Passive House-Level Insulation for House of Glass
Timbo_K
| Posted in PassivHaus on
How much should I insulate and leverage PassivHaus principles when I have a house of glass? When does it stop making sense?
Planning a new 3,500 sf bungalow lake house. Will have solar PV, Batteries, and geothermal for HVAC. but the front (southern exposure) is floor to ceiling glass.
I want to insulate the crap out of the house, maybe double stud walls etc, but at some point, when the whole front is (triple pane) glass… adding more R value to walls becomes pointless. What is that point? Located in southern Ontario Canada – Zone 5a.
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With a wall of glass facing south, your bigger concern isn’t staying warm, it’s the potential for over heating.
That’s what you need to address first.
That's a great point Oberon. Actually, one of my primary concerns as the lake house is mostly a spring/summer/fall residence. Our current place is surrounded by trees whereas this one has no southern shading. The overhangs (not shown well here) should extend far enough to keep the sun off the floor most of the time - based on the shading model.
Timbo - based on your latitude and thus low sun angle, you may be surprised at potential heat gain from those windows, particularly in the swing seasons. Here's an excellent and easy-t0-use tool that will calculate % shaded, % in sun, and heat gain:
https://susdesign.com/overhang/index.php
This might help you decide on e.g. SHGC for your glass.
Thanks - That is a great tool. I had used it months ago but forgot about it - and I think you are right; Its the off season Oct onwards where the solar gain may be the worst. I was concerned about the summer and recently only focused on those months.
You might want to look at quad-silver layer LowE coatings for those windows to minimize the solar heat gain. They will be darker than clear glass, but with all that glass area it shouldn't be all that noticeable in bright sunlight, and you will notice the lack of being cooked when the sun is hitting the glass.
Energy efficiency starts at the design stage, and the design is really the biggest make or break factor. Of course you should insulate and air seal well, but you obviously have 2 competing ideals at work- the aesthetic, and the energy efficiency. You need to decide which is more important.
A house like this that is very well insulated could get uncomfortably hot on a sunny day, how do you plan on dealing with this? If you have to run A/C all summer, not too efficient...
Passive house standards are arbitrary, and kind of political. In my opinion, a better design philosophy is the Pretty Good House. More rooted in common sense.
Thanks Plumb_bob. This is the real essence of my question ... is how do I get "pretty Good House". I probably should not have said passive house as it seems to evoke a lot of reaction. I just want to borrow some ideas - such as good insulation, good airtightness, high performance glass etc.
I hope to not cool it during the day - but ideally have enough natural ventilation to keep it comfortable. Bedrooms would be cooled at night - on the occasions that warrant (~10 days per summer at our current location)
From my reading of the Pretty Good House recommendations, they recommend moderation in both the size of the house and the percentage of glazing.
https://www.prettygoodhouse.org/design
So from that perspective, 3500sqft with a wall of glass doesn't really match the spirit of the PGH idea, which recommends 1875sqft for a primary home for a family of 4. Either you should reconsider the design or accept that any passive or PGH techniques you use will be marginal against the impact of the initial design choices, which seem to be for a lakeside palace.
That said, it can still be a comfortable and low-utility-bills house if you install a ton of solar to power the HVAC to control the heating/cooling issues inherent in the design.
I get the criticisms here about high cost, ROI, excessive glazing, needing a structural (and probably/hopefully mechanical) engineer, etc - but I think (and Timbo_K, please correct me if I'm wrong) it's probably safe to assume that we're talking about a pretty high-end home here, yeah?
We can probably all agree that there's a fundamental disconnect between wanting a home to be efficient/"green" and wanting it to be on the large side in a cold climate with huge amounts of glazing - but I think it's a totally valid question to ask, "okay, we're GONNA do this - given that, what are steps we can take to do it as efficiently as possible and do the least environmental damage?" So maybe it won't live up to all the "pretty good house" standards, but if it approaches them where possible (quality windows, tight air sealing, etc etc) then it's more of a win than building a code-minimum luxury home.
To that end, think really critically about that glazing. Does it need to be floor-to-ceiling? Granted, it'll probably look pretty badass if it is - totally get that. But how often will you be looking out at the lake from knee-level? Or from 8' up?
