Feedback on my ICF double stud wall detail
Last Post 23 Oct 2014 03:49 PM by tomk358. 31 Replies.
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tomk358User is Offline
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29 Sep 2014 07:00 PM
Hello, I'm building an 800sf 2 story house in wet Portland Oregon, Climate zone 4 I believe. After doing lots of research I've become enamored with the idea of a superinsulated building to keep energy costs down, comfort up, and use as little foam as I can to keep the environmental embodied energy down.

Here's what I've got so far for a wall system- one side of the building is 3' below grade, the other side is at grade.

Can anyone suggest improvements to this that won't raise the cost dramatically? Am I making any obvious mistakes with this design?

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LieblerUser is Offline
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29 Sep 2014 09:23 PM
The top of the concrete in the ICF is a HUGE thermal bridge! Consider a PWF insulated with mineral wool instead of ICF. It probably is more economical to build PWF walls from the ground to the first floor ceiling rather than stacking walls atop one another. With PWF you can have ZERO foam! The 4" of gravel needs to have minimal "fines" to be good drainage. Consider using a "filter fabric" under the perlite, loose perlite compacted perlite but no sand Why do you want r36 under the slab?
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30 Sep 2014 12:01 AM
You definitely don't want fiber cement board in contact with the ground as shown in the diagram above. The fibers in the board will turn to mush if kept continuously damp as would be the case if in contact with or buried in the soil. For the exposed foundation foam above and just below the soil you can apply a cement parge coating to protect the foam.

Also, I don't think that 2" of XPS between the ICF concrete and footing will fly from a structural standpoint - too much load concentrated in too small of an area.

2" of sand between the vapor barrier and the concrete slab? I thought the use of blotter sand was no longer recommended. Here is a link discussing the subject: http://www.greenbuildingadvisor.com/blogs/dept/musings/polyethylene-under-concrete-slabs
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30 Sep 2014 11:32 AM
@ Liebler: Yeah, the top of the ICF is definitely a weak point in the insulation strategy. I'm trying to work a way around that- maybe go larsen truss on the outside wall, adding OSB to the inside of the inside stud- which could also help w air seal if I wanted to do a utility chase between that and the drywall..

I hadn't heard about PWF, and while it definitely would have less embodied energy and petro-chemicals, and go up quick, I don't trust it for the long term where it rains so much here.

I forgot to add the details about the gravel, thanks for the reminder.

R36 underslab was based on another local passivehaus I came across. (https://sites.google.com/site/trekhauspdx/home/assembly-diagrams) It's still kinda arbitrary, and maybe just 1 layer of perilite bags at 6" for R18 would be more than adequate. I can't afford do a real passivehaus.

The compacted sand layer was from the PDF on the Perlite industry webpage. https://perlite.org/library-perlite-info/insulation-perlite/Perlite-underslab-insulation.pdf I'm not sure why they are doing that. I asked them on their facebook page, and will report back if I get an answer.

@arkie- I'll look into cement parge coating. I remember people were using something outside of the foam to protect it from fill dirt damage, but need to look up what it was, since it must not have been cement board.

I've also seen some examples where they said they used 2"XPS at the bottom of the ICF, and I thought the same thing, but threw it in here to see what folks thought. I'll have to do some math and figure how they were able to pull that off.

Thanks for the link about sand. Definitely won't be doing that.
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30 Sep 2014 01:17 PM
I lived in the Portland area for over 20 years. We all know that the "rain" in Oregon is a myth designed to keep Californians out! PWF is well proven and will outlive all of us and our children and grandchildren and then some! Regarding perlite: http://www.schundler.com/underfloor.htm   And     http://www.idosi.org/wasj/wasj25%2812%2913/19.pdf
Which shows that even slight compaction of perlite makes it stronger than foam.
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30 Sep 2014 02:07 PM
You'll also need a capillary break between the dense-packed cellulose and the concrete, as well as between the studwall bottom plates and the concrete. (EPDM sheeting spanning from interior foam to exterior foam on the foundation works.) With real capillary break (not just a foamy sill gasket) you don't need to use pressure treated bottom plates.

