REMOTE with SIPs??
Hi GBA,
Apologies if this has been asked before, but as far as I caould see it had not.
I am planning a Passivhaus self-build in a cold, wet, maritime type climate (Scottish Island), probably an equivalent ASHRAE climate zone 5C. It is a windy wet area, so dry building season is short, and I am planning to do much of the work myself (I am an HVAC engineer, limited building experience). So with this building experience in mind I am trying to use “dumb” technologies where I can, a balance of cost and build speed.
I was initially attracted by SIPs for it’s fast erection time, and buildability. I can get it made-up to my specs, delivered to site, and have it up fairly quickly. I really like Jo Lstiburek’s perfect wall, and CCHRC’s whole REMOTE wall assembly ideas. However I would like to know if you could marry the two technologies, SIPs and REMOTE together??
An inner 1/3 SIPs super-structure, wrapped in a breathable membrane, and then a complete rockwool exterior insulation to make up the R-value to Passiv standards. My WUFI calcs suggest this assembly would work well. The vapour drive is nearly always outwards here, and with the driving rain as well I really want an assembly that can breathe, and dry itself.
The added advice from Ecohome.net (https://www.youtube.com/watch?v=Z-yasiKVRms&ab_channel=TheEcoHomeNetwork%3AGreenBuildingResources) for reducing thermal bridging from insulation fixings by using a split set of furring strips looked sensible. My biggest issue with this proposed assembly is attaching all this rockwool (~6″), and wood siding, back to 15mm (about 9/16″) OSB SIP sheating. My alternative idea might be to use Larsen trusses outside the SIPs, thereby transferring loads a bit better, but it’s just more cost right.
I would love to hear anyones advice, experience, or alternatives for this wall assembly.
Thank you for your time, and any replies would be greatly appreciated.
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Replies
Supermull,
I don't know enough about either system to be of much use, but I suspect that the main problem is the lack of thickness of the SIPs exterior skin to provide attaching for the long screws that will be in shear, and I d0n't know how you would overcome that.
Here is link showing attachment of furring through exterior insulation (see page 10 and on):
https://www.victoria.ca/assets/Departments/Planning~Development/Permits~Inspections/Example~Plans/Illustrated-Guide-R22-Effective-Walls-In-Wood-Frame-Construction.pdf
Good luck with your build.
Thanks Malcolm,
Agreed, the fixings to the OSB seem to be the biggest obstacle I am encountering. The mineral slab insulation would have some support from the slab foundation. I may have do do some rough structural calcs to examine this issue :S Thanks for the document link that looks useful, and I will have a good read through it.
I would flip the wall around. Go for the structural SIP for the exterior, once up frame an interior stud wall with mineral wool batts.
Even adding in the cost of the extra lumber, this would be significantly cheaper than rigid mineral wool. It would let you run all your services inside this inner wall intsead of having to deal with wiring through the SIP and you would not have to deal with 10" screws.
If I understand your stackup, you have 3" SIP + 6" of rigid mineral wool, you are looking at around an R45 (yankee units) assembly. In zone 5, that need around R14 exterior insulation for condensation control, so your 3" SIP is more than enough for the job.
Akos,
That's a great idea.
Thanks Akos,
That is a good idea, however my issue with this is that it would seem to me the same as if a vapour retarder (SIPs with ~1+3/16" OSB and 3" EPS is fairly vapour impermeable) were to be applied to the exterior of the thermal layer. Then suffering condensation problems on the SIPs OSB during the heating season. In your case I would do away with the SIPs altogether and just bulk up the mineral wool (or similar breathable insulation), then apply a ply sheathing to the outside. Essentially a deep stud or double stud wall assembly? I feel I may be erring towards this solution given the highlighted issues of my original proposal.
Thanks again for your suggestion, much appreciated.
Supermull, if the condensing surface--the first fairly impermeable surface water vapor "sees" on its way from the interior to the exterior--stays above the dewpoint temperature, the water vapor won't condense and you won't have problems. In climate zone 5 you only need about 30% of the total wall R-value to be on the exterior to keep the condensing surface warm for all but a few cold stretches each year, assuming the interior humidity is kept to a moderate level. The more insulation you put on the exterior, the safer the assembly.