If cost of construction is a major driver (ie, it needs to go down to build to the level you want), cutting square footage is probably going to reap far more savings than any other changes you could make.
It looks (to me) like those renderings posted up top are pretty schematic - I assume this conversation is relatively early in the design process?
> So maybe it won't live up to all the "pretty good house" standards, but if it approaches them where possible (quality windows, tight air sealing, etc etc) then it's more of a win than building a code-minimum luxury home.
I think the point that some commenters are making is that if one is set on building a huge house with massive glazing, there is no point in trying to use PassiveHaus or PGH high performance approaches (vs a code minimum house) because the design is working against those approaches, and therefore they are probably not worth it, answering the original question.
It's a bit like trying to get better fuel economy in a Hummer by using low rolling resistance tires . Sure, you could get marginally better fuel economy, but it's a Hummer, so the overall effect on fuel economy minimal at best. If fuel economy is an goal, you need a smaller, lighter, and more aerodynamic vehicle.
But as DC said, the best thing would be to just do the math and see what it says.
High end home? Yeah, I hope so! This project is our legacy. Been in the lake for 40 years as are my siblings, so creating a nice place to enjoy with family and friends. We have 15-20 for dinner on a regular basis and that is growing as the family grows, so there is some method to this madness of space. There are small cabins already, but this is the main lodge / cottage / lake house.
You captured the spirit better than I did. And yes, we are early, but finalizing design so we can start working in construction drawings … thus the interest at this point.
Most of the energy usage is going to be locked in by the design and not by the construction details. So you really want to be doing energy modeling before construction drawings.
A good mental exercise is to set an energy budget beforehand.
Incidentally, I don't now how much you've talked with your architects about costs, but the same is generally true of overall construction cost. Most of the cost is baked into the design. It's very hard to find construction savings once the design is set, and almost impossible to find them once construction begins. So if you haven't given your architects a budget it would be a good idea to do so before work begins on construction drawings.
DC,
That's an underappreciated point. The only cost savings available once the contact drawings are complete are through compromises.
I'd say that cost savings are always only available through compromises. It's just that once the design is set the compromises available to you become a lot more limited.
There is no one-size-fits-all answer. You need to do energy modeling. I recommend the BEopt software from the US Department of Energy. It allows you to plug in different design elements and see how they affect comfort and energy usage.
Yes. We are actually contracting with a company now to do the energy model - should have it in the next few weeks - and can post the results.
Most of your cost savings in terms of initial cost and operating cost won't be wall selection.
Anything code min (2x6+r5 rigid) is good enough. Going up to R40 with a double stud would be nice but I doubt it will budge your overall energy usage much. Since it looks like you are looking at some tall walls, 2x8 24" OC might also be a good alternative over the 2x6+R5.
For large windows one important item is selecting the appropriate coating such as low SHGC windows for sides with no shading. Even better, getting proper shading for all your windows especially the large south facing one will make a huge difference in energy usage.
Unless you are into well drilling business, ditch the geo. There is a lot of misconceptions out there that geo is free heat thus green. A well set up geo might hit a COP of 3, since it is such a niece market, chances of you getting a well designed and set up system is pretty much nil.
At one time, geo was really the only electric heat option in cold climate but that changed with hyper heat air source heat pumps. For example, I heat a 550sqft cottage in northern Ontario with a single 1 ton Midea hyper heat mini split. The equipment cost on that under $1500. A 2.5 to 4 ton ducted version of it might be a couple thousand more which would still be less than the cost of just your pipes, fittings, plumbing manifolds and pumps. On paper the COP of an air source heat pump might be a bit less but it will be close enough that the ROI on anything geo is never.
I am waiting for the quote for the geothermal. I will not dig a well - It will be a horizontal closed loop (my neighbour has an excavator) or we will do a lake loop. This is our 100 year property - it will likely serve multiple generations, so I am playing the long game - dont mind paying a bit more capital, if it makes sense over 25 years or longer. Having said that, I am also looking at the air source heat pumps as you point out - seems they have come along way - and would be great for bedroom cooling - which is the most to have good room to room control.
