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Triple-pane windows? Cathedral ceiling with Roxul? Minisplit? Affordability?

edkrause | Posted in Green Products and Materials on

This might not be the forum for this long question, but, I’m driving myself a bit nuts with all the environmental/aesthetic/monetary values I’m trying to blend in designing and building our house. Maybe there are a few builder-psychologists willing to give council so as to arrive at a workable blend… and to quiet the competing voices bouncing around in my helmut.

We are trying to juggle the long term benefits and values (both environmental and economic) of making the house extremely easy to heat in a climate that often feels like Siberia, while we keep in mind a wish for frugality as we realize the work that we find most fulfilling often pays the least.

My main questions concern windows, the roof and a mini-split in this context:

Existing house

a.) 650 sq.ft. relatively energy efficient house in the woods of western Wisconsin, zone 6.
b.) 5 double glazed-argon filled Kolbe windows, and fixed pane windows I built using Cardinal Low-E argon gas filled patio door glass blems — passive solar design, tightly built 6 years ago.
c.) hydronic in-floor heat, off-peak electric micro-boiler I installed, acid-etched slab on grade, 2 inches foam beneath. No HRV.
d.) R-26 walls, but only R-40 closed cell rigid XPS foam sealed with foam, a hot roof.
e.) AC window unit for the worst days of summer, cumulatively about 3 weeks a year.

f.) 690 sq.ft. addition spread over 2 floors, slab on grade with an upstairs bedroom.
g.) R-36 walls, R-58 cathedral ceiling/roof 3/12 pitch with standing seam metal roofing, 4″ under slab and on foundation.
h.) Hydronic heat in slab (same boiler, which was sized for this originally) I’ve installed, so for a few $hundred in tubing we have comfortable back up heat on the ground floor. There will also be a wood-stove which will get used daily. I’m also considering a mini-split installed in the upstairs bedroom for occasional back-up heat and AC. I will add an HRV.
i.) I’m doing most of the work, as I did in the original small house. (I was a builder for 12 years and am now a furniture maker/designer).

Where I’m stuck:
1.) I’ve been planning on triple paned windows and have gotten bids from Intus and Kolbe. The Intus windows (open in) and are very impressive… expensive, but for what they are, a good value. Kobe’s (open out), are a 5 $thousand less, but not as carefully engineered, though aesthetically more consistent with the existing windows. In the first phase of construction I installed 5 Kolbe openers and built 5 fixed panes (costing $140 each in materials). I could do the same in 2nd phase. I could also well as spec dbl. panes rather than triple.
2.) We will have a woodstove which will be our primary heat source, so I imagine triple pane windows don’t make sense in terms of payback, but I feel some responsibility to the long term energy use of the home from an environmental perspective…. and they’d be more comfortable and condensation would be eliminated in below zero weather (over 50 nights this winter).
3.) we don’t have a lot of money (but we can find a way to buy triple panes) and want to pay off our home equity loan quickly.

I’ve boiled down this scenario to these Questions:

~ Is it worth getting a mini-split for the bedroom, when back up heat upstairs would be very sporadic and we only need AC 3 weeks a year? A better option?

~ Triple pane windows even though (in the short-term) they don’t make sense economically?
Anyone have experience with Kolbe triple panes?

~ I don’t like to use foam where I don’t need to (mostly for environmental reasons and toxicity potential) and am considering an “unvented roof”, using Roxul batts and rigid foam on the warm side, carefully sealed. Am I looking for trouble down the road?

Other thoughts?

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  1. Expert Member
    Dana Dorsett | | #1

    A mini-split will be ridiculously oversized for a single bedroom, unless it's a super-sized bedroom with LOTS of window area.

    If the wood stove can heat the whole house, a mini-split located in the same area as the wood stove is pretty good backup, and will be cheaper to run than the radiant floor. Using a floor thermostat to control the floor temp, keeping it at ~72F for bare-foot comfort and using the mini-split to control the room temp (when the woodstove isn't running, or is dying down) works at fairly high efficiency.

    Sometimes a mini-split can work with the head located just above the lower-floor ceiling level in a stairwell, but it depends a lot on the stairwell. Since the louvers can be directed, you might have sufficient upstairs cooling by the direction of the output air stream (probably not if you have big west-facing windows.) But in heating mode convection will do a lot for you even if the air flow is directed down stairs.

  2. oberon | | #2

    Good afternoon Edward,

    What is your window orientation? Based on your post, I am assuming that you want to maximize solar gain on your southern exposure, but what about the other three directions?