Using 4" of XPS isn't very green, given that it's blown with HFC-134a, which has a global warming potential ~1400x that of CO2. As it leaks out over the next 30-50 years doing it's climate-thing the R value slowly drops, eventually hitting the identical R-value of EPS of identical density. (By contrast, EPS performance is about the same at year 50 as it was on day 50.) From a lifecycle point of view it's better to use something that does less damage, since the performance will be the same in the end.

If you make the footing wide enough you can usually insulate UNDER the footing with Type-IX EPS (2lbs nominal density), and with (cheaper) Type-II EPS on the sides top of the footing to get that thermal break. As others have pointed out, there is no foam than can manage the weight of the foundation & house bearing on it at the width of the foundation wall itself. You will probably need an engineer to sign off on a sub-footing EPS solution though.
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30 Sep 2014 04:55 PM
@ Dana1- thank you. I'm changing the XPS to mineral wool, and added EPS type IX under the footing.

Here's the latest iteration (larsen trussified) including changes from you guys. Thanks so much for your help! javascript:amaf_insertHTML('');amaf_toggleInline(4902,28811,0);
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30 Sep 2014 07:19 PM
Tom,
Perlite left in bags will not result in a uniform solid surface, there will be gaps and voids where the bags abut each other this can compromise the structural integrity and insulating value of the perlite. My suggestion is leave small deliberate gaps then fill the gaps with loose perlite then run a plate compactor over the "composite" (mostly bags) surface. To prevent the loose perlite "infiltrating" the gravel drainage layer, I'd recommend a layer of "geotextile" filter fabric between the gravel and perlite.
tomk358User is Offline
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05 Oct 2014 04:14 PM
Here's the latest update to my evolving wall section design. I switched to a regular poured wall instead of ICF. Just seems to make more sense, and I'm figuring that renting forms will cost in the same range as buying the ICF blocks that I would then need to waterproof and protect from fill.

Other notes- air barrier on external side to avoid homeowner holes, and having to deal with air seal on electrical boxes and such. Also means less wood since I can do without the utility chase.

I know this kind of wall means cold and possibly wetter OSB on the outside, but have seen so many conflicting opinions on this system vs XPS/EPS on the outside making a vapor barrier, seems like there is no answer that everyone agrees with. There have been several passivehaus projects in my area that use OSB or ZIP on the outside, so I'm assuming if its working for them, it'll work for me..

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jonrUser is Offline
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05 Oct 2014 05:44 PM
I'd consider a frost protected shallow foundation of the unheated building/thickened edge slab variety. Then pour a wall on top of it.

An air barrier only on the outside means that in theory, air can enter the wall on the interior side, approach the exterior side, drop moisture, then exit back to the interior side. Both sides is best.

You are planning to use "mineral wool board" below grade?
tomk358User is Offline
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05 Oct 2014 10:02 PM
Thanks for your feedback Jonr.



My only problem with FPSF foundations is the fear that the perlite or the EPS would settle at different rates, causing the foundation to crack. If I were go go with EPS under the entire foundation, it would probably be safer in that regard... but I'm hoping to using perlite instead of foam where possible. Still thinking about it for sure




As far as air barriers, I didn't realize that could happen- maybe I will detail the inside shear wall of OSB to air seal, which would solve that problem with minimal issue.



I was planning on Roxul mineral wool board (or another brand?) for the below grade insulation- for several reasons. First, it's insect proof. Second it doesn't degrade in water, and third, it is less environmentally bad.
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07 Oct 2014 10:59 PM
Posted By tomk358 on 05 Oct 2014 10:02 PM
Thanks for your feedback Jonr.



My only problem with FPSF foundations is the fear that the perlite or the EPS would settle at different rates, causing the foundation to crack. If I were go go with EPS under the entire foundation, it would probably be safer in that regard... but I'm hoping to using perlite instead of foam where possible. Still thinking about it for sure




As far as air barriers, I didn't realize that could happen- maybe I will detail the inside shear wall of OSB to air seal, which would solve that problem with minimal issue.