I second the geothermal part - any savings that far in the future are essentially worthless and that's if you hold onto the house - the resale value of geothermal is nil.
If you are looking for a 100 year property, I would rethink the amount of glass. When all of those windows need to be replaced in 30 years, the cost will likely be equal to the value of the entire property in today’s dollars. I would not want it be the family member footing that bill.
Why would you assume that all the windows would need to be replaced in 30 years?
My guess is your question is not even a possibility.
The last time I looked at the Passive House rules they boiled down to thou shall not use more than X number of BTUs per square foot. Given your location on earth and the fact the best glass know to man is less than R5 no house with that percentages of glass area in your location can win the passive house approval.
When I look at your drawing, I see a monument to architecture at the expense of everything else.
Windows are the biggest line item in most construction budgets when they are less than 15% of the wall area your drawing looks more like 55%.
If the plan is to buy enough solar to get that house to net zero my guess is the house + solar budgets will be almost $700 a square foot. If this is off grid batteries and ground source HP in a remote location lets double that budget.
Walta
Run of the mill triple pane is R10. Quad pane, krypton filled, etc is pushing R16.
https://global-uploads.webflow.com/5cb62ed8577ebe3b6d03de1a/61e70c1dae5044b45aa11293_Zola%20Arctic.pdf
A "run of the mill" triple pane IGU is going to run around R6 or a bit better. A top-end triple pane IGU with argon fill is around R9. Those are center of glass values according to Cardinal's technical glass guide (linked below), so a window incorporating the IGU will measure something less than those R value numbers.
If you want better than R10 for a window (U of 0.10), you need to get into fancy assemblies with more than three panes, heat mirror films, etc. That means $$$. If you have entire walls of glass, the glass will dominate in terms of total heat loss from the home, so you don't gain as much by heavily insulating the walls since the walls aren't where the majority of the energy loss is coming from. The PassivHaus people discourage large amounts of glass for this reason.
Cardinal's Technical Glass Guide is here: https://www.cardinalcorp.com/source/pdf/Technical_Glass_Guide_Web.pdf
Bill
"Those are center of glass values according to Cardinal's technical glass guide (linked below), so a window incorporating the IGU will measure something less than those R value numbers."
Given that the area between the frames is going to have to be structural, the whole wall R value will end up being a lot lower too.
Windows aside, there are a lot of really worrying design elements - like the transition from the sloped to the flat roofs, and the lack of overhangs immediately over the windows - which will make the successful detailing and execution of the house really challenging.
This reminds me a little of an old joke from college about engineers and architects. The architects didn't always get along with the engineers, and it went something like "the architect says "it will be beautiful!", then the engineer says "forget it, we can't build it like that"". Architects tend to be a little more towards the artist side of things, and engineers a little more towards the practical/physics side.
Aside from the proposed design being a fishbowl (it's nice to have places to "get away" too), I certainly agree that there needs to be a little more attention to practical construction details given to this design. All that glass will make it difficult to place mechanicals properly too, since there aren't many places to hide wires, ducts, and pipes in such a design.
Perhaps try putting in more walls, but keep some big glass corners to keep some of the panoramic views? Use the glass more strategically instead of just big swaths of all glass? That would help with overall R value, and also provide places to hide mechanicals.
Bill
Agree with R6 to R7(+) for a good triple pane window, but R8 is pretty much the high end (and relatively rare) unless you are going with a more exotic (as you pointed out, quad panes using glass or heat mirror, vacuum glazing in an IG, etc.).
Keeping in mind that center of glass is the entire glass face except the outer 2 1/2" from the edge. That is the edge of glass.
The warmest part of any window is CoG so a larger window actually has better U factor (or R value) performance than a smaller one because of the higher percentage of warmer glass, but when you have an R3 at 5 square feet versus R5 at 50 square feet...it becomes less simple.
The R10 number I mentioned is center of glass for every European and Chinese triple triple pane IGU I’ve seen. With glass this size, cardinal can’t even make these units in triple pane, hence the reference to European numbers.
You have to be careful with European windows. They publish U factors using metric units. You have to make sure the numbers you're looking at are using US units.
