll talk to my structural engineer about this if the Thermalbuck side doesn't pan out.