    Not all triples are created equal - not just "windows", but glass package as well.

    When looking for maximum solar gain, and if you are willing to compromise U value a little bit (without throwing it out the window completely), then you might choose just a single high solar gain coating on either surface 3 or surface 5 of the triple pane IGU versus an IGU with coating on two glass surfaces.

    For example, glazing Cardinal's LoE-180, high solar heat gain coating to surface 5 only of a triple IGU results in a U .184 and SHGC .615 (IG - 4mm / 13mm 90% argon fill / 4mm / 13mm 90% argon fill / 4mm). While applying 180 to just surface 3 results in U .193 and SHGC .628.

    Slight advantage in U value with coating on surface 5 and slight advantage in SHGC when coating is on surface 3.

    For comparison, if the coating is on surfaces 3 and 5 then the results are U .133 and SHGC .586. An excellent U value and not half bad SHGC either. While no coating triple has U .288, SHGC .673. Great SHGC when heating, but U value more in line with coated dual pane.

    Triples can make a lot of sense in your environment - which also happens to be my environment - and as soon as the rain stops and the ground dries up a little I will also be adding a "room" to my home and I will be using triple panes throughout.

    I will be building my own windows for the new addition but at the moment I haven't decided yet on single coating surface 3 or two coatings on 3 and 5 for the IGU's....I am leaning towards 3 and 5 though. I won't be building the IGU's, just the sash and frame components. IGU's are not a DIY project.

    Per your specific question on Kolbe triple, first question I ask is what's the glass package?

    It will come from Cardinal, so that part is a given.

    Also, from your post these are casements? I am not a fan of casements in a heating dominated environment primarily because they are installed pround of the exterior wall which, in my opinion, is a very bad design feature.


  3. GBA Editor
    Martin Holladay | | #3

    I agree with Dana on the minisplit.

    Concerning the triple-glazed windows: only you can decide. We all know that triple-glazed windows are more comfortable and save energy. They are also more expensive than double-glazed windows and have a long payback period. You have to look at your bank balance and make your own decision.

    Q. "I don't like to use foam where I don't need to (mostly for environmental reasons and toxicity potential) and am considering an unvented roof, using Roxul batts and rigid foam on the warm side, carefully sealed. Am I looking for trouble down the road?"

    A. You can't build an unvented roof that way. If you want an unvented roof with Roxul and rigid foam, the rigid foam belongs above the roof sheathing -- and it has to be quite thick for the system to work. For more information, see How to Build an Insulated Cathedral Ceiling.

  4. wjrobinson | | #4

    We found for a log home being built this summer some very low cost triple panes framed in vinyl. The owner is OK with the vinyl which some may not be. The price we are paying for sash windows is really low under $300 each. I normally have installed Andersen 200 and 400 series some Pella and some custom Marvin.

    The triples are R-5.

  5. edkrause | | #5

    Thanks, Greg, Dana, Martin and AJ for your helpful perspectives.

    The house is sited for solar heat gain and I've designed the window placement to optimize heat gain on the south (not too much) and minimize heat loss and heat gain elsewhere, while letting in adequate natural light. We are in the woods so in the shade during the warm months... and the house will also have large overhangs. Kolbe's glass does indeed come from Cardinal and I'm working with them to optimally spec glass based on orientation, though the salesmen are astoundingly uninformed. In contrast, the Intus people are very conscious of the issues. I know I must make the difficult double/triple pane decision, and I've pretty much decided to go with Kolbe triples. They now have a casement than has inbetweeny mounting which will improve the overall performance. Their triples will be significantly less costly and the factory is just 3 hours from here, rather than thousands of miles over an ocean. The Intus are undoubtably better windows, but this is a cost/benefit compromise I think I'll make, all things considered. It has been the most challenging decision by far.

    Dana, I like very much your suggestion to mount the mini-split in our stairwell. That will work well in our case. Thanks.

    As for the the Cathedral ceiling, I'll probably make a continuous baffle out of plywood fastened to the underside of the TJI rafter's top cord, fill the cavity below with roxul batts and then install 1.5" faced iso across the bottom plane, taped. Any suggestions about materials/details with this program?

    Greg, good luck with your addition. Mud here too, but the weeks of rain (and snow) has finally broken and today the sun shines.


  6. wjrobinson | | #6

    That's how I build cathedral ceilings vented insulated and rigid foam inside then finish T&G or your choice. Roxul needs to fill the space it is in. Foam needs to fit right and be taped edges sealed to edges of plane.

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