I think it was Joe Lstiburek that said the very best windows make lousy walls.
Walta
R10 is possible with quad panes, not happening with triples, especially not "run of the mill" triples. But even basic triples will hit R6 and many companies offer R7 or R7.5 windows, with a few legitimate R8's on the market.
Plus R value (or better U factor), is only part of the equation, blocking or allowing radiation, solar or otherwise, coming in or going out, and air tightness are major considerations as well.
Like stated above, I was referencing center of glass numbers which is the majority of the wall in this case. Whole window NFRC u values are based on a window that is relatively small and the frames hammer the overall u value.
I have attached 4 different examples. GLO windows, European Architectural Supply, and Zola Windows, who are using european IGUs and Fenstur from Canada who is using IGU's from China.
The Zola "Arctic" is R 16.7 center of glass.
Fenstur will also do a window using LiteZone. With 4 layers of film inside interior & exterior glass would give R14, U 0.074 (imperial) & SHGC 0.167. As per my post #40 below I'm curious anyone has looked into or tried these.
I was going to use Fenstur on my project but they do not provide a warranty on the oversized units. That was a deal breaker for me.
Passive House is a whole home standard. They don't have specific requirements for each component (unless you count airtightness as a component), so "passive house level of insulation" is a meaningless term. Given your wall of windows, you can't meet the passive house standard, so there's no point in looking to it for guidance on insulation levels.
1. First consideration needs to be comfort. To that end
a. pursue above comments about overheating, and ensure you have sufficient cooling capacity.
b. be obsessed about selecting windows with very little air leakage, and then be obsessed about how they are installed.
c. be obsessed with general air sealing of the house, perhaps even using Aerobarrier.
Even if your heat/cool system can keep up, the biggest comfort issues in that living room will be drafts.
2. Challenge your designer a bit on the glass. This seems like the easy way - surely there is an awesome stop dead in your tracks design that uses less glass...
3. Make sure you rough in for low voltage wires for motorized shades, and if you can afford it, build pockets for cellular shades to disappear into. Someone, at some point will want/need them, and will not want to be constantly climbing up there to change batteries. I would home run every wire the shortest distance possible to a power supply - maybe something in the ceiling below, or a box recessed in the floor with a cover, or other.
Thanks. That's helpful. And yes, we will run low voltage for blinds to all windows and it will be automated (Lutron). I have used the automated roller blinds before; will check out the cellular shades.
Timbo,
Architectural critiques temporarily aside, a more fundamental issue is the glass sizes shown. I don't know code for your country or locality, but where I am, tempered glass through the whole window unit is required where sill heights are less than 20", at stairways, within 24" of exterior doors, etc, etc.
My current build for a family member is a bit similar in that the east-south elevations have large (~200sf total) glass areas for bringing in views of the 90' pines. Whole window values of R-10 are possible with krypton filled triple cavity 4 pane though the current cost of krypton makes this a pricey option. More relevant is the size limit of the units - roughly 4'x8'. In your presented views I see glass panels that look more like 5'x10'. I am ignoring the likely impossible triangle elements.
Your architect may have access to such large glass, but are they very heavy and special order thick glass single cavity? Sun control options may be very good in the near commercial glass realm though I suspect the R values will be lacking. The support framing for such heavy glass will severely limit your materials choices and that is ignoring the rather severe structural limitations your engineer will face keeping the "porch" stable against wind and snow.
Others have noted the odd roof intersections and lack of water management, so I will only add that it may be better to back up and think about whether you want a home to look out of or a home to be looked at. You may find the feeling of being in a fish bowl quite comfortable or not. Either way, the sun will be blinding without extensive blinds or curtains which kinda makes the large glass moot most of the day. Come winter the cold wash off the large glass will be a constant reminder of how much energy you will be pouring out the glass.
Code seems to allow these large windows - the architect uses them often. In our current house (using the same architect ... rightly or wrongly) , which has a lot of glazing, many of the windows are floor to ceiling 9' (107") triple glazed Marvins - so I am confident we can do something similar here.
