I was informed by the soils engineer and the structural engineer that I could not use the thickened slab edge with continuous rigid insulation on my project. The structural engineer wants a 36" deep perimeter stem wall which would preclude installation of continuous insulation wrapping the foundation. Has anyone doing passivhaus design run into this problem and, if so, how have you solved the problem?
What does your soils investigation report say about the strata immediately below the slab in question (not the footing)? If it is organic soil or less firm soil, then perhaps the problem is that there is too much load being superimposed on the slab... This may either be due to a number of reasons, including, but not limited to:
1) There may be a bearing wall that rests on the slab and not on the perimeter walls.
2) Differential settlment in the top 36" layer of soil may result in cracking of the slab and ultimatley flooding and other undesireable effects.
3) The dead or live loads planned for the slab are too great to produce an economical design. The structural engineer probably thinks that instead of pouring one monolithic slab, it may be more economical to pour footings on the perimeter 36" below grade and have all loads transfer to them...
Again, There are many factors to consider, but the above are some preliminary assumptions about without knowing more about the project...
The issue is the clay in the soil causing uplift in the slab when it becomes wet (could be as much as 1/2"). Normally in Santa Fe we only have to go down 24". I would like to totally wrap the slab and perimeter footing in insulation but this seems impossible with a 12" thick x 36" deep stem wall.
I should also add the structural engineer wants an 8" thick slab. This is a 2600 sf, one story, double exterior wood frame, residential building. It's hard to argue with a structural engineer. They've got the numbers!
Why can't you just apply the insulation to the exterior of the stem wall? it is only 12" deeper than a thickened slab edge?
Argue with your structural engineer...or just get somebody that is less conservative...you can probably save like 25-50% on raw materials in the selection of a less CYA, chicken-shit engineer.
In a passivhaus foundation design the insulation runs continuously on the exterior and underneath the thickened slab at the perimeter as well as under the floor slab. Of course I will extend the insulation 36" down along the exterior and interior of the stem wall. I'm wondering if it would be possible to place the insulation at the bottom of the 12" x 36" stem wall. I'll have to talk to the engineer about this. Seems doable as long as the insulation holds up to the compression at the bottom of the stem wall. Structural engineers have had lots of problems in the Santa Fe area due to foundation slabs lifting due to the expansion of clay soils. So in all fairness, I don't think it reasonable to refer to them as "chicken-shit".
Post tension slab is too expensive. Structural engineer suggested that until I asked about the cost. Helical foundation piles will not provide the "beam" strength needed to span soils below. Thanks to all for your interest in this technical question & thanks to Archinect for this forum.
chigurh, I don't know if the OP will respond, but the key is this is a Passivhaus project, so insulation is very different from typical US construction.
I looked that up...and it just had a note "super insulation via continuous thermal envelope"
I get it...but regardless of the marketed name of the construction/design type, in this case passivhaus, there might be some retarded suggested detailing associated with that system.
In addition, since the OPs engineering/soils report is not even going to allow for the system to be built as suggested, he is going to have to deviate from the plan...Either way, as an architect, I look at that system and it makes no sense whatsoever...Heat loss under a 36" deep embedded footing? Just use some common sense.
The temp 36" below grade is likely equal to the yearly average temp...65 degrees or so. Why would you need to insulate 36" below grade? In Santa Fe why are you so worried about heat loss? Its a pretty mild-cool climate.
Donna, I can't tell if you're kidding or not. Let's not be too politically correct here. Next thing we know they are going to take the term "Ritardando" or "Ritard" off of sheet music because it's too offensive. It means slow down. It's not like it's always been an insult. I agree Miles, this sounds retarded. It's California, coastal California.
We've decided to go with an 8" thick slab with a poured 12" x 30" stem wall. The stem wall will have 2" x 30" rigid insulation and under the slab will be 2" of rigid insulation. No need to insulate under the stem wall. I started this discussion because I wanted to hear feedback from "passivhaus" builders. Not all passivhaus details work with the soil condition I have. However, since no passivhaus builders have responded this issue is settled. Time to move on.
Not joking at all, CD.Arch. "Retarded" as a pejorative is completely, offensively unacceptable in the social circles in which I move, which includes parents of children with developmental disabilities.
jcrarch - Why the 8" slab - does the engineer think the extra weight will keep it from heaving? And why not let the wall go and isolate the slab? The detail you are using allows conduction between the wall and the slab.
Passivhaus Foundation Design
I was informed by the soils engineer and the structural engineer that I could not use the thickened slab edge with continuous rigid insulation on my project. The structural engineer wants a 36" deep perimeter stem wall which would preclude installation of continuous insulation wrapping the foundation. Has anyone doing passivhaus design run into this problem and, if so, how have you solved the problem?
