Opinion on a wall assembly
Salibonz
| Posted in General Questions on
I have a question regarding a wall assembly. I am in climate zone 6a. My intended wall assembly from outside in is vinyl siding, furring for air path, 3 inches of gps @ r14.1 , zip sheathing sealed and taped, 2×4 framing with r15 eco batt, 1 ich r5 gps, proclima intello plus vapor barrier,2×3 service cavity on horizontal and drywall/paint. All this on a frost protected shallow foundation as a unheated structure. I’m looking for any input and advice. I appreciate all input, thank you.
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Replies
I’m curious, what you mean about an unheated structure?
Regarding your wall assembly, do you have sheathing or wind bracing against the exterior of your 2x4 wall?
Is your exterior GPS foil faced with seams taped to act as WRB/air control? You. Could probably eliminate the interior layer of 1” GPS as you have an effective R-value of 26 without the interior Ci which easily exceeds code levels in CZ6. Intello is an excellent material and a 2.5” service cavity sets up your wall for future alterations and maintains integrity of your interior air/vapour control material.
Sommerbros, in the "Builder's guide to frost protected shallow foundations" it specifies 2 ways of designing the foundation. As either heated or unheated structure. In a heated structure foundation is effective by maintaining a consistent 64 degree interior temp to keep frost at bay, and unheated is to add specified underslab insulated to deal with frost. As far as sheathing I'm using 7/16 zip taped seams as wrb. Exterior gps is an unfaced and vapor open affordable brand called half back. The 1 inch is up for debate as to whether to install it or not( that is 1 of the inputs I'm looking for, so I appreciate your advice, thank you). The purpose of the intello is to maintain vapor control. With all this assembly I'm trying to deal with the control layers effectively and still be affordable. I was able to find a decent price on 3" gps and also offset the cost by using 2×4 vs 2×6 framing. I'm thinking ci is better investment than deeper wall cavity. I appreciate your help!
As air displaces the blowing agent in the cell walls, GPX's performance will decrease, likely to around R-4.3/in, just like XPS.
What is the interior GPX for? Seems like a complication to me, when 2x6 walls with R-12+ exterior insulation will perform pretty much the same.
Service cavities can be helpful but in most cases the majority of the space will be unused. I think it's more valuable as insulation space. If the sheathing is detailed as a really good air control layer, you don't need to be obsessive about making the interior airtight as well.
I recommend buying and reading ASCE 32-01. I have it but it's not sharable. It includes various "in-between" options so you may not need to go with the fully unheated option. In an unheated space, the interior will be the same temperature as the exterior. With the Passive-House-like details you are proposing, as long as the windows are high-performance and well placed, the interior should never freeze.
Michael, I briefly read asce 32-01. Correct me if I'm wrong if the interior temperature is maintained between 41-63 degrees it is considered semi heated? And with that said, if I increase the depth 8 inches( 14" + 8" total 22" depth) and otherwise follow as a heated building design( air freezing index below 2000 and a mean temperature of 50 degrees) I would not need sub slab insulation? And wing insulation? Just vertical at r 5.6? If that is the case that is a huge help and thank you!
I don't have time to dig into it right now but that sounds about right. Be aware that there are tradeoffs, however. I would not forego sub-slab insulation in CZ6. Also be aware that the air freezing index zone can be unintuitive; it should be the coldest that your zone gets.
Ok Michael, thank you for the information. I will continue to read on it and I will take head on your input about sub slab insulation. Green building advisor has been a great source of information and it's a great source to seek advice. Thank you
That is a ton of insulation, money and materials that more or less does nothing when the building is unconditioned. Wanton waste is the most ungreen thing one can do.
Without a heat source the inside temp will equal the 24-hour average temp regardless of the wall R value.
Walta
Walta,
The house will be heated, it’s just that the FPSF is designed as though the house may sometimes not be heated, as insurance against frost heave.
Yes, as Malcolm states. A frost protected shallow foundation can be done without sub slab insulation but it requires a constant 64 degree temp. The other option is to design it as unheated space even though it will be heated as Malcolm stated as an insurance. The purpose for the service cavity is to maintain my vapor barrier from anybpenetrations. Wouldn't that be beneficial for not such a high cost? Its an easier vapor sealing detail to get right. I suppose I can eliminate the interior gps insulation.( not really necessary, just want to also build slightly above code minimums). I do plan on shuco triple glazed windows. So I guess i am essentially at a decent design? My goal is diligent air and vapor control and reduced thermal bridging. That is why I opted for unfaced gps, good r value and vapor permeable (also happens to be cheaper). Thanks again, to everyone, for all the helpful insights.
Salibonz,
Do draw out your details for the interior horizontal service cavity prior to framing. You will need to change the standard ones for inside corners and interior wall intersections to provide backing.
I'm trying to understand, are you saying that when I do horizontal service cavity framing I will need to add an extra corner vertical so I can attach the horizontal? Is that what you are saying?
Thank you Michael, I will check that out. I have been reviewing the builder's guide to frost protected shallow foundations that you recommended. Very helpful but I will check this out. If I am overdoing it , and could simplify it I will definitely go that route. I am placing shuco triple glazed windows but the house is south facing and unfortunately (kind of stock plans) most of my windows are north facing. Thank you for your help.