Re: Fishbowl - appreciate it not for everyone - but in this case, only thing in front of us is the lake. The other side of the lake is wilderness park (no cottages) - and I have no neighbours within a mile or two. So therefore, its about the views, being connected with the lake.
Which direction does all that glass face in your current home?
You may have noticed the OP has never responded to anybodies replies. By the sound of it, I'm guessing he drew this drawing thinking he could maximize his "bungalow" lake views by using huge amount of glass and then compensate by using huge amounts of insulation. This is not a building an architect would design, although anything is possible, especially in the age of bigger is better.
nick,
I look at GBA questions as having two functions. One is of course to offer (what is hopefully) useful advice to the OP. The other is to spur discussion on things we haven't had occasion to think through. I learn a lot from questions that seem irrelevant to me.
Good advice. Thx Malcolm.
We also get design ideas this way. If you watch the home renovation shows and ignore the made for TV nonsense, you can get some good design ideas and also see things that might sound good on paper but not work out in practice. The Q+A forum offers a similar testbed for ideas.
It's always my hope that through the collective knowledge of these forums that we all benefit and hopefully arrive a bit closer to the ideal solution to the presented problems than we would have on our own.
Bill
Yes, we are trying to maximize the lake views of course. And yes, I would like to maximize the insulation ... where it makes sense. That was the gist of putting this out there - and getting the feedback - which has been excellent and helpful.
"This is not a building an architect would design..." uh... I wont share that comment with ... the architects :)
If this came across my desk I would require engineering. Light frame construction is partly defined as having sheathing or bracing on one side, the front wall has neither. It looks like it would be built basically as post and beam with glass infill.
The views would be stunning with all that glass, but there are negative implications as well.
Are you up for a 2.5 million dollar build not counting the land?
If not you and the you the builder and the architect need to have a budget meeting and make some hard choices the sooner this meeting happens the less of your money gets wasted planning unaffordable pipe dreams.
It is all to easy for the architect to draw your every desire collect his money while knowing only the richest few people on earth could afford to build what was drawn. Then you have no choice but to pay the architect again for a totally new plan you can afford.
I know you won't be talked out of the idea of so much glass, and it does look nice. Could you (and your architects) extend the overhang? Physically blocking the worst of the sun's rays spring through fall will go a long way toward reducing unwanted solar heat gain. BEopt is a free energy modeling tool that can show you the effects of the overhang and different window and wall insulation details.
As always these discussions are so helpful.
We are also in the process of designing our “forever house” and retiring to Revelstoke. We want to highlight views south across the Columbia Valley so we are planning lots of glass in our Great Room.
One of the options that we’ve come across is LiteZone IGUs. https://www.litezone.ca/construction.html
These use internal layers of film rather than glass to provide more insulation. The product with 4 layers of film inside interior & exterior glass would give R14, U 0.074 (imperial) & SHGC 0.167. These would be a considerable improvement over other high quality products. The company offers a 25 year warranty and states the windows will last 60 years. Lifespan is as important as a bump up in the U & SHGC.
They have been discussed here previously: https://www.greenbuildingadvisor.com/article/the-road-ahead-for-insulating-glass
And there is a summary video on the Green Energy Futures channel: https://www.youtube.com/watch?v=sCACGXAT6ik
We are just starting to get pricing, but it sounds like they may be 30% more expensive that an argon triple pane. We are awaiting Energy Modelling which will help us decide on this and other issues like size of overhangs, HVAC, etc.
I’d appreciate hearing if others have found any objective research or practical experience on how they perform. What I’ve been able turn up has mainly commented on the intriguing design rather than actual real life performance.
Thanks!
Rog
(I'm the OP ... ad we are progressing but ...)
One item that came up and I don't know how to resolve is if we should have vapor barrier ... as the entire house will be spray foamed. Apparently the spray foam acts as a vapor barrier... so some say that adding a regular plastic barrier would be a 2nd layer and could result in vapor lock between the 6ml and the foam. others say the 6ml will be up against the 6 inches of foam - so it is all part of the same barrier - so add the plastic just in case the house shifts and the foam cracks.
Weird - thought this should be straight forward. Thoughts?
No vapor barrier. If moisture gets between the foam and the interior surface you want it to be able to dry to the interior.