What Ratio should I use for interior / exterior insulation in a Steel Frame house ? (Cold winters / Hot Summers area)
SebMaz
| Posted in Energy Efficiency and Durability on
Hello,
I’m building a new house using Light Gauge Steel Frames, and I’m worried about the insulation, there are no experts in my area it is Cold and snowy in winter and hot in summer, and rarely does someone build a house using Steel, to not get condensation and dew point into my interior or inside the wall, what ratio of rock wool / Mineral wool should I use Interior to Exterior?
my wall thickness (metal studs) is 9cm
my planned layers from inside to outside: Gypsum board, vapor barrier, 5-10cm rock wool insulation, OSB sheet, wind / water barrier, ??cm rock wool insulation, vented facade fiber cement boards.
so now does it matter if you have more insulation inside or outside? type of insulation outside like XPS vs rock wool? and wind/water barrier be over the insulation or the OSB? and should I avoid vented facade ?
Thank you.
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Replies
100% exterior zero % interior!!!
Any insulation installed between the steel framing will be almost total ineffective from a thermal point of view. The steel frame has an R value of zero and when you calculate the whole wall R value the interior insulation with the zero from steel in wall becomes almost zero. It may dampen some sound but mostly a waste of money.
100% of your thermal insulation must be on the exterior of the steel framing.
Generally steel and pole buildings do not get much love on this forum from an energy efficiency point of view most of us do not see them a high-performance building.
I am sure from construction dollars per square foot point of view some do think they are great and many that read the garage journal form likely to offer more positive advice.
https://www.garagejournal.com/forum/forums/general-garage-discussion.3/
You may find this article interesting.
https://www.greenbuildingadvisor.com/article/insulating-a-metal-building
Walta
Have to agree.
I don't know if this is a pole building or something different. With pole buildings, what ends up happening if you want to finish it like a house you end up having to build a house within a house and the whole thing ends up being more expensive and more problematic than if you'd just built a house in the first place.
Thank you for the advice, I'm located outside US, in Armenia and here nobody builds houses with wood importing will be too expensive, 99% are made using Tuff (a kind of rock) or cement blocks + concrete, and almost no one cares about correct insulation.
I'll check out the links you've mentioned.
Thanks
Barev! When I worked in Armenia after the 1988 earthquake, we built wood-framed multifamily buildings in Stepanavan. All of the framing lumber was imported from the U.S. -- shipped by containers through Moscow. Not very practical -- but wood-framed building has happened in Armenia.
Good luck with your project! Exterior rigid foam is definitely the way to go, whether you end up using steel framing or concrete blocks.
Barev! nice to hear that :) yes, the wood price is high and you can't find a treated wood. thank you
If you are in the land of masonry construciton, I would look at a block load bearing interior wall, vapor/air barrier over the outside of the CMU, rigid insulation then either a brick or stone veneer for cladding:
https://www.gobrick.com/media/file/28d-tn28d.pdf
The rigid insulation in this case can be any type, whatever the cheapest to source locally.
If you stick with steel, you can also skip the interior vapor barrier if you have enough exterior rigid.
Assuming that 89mm stud wall will get something like R10 batts, assuming you are somewhere in zone 5 equivalent, you only need about R5 exterior insulation. If you final cladding is a reservoir type (ie masonry or stucco), you are best to use foam instead of mineral wool because of solar vapor drive.
R5+R10 steel wall (so about an R10 assembly) is not great in cold climate. You want to shoot for about 2x that value so R15 or so exterior rigid.
Thanks for the resource, I left with 2 options after calculating the budget.
1- 2" continuous XPS outside = ~R10 + 1" EPS thermopanel siding = R4 (non-ventilated facade)
2- 4" continuous Rockwool (75kg/m3 or 4,6 lb/ft3) = ~12R + Fiber cement (ventilated facade)
not sure which one will perform better and won't cause issues later.
2nd option is more expensive but if it will be the better one I will go for it.