Thank you
So lets think through this logically...
What does your soils investigation report say about the strata immediately below the slab in question (not the footing)? If it is organic soil or less firm soil, then perhaps the problem is that there is too much load being superimposed on the slab... This may either be due to a number of reasons, including, but not limited to:
1) There may be a bearing wall that rests on the slab and not on the perimeter walls.
2) Differential settlment in the top 36" layer of soil may result in cracking of the slab and ultimatley flooding and other undesireable effects.
3) The dead or live loads planned for the slab are too great to produce an economical design. The structural engineer probably thinks that instead of pouring one monolithic slab, it may be more economical to pour footings on the perimeter 36" below grade and have all loads transfer to them...
Again, There are many factors to consider, but the above are some preliminary assumptions about without knowing more about the project...
The issue is the clay in the soil causing uplift in the slab when it becomes wet (could be as much as 1/2"). Normally in Santa Fe we only have to go down 24". I would like to totally wrap the slab and perimeter footing in insulation but this seems impossible with a 12" thick x 36" deep stem wall.
I should also add the structural engineer wants an 8" thick slab. This is a 2600 sf, one story, double exterior wood frame, residential building. It's hard to argue with a structural engineer. They've got the numbers!
Why can't you just apply the insulation to the exterior of the stem wall? it is only 12" deeper than a thickened slab edge?
Argue with your structural engineer...or just get somebody that is less conservative...you can probably save like 25-50% on raw materials in the selection of a less CYA, chicken-shit engineer.
In a passivhaus foundation design the insulation runs continuously on the exterior and underneath the thickened slab at the perimeter as well as under the floor slab. Of course I will extend the insulation 36" down along the exterior and interior of the stem wall. I'm wondering if it would be possible to place the insulation at the bottom of the 12" x 36" stem wall. I'll have to talk to the engineer about this. Seems doable as long as the insulation holds up to the compression at the bottom of the stem wall. Structural engineers have had lots of problems in the Santa Fe area due to foundation slabs lifting due to the expansion of clay soils. So in all fairness, I don't think it reasonable to refer to them as "chicken-shit".
any thoughts for a post tension slab?
Or how about helical foundation piles Then you can do what ever you want with the insulation.
Post tension slab is too expensive. Structural engineer suggested that until I asked about the cost. Helical foundation piles will not provide the "beam" strength needed to span soils below. Thanks to all for your interest in this technical question & thanks to Archinect for this forum.
let me get this right...you want to put insulation under a footing?
why? even if you could get insulation with a super high compressive strength, what is the benefit?
I'm wondering if it would be possible to place the insulation at the bottom of the 12" x 36" stem wall.
Yes, it's possible. LOL
Helical foundation piles will not provide the "beam" strength needed to span soils below
add more piles
but it can't be that easy or you all would of thought of that.
what part of sata fe is this in i am down here in burque
chigurh, I don't know if the OP will respond, but the key is this is a Passivhaus project, so insulation is very different from typical US construction.
I looked that up...and it just had a note "super insulation via continuous thermal envelope"
I get it...but regardless of the marketed name of the construction/design type, in this case passivhaus, there might be some retarded suggested detailing associated with that system.
In addition, since the OPs engineering/soils report is not even going to allow for the system to be built as suggested, he is going to have to deviate from the plan...Either way, as an architect, I look at that system and it makes no sense whatsoever...Heat loss under a 36" deep embedded footing? Just use some common sense.
^ Retarded is exactly the right word.
A cost-benefit analysis would be amusing but isn't worth the time it would take to perform.
The temp 36" below grade is likely equal to the yearly average temp...65 degrees or so. Why would you need to insulate 36" below grade? In Santa Fe why are you so worried about heat loss? Its a pretty mild-cool climate.
You guys, please stop using "retarded" as you are using it. As I know you both know, it's not acceptable to use that word in that way these days.
We've decided to go with an 8" thick slab with a poured 12" x 30" stem wall. The stem wall will have 2" x 30" rigid insulation and under the slab will be 2" of rigid insulation. No need to insulate under the stem wall. I started this discussion because I wanted to hear feedback from "passivhaus" builders. Not all passivhaus details work with the soil condition I have. However, since no passivhaus builders have responded this issue is settled. Time to move on.
Not joking at all, CD.Arch. "Retarded" as a pejorative is completely, offensively unacceptable in the social circles in which I move, which includes parents of children with developmental disabilities.
jcrarch - Why the 8" slab - does the engineer think the extra weight will keep it from heaving? And why not let the wall go and isolate the slab? The detail you are using allows conduction between the wall and the slab.
Don't forget the 12 mil plastic for a radon barrier, Santa Fe is a zone 1.
Here is some information for slabs.
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