Breezeway Home Foundation Design
Zone 6, wet area. We are working with a local contractor and have finalized our home main floor layout and are now in the process of approving the foundation design. What complicates our build is that it is a breezeway home similar to this link… https://designwithfrank.com/products/2-bedroom-dogtrot-house
The current plan is a shared 4’ frost wall with dividing walls where the patio begins for each side of the home (I created a quick sketch to help explain what I mean by this). The main home side of the home would be a finished mini basement that is conditioned. All utilities would be in the basement of that side. On the smaller unit side, the foundation would be filled and it would be a slab on grade conditioned by a mini split unit.
Our builder is thinking it may be best to apply a paintable moisture barrier to the exterior of the entire foundation perimeter, then wrap it with XPS insulation, then drainage mat. We’d repeat this on the inner walls of the patio area that connect to the interior spaces. Something I wonder about is the central patio area as there would be an area we could not wrap at the four patio corners connecting to the interior sections. Would this lead to issues with thermal breaks? Also, what would be the best way to hide the exterior foam layer for the above ground portion of the patio? We would appreciate any thoughts or advice on what may be best in this situation.
Also, I’m wondering if it would be best to run the foundation plans by someone in the building science realm. Maybe an architect or engineer? Does anyone have a suggestion for this as well?
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Replies
Emil,
Unless I was in a high seismic zone I would not join the two foundations, and would treat them as separate structures. This would simplify drainage, damp-proofing and insulation, as well as use less concrete.
I would build it as designed but place the foundation insulation on the interior. That avoids the need for thermal breaks or for the need to use XPS which is has the highest GWP of the rigid foams. Thermax polyiso can remain exposed on the interior of the mini-basement and you can use EPS or GPS inside the raised slab foundation.
Concrete is an ideal exterior material. There is virtually no benefit from the often-touted "thermal mass" of having the concrete on the inside of the insulation layer.
For waterproofing, the code-minimum approach is to use an asphalt emulsion damp-proofing compound on the exterior, up to grade. I like to upgrade that to a heavy-duty elastomeric compound and/or add a dimple membrane for drainage. By code you usually need a footing drain that extends to open air, though it's not required for a raised slab as you describe. I prefer to upgrade the drain to a perforated PVC drain which is less likely to crush over time, a common problem with corrugated drains.
Michael,
Your comment got me wondering...
What are the advantages of exterior insulation? I can't come up with any compelling ones.
Malcolm, good question. Exterior insulation is good for thickened-edge slabs and required for frost-protected slab foundations. Unless interior basement space is extremely limited, which has me considering it for an addition I'm designing now, I can't think of a good reason to use it other than "that's the way we always do it."
Also, I hadn't seen your comment when I posted mine, so my first sentence wasn't a passive-aggressive response to you. Separating the foundations makes sense to me as well, though it leaves fewer options for the breezeway flooring or the potential to enclose it in the future, as I bet will happen eventually.
Michael,
When we all give the same answer to a question, it will probably be time to relinquish the function to AI.
That's a very good point about future-proofing it by extending the foundation through the breezeway - although I think houses with that feature should come with covenants prohibiting filling them in as I adore breezeways. Many of my favorite house have them.
Lake Flato: https://www.lakeflato.com/ranches/cross-timbers-ranch
And this one that is ingeniously linked below grade: https://www.basisdesignbuild.com/projects#/project-22-1/
I agree on all counts and those are beautiful examples! I have just had uncountable experiences with clients and potential client who see any volume with a roof over it as potentially conditionable space. Neary every renovation client with a garage wants to use its attic space for living, though it's rarely feasible; many want to use their damp, dark basements with low headroom as living space, and some want to enclose covered porches, if they aren't already covered, which is very common around here. I think it goes along with the fact that most clients don't have much imagination, which is why they hire me, but they can imagine a space that is already defined as being finishable.
Additionally, I'm sure it's different in more mild climates, but when it's cold and windy, sitting in a breezeway with accelerated wind and no direct sunlight is not particularly pleasant.
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I agree the insulation on the inside would be a good way to easily avoid the thermal breaks. And I love the breezeway examples included in the responses.
Our contractor was hoping to install insulation externally due to the way they are recessing the trusses in a reverse brick ledge (similar to the photo attached) for the mini basement side. They were hoping this would give a consistent insulation around the perimeter without having to navigate spray foaming or insulating around all the trusses internally. The problem is, if we go with the external insulation, what would be the best way to handle the insulation around the breezeway area? Maybe we insulate the entire exterior foundation perimeter, then also the inner walls of the breezeway area and under the breezeway slab? I think to our current home built in 1960, which has no foundation insulation internally or externally and we’ve never had a condensation issue on the inside of the basement concrete walls. I’m not saying that best practice, but it leads to some confusion in knowing what would be best practice in this situation.
Emil,
You have a good example of where exterior insulation may make sense.
Three suggestions:
- Omit the stem-walls in the breezeway, which also gives you flexibility in what materials you can use there as a surface, and how far you may want to extend them out each way as part of the landscaping.
- Pour those stem-walls with a gap for the foam. They can be tied to the other walls with re-bar.
- Increase the width of the interior ledge the trusses sit on so it can accommodate foam between the trusses and stem-wall.
I think I’m follow you. Are you saying to pick 1 of those three options to do? For example, eliminated the stem walls completely leaving only the footings shared, or use rebar connected stem walls in the breezeway that are spaced apart for insulation, or space the trusses away so we can easily insulate behind them. It sounds like any of those options alone might solve the problem, correct?
Emil,
Yes. Sorry for not being clearer.
It sounds like the builder has a plan to collect the water the question is where do you put the water once you have collected it?
Do you have someplace on your property that is several feet lower than your basement floor will be to drain this water too?
I want to live high on the hill not down by the creek. It is all to easy to buy that low spot in the corn field where it is to wet for the corn to grow sometimes.
As always, I am ready to build the ark.
Walta
Yes, haha. Belt and suspenders approach is always what I would rather do than take chances with water. Because we are in a wet area, we are digging down only minimally (about 1-2 feet), and then building up. We have plenty of dirt from an irrigation pond that services our farm and that dirt will be used for grading around the exterior of the home essentially creating a small hill. Even without the man made hill, from the house location down to the pond there is a 7-8ft grade over a 700 foot distance. The pond itself then overflows down another 8 feet from there into a low lying area that flows out eventually off property toward the river. We were told that should make it easy to drain everything to daylight from the drain tile.