In both cases I will insulate the cavity also 4" with Rockwool
Walta does have a point but there is still value in insulating the steel. You just don't want to use expensive or high R value insulation. Depending on the stud depth you will only end up with an R5 to R8 assembly regardless of your center of cavity insulation R value.
There are a lot of other issues with steel studs, I would only use it if you must have non-combustible construction.
I once went on a deep dive to find exactly what the r-value of steel was. I couldn't find a number, every source I found just said it was virtually zero. I did find a source that said it was zero to three decimal places. But what that means is that whatever the rest of the wall assembly contributes -- which normally you just kind of ignore because it's negligible compared to the insulation -- is all of the insulation for that section of wall. In this wall it's drywall, OSB sheathing and fiber cement siding. Let's say for the sake of argument all of that together comes to R1.
Just to run some numbers, let's say the rest of the wall is R20 and the framing fraction is 15%. So you get 15% with a U-factor of 1, and 85% with a u-factor of 0.05, that give a combined u-factor of .1925 or an R-value of 5.2.
An inch and a half of continuous insulation, on either side of the wall, gives R6, with nothing in the cavities. Add R13 in the cavities and you get R10.6 for the assembly.
So even modest amounts of continuous insulation contribute far more than anything in the cavity.
Thank you for the advice and the calculation!
what do you suggest using XPS or Mineral Wool on the outside? If I Use XPS and have a vapor retarder inside will that cause an issue if some amount of vapor gets trapped in the cavity? or I don't need to use a vapor retarder in that case?
PS: my climate zone is close to zone 5 (Utah, Colorado) but I'm outside the US.
You can't have vapor retarders on both side of the wall, it has to be able to dry to one side. In a cold climate it's preferable to dry to the exterior. How about rock wool board?
DCcontrarian- yes understood, so I left with 2 options :
1- 2" continuous XPS outside = ~R10 + 1" EPS thermopanel siding = R4 (non-ventilated facade)
2- 4" continuous Rockwool (75kg/m3 or 4,6 lb/ft3) = ~12R + Fiber cement (ventilated facade)
in the case of XPS, I won't use any vapor barrier.
but in the case of Rockwool should I use a vapor barrier inside and an Air barrier outside?
In both cases I will insulate the cavity also 4" with Rockwool
Seb,,
I don't think it matters much which way it is designed to dry if there is no cavity insulation, and it is quite vapour-open on the other side. I agree rockwool would be a good solution, but if it's a lot more expensive than foam, I'd use foam.
https://www.energyvanguard.com/blog/combobulating-the-perfect-wall-the-basics-of-control-layers/
Here is an article based on ASHRAE 90.1 calculations for bridging, on page 3 it says there is about 60% benefit to adding batt insulation between metal studs. Assumes you have exterior insulation and still need more thermal resistance.
https://www.pacerepresentatives.com/uploads/MORRISON_HERSHFIELD_Z_GIRT_THERMAL_SHORT_STUDY.pdf
Thank you! I will check it out
To the question that was asked, your wall assembly is safe, it has the vapor barrier on the warm side and can dry to the other side so you don't have to worry about permeable/impermeable ration.
Thermal Resistance of steel
https://www.engineeringtoolbox.com/thermal-conductivity-metals-d_858.html
Tangent--
copper piping is about 10x more thermally conductive than steel, but still less heat loss for DHW piping when compared to PEX.
That shows K-value, which translates to roughly R-0.003/in for carbon steel.
I'm not sure I understand your PEX/copper comment. Pex is about K-0.4 while copper is K-400. In other words, copper is 1000 times more conductive than PEX.
I would think hard before going with a construction that isn't what everyone else is doing locally. You can insulate and finish it differently, but at every stage of construction it's going to slow things and make them more expensive if you have to explain to everyone how the walls are supposed to work.
yes, that was my mistake going for a steel frame, didn't think that It would be very hard to find the right materials and lack of experts, that's why I'm trying to arrange everything myself and telling people how to do it.