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Building a Budget-Friendly Unvented Cathedral Roof

JWalker1511 | Posted in Green Building Techniques on

Hello all,

I have been researching for hours and have ended up with more questions than answers. I am looking for the most efficient to build/least expensive for labor + materials/easy to build way to build an R-60 cathedral roof in zone 6 (coastal Maine waterfront). The house is 2900 sf partial timber frame and will have cathedral ceilings everywhere. I understand from poking around on GBA that the vented roof I wanted is not possible because of the intersecting gables, dormers, and skylights. So that means unvented cathedral roof. I also want to do as much as I can to reduce thermal bridging. And Covid has tripled the cost of wood as I understand, so I want to do this with only 1 layer of OSB for the roof deck. The timber framer usually uses SIPS for the whole building but they quoted over $110k for materials and $50k labor just for the shell (no windows) which is just too much. How can I do this cheaper?? My budget is ..stretched… to say the least.. if not completely impossible.

So, I was thinking parallel chord roof trusses – do those help eliminate thermal bridging at all? I have read they are easier to work with, save labor, don’t know about cost though. Those could be filled with either closed cell spray foam or maybe for something way cheaper, dense packed cellulose with the intello on the inside? Can you make the trusses deep enough to get enough R value though? Then I was wondering if you can put say 2″ EPS directly on the exterior of trusses then put OSB on top of it? Then underlayment and asphalt shingles. Huber R sheathing does it, but I haven’t found another nailbase doing the same thing using EPS. I don’t want to use Huber because it’s Polyiso and pointless for my climate. I don’t want to have to make a foam sandwich with two layers of OSB, might as well use the SIPS in that case. Another consideration is in the great room which is timber framed, I may go with tongue and groove ceiling, but probably just sheetrock elsewhere. I’m not sure how this will work either with having two different types of framing going on.   And I gather there are rules on how much exterior foam you have to use to pair with cellulose, but not with spray foam.

For the walls I’m going for R-30 so I was thinking painted sheetrock, 2×6 advanced framing with d.p. cellulose (do i need Intello?), OSB, 3″ EPS in 2 layers taped, (do I need a WRB – what about insects??), battens to vent, then composite siding like Everlast. Or maybe an EPS nailbase product instead? I could also try to find recycled EPS to save money, if there is any around.

Thanks for any info you can help me with!!

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Replies

  1. insaneirish | | #1

    > So, I was thinking parallel chord roof trusses – do those help eliminate thermal bridging at all?

    Certainly to some extent as the amount of wood connecting inside to outside is lower than a solid sawn rafter. You'd probably have to do some modeling to understand the difference. I suspect the rafter spacing to be more significant than the construction of the rafter, overall.

    > Those could be filled with either closed cell spray foam or maybe for something way cheaper, dense packed cellulose with the intello on the inside?

    You can do some mixture of closed cell against the deck with the remaining portion being whatever [type of insulation material] you want. Using only 'fluffy' (e.g. dense packed cellulose) is risky in your climate (due to the potential for humid interior air to reach the cold sheathing).

    > Can you make the trusses deep enough to get enough R value though?

    Yes.

    > Then I was wondering if you can put say 2″ EPS directly on the exterior of trusses then put OSB on top of it?

    2" of EPS is definitely not enough in climate zone 6 with an R-60 target. This article (https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation) shows you need 6.5" of EPS in climate zone 6 with an R-49 target. You would need even more foam for R-60.

    > Huber R sheathing does it, but I haven’t found another nailbase doing the same thing using EPS.

    You cannot put R-sheathing directly on roof rafters and roof over that. You need structural roof sheathing, then R-sheathing. As you allude to, you're basically making a SIP when you do this.

    > Another consideration is in the great room which is timber framed, I may go with tongue and groove ceiling, but probably just sheetrock elsewhere.

    You still need drywall under the tongue and groove ceiling, for air tightness and fire safety.

    My commentary:

    As many of us have realized, there is an incredible tension between design elements, energy efficiency, and cost. If cost is weighing on you, you need to decide what you value most: energy efficiency or design elements.

    Can you eliminate dormers? Can you eliminate cathedral ceilings? R-60 won't yield you all that much energy savings over R-49 in the roof. The tongue and groove ceiling... forget it for now; you can always add it over drywall later.

    As for reducing thermal bridging, consider the 'Bonfig' approach (https://www.finehomebuilding.com/2014/11/26/breaking-the-thermal-bridge) on both your roof rafters and studs, especially if you're putting in sweat equity. This is very straightforward work that yields an appreciable benefit in efficiency and doesn't require asking a builder to do anything out of the ordinary.

    1. JWalker1511 | | #6

      Thanks for your detailed reply! That is interesting, I haven't seen the Bonfig idea anywhere before.

      I didn't know the Huber zip R couldn't be used on roofs alone. I just assumed since it can be done on walls. And yes that makes sense I would need more exterior foam with cellulose, been reading about it all day for walls and didn't even think about it being the same for the roof, I guess I had the spray foam in mind. Trying to learn all these different scenarios is tough as things change every year!