I was planning on Roxul mineral wool board (or another brand?) for the below grade insulation- for several reasons. First, it's insect proof. Second it doesn't degrade in water, and third, it is less environmentally bad.


Roxol doesn't degrade in water, maybe, but how much insulation value will it have if it is saturated?
Will the structure integrity of the Roxul board stand up to wetting and the lateral load of the compacted soils?
tomfrischUser is Offline
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07 Oct 2014 11:40 PM
According to unverified internet posts, it seems only the outer 1/4" of Roxul drainboard gets saturated, so that would mean loosing about R1 from the stack. This was from here: http://thepathtosustainableliving.blogspot.com/2013/09/designing-walls-that-are-not-vapour.html



The structural integrity will NOT stand up to fill without a rigid board on the outside, I was planning on that 'asphaltic' board for that very purpose.

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08 Oct 2014 09:42 AM
If Roxul is free draining, it would seem to me that it would be impossible for only the first 1/4" to be saturated.

Some years ago there was a fair bit of Roxul Drain Board installed in southern Alberta. Now I doubt you can even find a distributor that carries it.

Apart from saturation, it does not seem to reflex from frost pressure, so every frost cycle crushes it a little more until it is to thin to be of any value.
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12 Oct 2014 02:28 AM
Can I ask a few questions?

I noticed that you don't have a radon system. Have you checked the radon levels on or near the site? Radon mitigation systems also have the welcome benefit of keeping the foundation dry. It might be smart to investigate it.

Also, if you're concerned about embodied energy, have you thought about just skipping the basement and doing a slab? It saves a lot of time, money and resources not having to dig and pour all that concrete. With insulation and air tightness levels like you're planning for, and for a tiny 800sf house, you could heat and cool that thing with a ductless heat pump and you could probably fit the water heater in a small closet.
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12 Oct 2014 08:38 PM
@ FBBP - I don't have enough information about Roxul Drainboard to defend it. I like that it's insect proof, and isn't styrofoam. I'd certainly need a rigid board to protect it below grade, but not sure if that would help with the frost pressure and lack of expansion after compression. I'll have to do more research. It certainly doesn't seem to be as readily available in the US as other Roxul products. I talked to the building inspector about it, and he said it's fine to use below grade as far as the city is concerned.



@ strategery
When I met with the building code people in my city they said I'll need a 3" pipe for radon remediation, and electrical installed to support an exhaust fan if the radon levels are higher than the limit. So I'll at least be putting in the pipe. As far as a basement, most of the building will be slab on grade, there is just one corner that has a higher grade, so it's really just a partial basement. (this is all assuming I can get permission to do this). I'm still debating the HVAC system, was thinking about a hydronic radiant ceiling for the first floor, and hydronic radiant floor for the 2nd, separated by insulation. Ductless mini-split might be another way to go, I'm just thinking it might be economical to use an on-demand water heater for DHW and a closed loop radiant system, if I'm going to DIY. But need to learn more before I could decide.
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13 Oct 2014 06:46 PM
Some thoughts...
I don't know if its done for the convenience of the drawing, but your outer drain tile should be against and slightly below the top of your footing with bleeders between inner and outer drain tile.

I've never used or seen EPDM sheet used as a sort of filter fabric, as drawn wouldn't it trap and direct moisture against your foundation?

The EPDM sheet as a cap. break under the plates seems like a likely air leakage point given the thickness of typical EPDM sheet. I'm sure you would still air seal the joint somewhere, but you would be missing the redundancy of a gasket, whether that is foam or rubber gasket.

I would also check with your building inspector if he will allow continuous dense pack cellulose as a fire block between floors since you have a balloon frame going on.

What is your exterior wall bearing on, if it's a larsen truss style, can it support the weight of the fiber cement.

Have you considered LP Smartside? Great product over fiber cement.

We put our radon pipe into the sealed sump pits, works great. Can leave it passive, if it tests high you can cut the fan in.

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13 Oct 2014 06:46 PM
Some thoughts...
I don't know if its done for the convenience of the drawing, but your outer drain tile should be against and slightly below the top of your footing with bleeders between inner and outer drain tile.