A couple of years ago I was the Passive House Consultant on a zone 6 house with SIPs on the exterior of a framed wall. I usually avoid foam for environmental reasons but in this case it was a very tight site and my client wanted to reach the PH standard. We ended up with 6" framed walls with cellulose, and 6" SIPs with GPX foam on the exterior, for about R-50 total. Due to other factors we couldn't quite reach the PH goal but it's an extremely efficient and comfortable house.
Michael,
Thanks for your reply. That is really interesting. Have you had any data from the build, have they had any issues with damp in the SIPs? I assume they had a very air tight vapour barrier within that build-up, inside the SIPs?
Thanks
I have not heard of any problems with that project, or with any of the projects I've done with less exterior insulation. Here's an article that goes into the subject further: https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing.
What is the thickest SIP you can get? For example if you can get a 4.5" PU SIP (~R27) you would only need about 3" of mineral wool insulation.
You can than strap out this 4.5" SIP on the exterior with 45x100 lumber on edge and insulate between with regular mineral wool batts. This would get you an above R38 assembly and be reasonably easy to build. This would also raise the temperature of the OSB facer and allow for drying to the exterior which I can understand as wanting in your climate. The siding can be directly attached to this lumber.
I still think the SIP on the outside + timber studs on the inside is easier to build but it does expose the OSB more to the elements.
I like this idea, and would be a more cost effective solution to my thought of using Larsen trusses for a similar approach. I totally agree with trying to keep that outer SIP OSB layer warm, and drier, I would be nervous to have the SIPs on the outside.
I am thinking an engineer would do the math and likely find an 8.5 inch R28 SIP would be more cost effective than the SIP+ plan as the second wall seems likely to double the cost of the walls to save less than 4% on fuel.
Build a BEopt model of the home. If you do take the time to build the model my guess is you will not build anywhere close to passive house standards as they make no finance sense.
https://www.nrel.gov/buildings/beopt.html
Walta
Thanks Walta,
That is probably the case, however a Passivhaus requires a wall assembly to be an R38 or there abouts to meet the standard. Unfortunately I cannot find a SIP thick enough that can achieve this without supplementary insulation being added. Which brings me back to my original issue. A major SIP and insulation manufacturer in the UK does use this approach of thicker SIPs with additional PUR boards to the exterior, however their assembly is very vapour impermeable, and in the climate of the Scottish West Coast I would be nervous damp getting into the SIPs OSB, and ultimately failing because they cannot dry out.
Thank you for the direction to the BEopt, I have never seen this. It looks like a useful tool, the UK government should do something like this!
Have you looked at SIPs from Kingspan? They have an article about building a passivhaus house.
Hi Jonathan,
Yes I have been talking to the SIPs TEK reps about they're solutions. However they're not being particularly forthcoming with their details... All their case studies are also based in milder and drier climates of the south/south-east UK. Also, with Kingspan's less than admirable hostory of testing their insulation and assemblies I am slightly wary!
Thanks
I understand you have drunk the Passivhaus Cool aid and it is very strong stuff.
Take a step back and do the math my guess is the R38 wall never save enough in fuel costs pay for the extra cost.
How much money are you willing to waste to have a Passivhaus plaque hanging on your wall?
Walta
You maybe right Walta, especially in our mild coastal climate, however the additional returns on the home resale value because of the Passivhaus certificate (should) make up for it. So to go so far and not do it does not work forme, from an investment decision.
Thanks
Your resale market and bankers maybe different but from what I can tell no certificate will change the banks appraisers’ value number for a home even a penny. Square feet , bathrooms and Stone count tops add value triple pane windows, fancy HVAC equipment and insulation hidden in the walls add no value if you spend your money on that stuff you have to keep the house in order to have any chance of getting back your money.
I will come back to do the math.
The Passivhaus formula does not consider cost to be a factor.
Walta