      So in theory , I will have to check with architect about the trusses, I could do shingles, (what underlayment would i need for a hot roof?) , osb/plywood, 12" deep trusses ? with 5" of closed cell spray foam against the osb, 9" cellulose, EPS backed furring strips (2"), intello, strapping, sheetrock, paint/tongue and groove. Correct? Wouldn't have the full thermal bridging protection as exterior foam but would be way cheaper and still better than none. Especially where I have a lot of dormers and things in the roof.

      Is there a concern about ice dams with this roof assembly? Or is the R value high enough to protect it? The eaves will have a flared detail and I wanted to put in gutters all around to protect the siding.

  2. mikeferro | | #2

    I built a SIPS 'enclosed timber frame a few years ago in Western Maine and ended up shopping my SIPs package to nearly every manufacturer in New England. I discovered that pricing varies significantly and rather than switch to an alternative enclosure that my builder wasn't accustomed to installing around a timber frame, I opted to go with a lower cost SIP manufacturer.

    Lots of New England timber framers like Foard Panel SIPS and I'm guessing that's who your timber framer may be using. Their panels are the best of the best, however they come at a premium price.

    I'd suggest working with your timber framer to get a quote from another manufacturer so you have a comparison. I've used Murus in the past and will be using them again for my next project. Their panels are top-notch and can be had at a reasonable price. The only downside is they don't have their own installation crews, so your timber framer will need to install the panels.

    1. JWalker1511 | | #3

      Thanks, it was a quote from Murus. It may be worth shopping around though.

      1. mikeferro | | #5

        Does your design have a complex roof and lots of corners?

        Complexity can be a big cost driver whether you go with SIPS or another enclosure method.

        1. JWalker1511 | | #8

          Yes, it has cross gables, a cupola, shed dormer and another gable dormer over the entry. My taste is fancier than my pocketbook I'm afraid haha

  3. user-723121 | | #4

    You need the help of a building professional, someone well versed in energy efficient buildings. At this stage of he game all of the construction details should be worked out, a lack of planning can only lead to trouble.

    1. Deleted | | #7

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  4. user-2310254 | | #9

    Jessica,

    I’m wrestling with the cost issue myself. Simplifying the design can help with your budget. But if you care about performance and aesthetics, that can be a tough pill to swallow. I’m finding that the best way to balance my wants and budget is to shrink the footprint.

  5. Expert Member
    AKOS TOTH | | #10

    What I find is trying to horseshoe an existing design into a green building is a very expensive proposition. You really need to start your design from the ground up with efficiency in mind, in that case the cost increase will be actually surprisingly small.

    Simplifying the design can usually go a long way of reducing both building costs and energy efficiency cost. A simpler design doesn't have to be ugly, you just have to take care and get the details right.

    In a good design the envelope costs are actually a pretty small portion of your budget (even with the current material price increase taken into account), so if this is not the case, you need to re-evaluate your design.

    1. Mark_Nagel | | #12

      Odd layouts can also present a challenge in deploying the various mechanicals. For myself, as I will be the builder [who has a huge learning curve to climb], I have sought to simplify this as best as possible: mechanical room that provides short pathway for venting and lines up with the basic core of the building such that I can run most everything down as trunks (in a chase) and then branch off laterally (from dropped ceiling).

      I'll also add that maintenance should also be a consideration. An episode of having a ladder start to twist/kick out on me while up 2 1/2 stories convinced me that I'd prefer to be closer to the ground. If you hire someone to do work it'll also be a lot cheaper if it's not placing them in a similar situation.

      Jessica, I'd hope that you'd be going for a metal roof. Costs to replace the roofing, for labor, could end up being very expensive. I know that it's hard to think about such things when you're building new, but thinking about them now/up-front could make a huge difference later on when the excitement of a new build is long gone and entropy has had its say.

      1. JWalker1511 | | #14

        I actually was just going to go with asphalt shingles, for aesthetics mainly but have been told the good architectural shingles last a very long time and isn't worth the price increase to metal. I'm planning to eventually put solar panels on the roof as well which I hope would protect the shingles some. I'm going for a little different styling than the typical modern farmhouse look, to a more French Country style, so if I went with metal or composite then I'd want something that looked like shingles/slate/etc. What is your take on the cost/benefit of the metal? Open to opinions! I am definitely going for low maintenance as much as I can, such as composite siding that never needs to be painted.

        Yes we designed the house to be centralized with mechanicals, I wouldn't call it an odd layout really but it's not a simple rectangle. It's basically a "t" with the master bedroom and screen porch added on as shed roofs, and the front porch as a gable roof. The rooms are oriented to the direction of the sun exposure and to have views to the water in the S facing great room, master, and screen porch. Garage to the North, kitchen & entry facing east to driveway. The building envelope is long and narrow due to restrictions so that limits the layout a lot. There is a utility room off the mudroom entry, which is near the kitchen, and all bathrooms are very close or above it. I wanted it streamlined so there is only one entry rather than a front door no one ever uses, so the mudroom entry is the front door and connects to the future garage. I will use mini splits for heat, and I haven't decided yet which HRV/ERV system to go with yet, whether I want to do a ducted one or the ductless through-wall units. I want to use trusses for the 2nd floor framing so it will be easy to run everything. It will be an insulated slab on EPS with 4' frost wall and may use 8" ICF for the frost wall (R 25). May add more EPS to the perimeter and below slab, not sure what a good R goal would be for that.