I've never used or seen EPDM sheet used as a sort of filter fabric, as drawn wouldn't it trap and direct moisture against your foundation?

The EPDM sheet as a cap. break under the plates seems like a likely air leakage point given the thickness of typical EPDM sheet. I'm sure you would still air seal the joint somewhere, but you would be missing the redundancy of a gasket, whether that is foam or rubber gasket.

I would also check with your building inspector if he will allow continuous dense pack cellulose as a fire block between floors since you have a balloon frame going on.

What is your exterior wall bearing on, if it's a larsen truss style, can it support the weight of the fiber cement.

Have you considered LP Smartside? Great product over fiber cement.

We put our radon pipe into the sealed sump pits, works great. Can leave it passive, if it tests high you can cut the fan in.

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13 Oct 2014 07:08 PM
greentree- thanks for the feedback. As I'm a DIYer, I don't have all the terminology correct, so please help me out.




For outer and inner drain tile- do you mean against the foundation wall? I thought the inner drain tile would be the green colored layer against the concrete labeled "waterproof membrane" I guess the outer would be the asphaltic board, though it doesn't need to be waterproof, just keeping the insulation from getting crushed. Maybe I need to find something else to use there so I don't trap moisture between those 2 layers?



The EPDM sheet below grade is for directing any rain away from the foundation and into the drain pipe, so it's not really meant as a filter, but more like underground eaves.



Can you explain what bleeders are?



As far as EPDM as a seal under the sill plate, I was told by another poster that it would take the place of a foam or rubber gasket as the EPDM is rubber. Are you saying that's not true? And if I also need to air seal this joint, how would I do that?



Exterior wall is larsen truss style, hanging off the interior wall w/ OSB and 2x2 gussets. I'll have to make sure it can take the weight, or look into alternatives. I'll check out the LP Smartside. The impact resistance vs fibercement video they have online is pretty neat, but I'm more concerned about moisture right now. Maybe that's ignorant of the issues that plague siding?



Now that I think about it, I can attest to the impact resistance of good old wood siding- once a stray bullet from a drive-by shooting grazed the siding, and you can hardly see the dent!



Finally, when you say sealed sump pits- I'm not totally clear on what you mean- in my case, I was going to bury the radon pipe in the gravel below the slab, but maybe that's not sufficient?
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14 Oct 2014 11:45 AM
The drain tile is the pipe you have drawn along the footing in the gravel. It collects and dumps subsurface water around your foundation either to grade if you had a sloped lot or into a sump pit. Those pipes need to be in rock alongside both sides of your footing. "bleeders" are pieces of pipe set into your footing before the footing pour which will create a free flowing channel that will allow water to go in either direction; either into your pit to relieve soil pressure against your foundation wall, or allow underslab water to drain to daylight. These are code requirements for my code, I would imagine IRC would be similar.

An EPDM gasket would (should) air seal your joint because it gives you the extra thickness in material so it will compress even in a foundation wall dip, I'm questioning how EPDM sheet you would use for say roofing would give you a decent gasket because in the real world you often need that extra thickness for compression against dips and humps in the foundation wall. Find a material that will give you a cap. break and compression to air seal above and below all in one. Most of the issues tend to be around pre-embedded anchor bolts and the lack of finishing poured wall guys do to get it truly flat. In your case where your foundation wall won't be available very long to air seal since it is part of a finished wall you need to get that area air tight right away.

LP is an engineered product, not really comparable to wood siding since it is pressed dense with resins, I thought the stuff would be garbage when I first saw it, so I took some pieces years ago and stuck them in a bucket of water. The result was impressive enough, I wouldn't hesitate to use it and its workability is miles ahead of fiber cement.

The sump pit is where your drain tile connects and dumps water and where a sump pump would be located. A sealed pit just means it has a bolted gasketed lid, the sump pump pipe and radon pipe go through the lid with gaskets. If you have drains to daylight you don't need one, I always put one in as a redundancy even if we are draining to daylight.
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