        I hear you on the height of the cathedral ceiling, the way the roof works it would actually be more expensive and complicated to lower the great room roof than to keep it in line with the rest of the roof.

        1. Mark_Nagel | | #19

          Sounds like you have a decent orientation and layout.

          Metal roofing is hard to beat for durability. For myself it's also a matter of installation ease compared to other materials: I have only minor experience with roofing, small, simple structures- rolled roofing, shingles and metal; I have some idea, though on a more complicated roof I cannot say. But, and here's the point, if it's easier for me then it's easier for the professionals too, and that likely means less labor costs. And metal is light. With heavier materials comes roof structures spec'd for that heavier dead load. Snow also slides off metal pretty well.

          Regarding the insulation for the foundation, proper calculations would have to be done by a professional. I've only, so far, run a quick PHIUS prescriptive (https://www.phius.org/PHIUS+2021/PHIUS%202021%20Prescriptive%20Project%20Specific%20Calculator%20v1.45/PHIUS%202021%20Prescriptive%20Project%20Specific%20Calculator%20v1.45.htm) and came up with an R-value of 23 for my "floors." It's their notion of "floors" that become a bit problematic (and it's noted as being a bit problematic by actual professionals); what about SLAB foundations? No real answer that I'm aware of, in which case R-23 is what I'll aim for (will have a PH architect review all). NOTE: I'm in zone 4C. Getting the foundation right is the most important part of any build as it's the one component that's nearly impossible to change (esp a slab!) after the fact.

  6. JWalker1511 | | #11

    Well it was planned from the very beginning to be energy efficient, my initial goal was to be a net zero home, and I had my goals in mind for R values based on the recommendation of a PGH architect, and have researched exhaustively to make every decision with energy efficiency in mind, but also wanting a beautiful house with character and a timber frame great room. I did cut down the size a bit already from what it was originally, and also removed the screen porch and the garage and bonus room above to cut costs, and will add them on later. But the way it's laid out is specific to the lot and there isn't much that can really be changed without completely starting from scratch, which I'm not doing since I've already invested thousands into this design process. The dormers are necessary because it's 1 1/2 stories with 6' kneewall rather than a full 2 stories. So the upstairs needs to be cathedral. My only other option to make it smaller at this point is to remove the 1st floor master bedroom to add later which isn't really a big savings and would result in an unfinished and inaccessible master bath which I'm sure the bank won't approve of! I will find a way to make it work but just wanted help on a cheaper building method than SIPS and wanted to know what people would do in my design situation where I can't vent the roof, to address cost, using less wood, and thermal bridging, and ease of building, and still get the performance I am aiming for.

  7. user-723121 | | #13

    Jessica, You can use vaulted parallel chord trusses with dormers, there will be extra or heavy duty members next to the dormers. How wide are the dormers and how many? Can you provide a drawing showing the proposed roof design? I have built many working and complicated roofs over the years, let's take a look.

    1. Deleted | | #15

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      1. user-723121 | | #16

        A lot of complexity in this plan, including the roof. I have used parallel chord trusses a couple of times in a similar situation. One was a master suite over a triple garage and the outside walls were a 6' height. The roof trusses allowed for a 12' ceiling height in the master bedroom, bath and walk in closet with no interior partitions. All of the glass in this area was on the gable end so no dormers were used. Another build was adding a functioning 2nd story to a 1 1/2 story home. For this I removed the existing roof, built 8' walls for 1/2 of the upper level towards the rear of the house. The street elevation maintained the typical neighborhood look of the 1 1/2 story with (2) 6' wide dormers and an 11/12 pitch up to the middle of the roof. The main roof was parallel chord trusses (R-70 blown) allowing a 12' ceiling height and a wall of expansive glass facing south. There was a 3 member girder truss that connected to the hand framed 16" TJI and Micro Lam street elevation roof half. The TJI rafters were 16" deep allowing for R-50 plus fiberglass insulation. There was a 1 1/2" dedicated air space above the fiberglass batts. Along side the dormers were double 16" Micro Lam to support the roof in place of what would otherwise have been common rafters. A complex roof but it worked beautifully and brought the praise of the St. Paul building inspector.

        I have to wonder if your house plan would not be more functional with a full wall height on the 2nd level. The roof could be redesigned to reflect this, keeping the look you want but making the dormers easier to incorporate.

  8. Expert Member
    AKOS TOTH | | #17

    You have a complicated plan, there is no way for this to be made cheap. The roof costs are probably the least of your worries. Since you have gone down the design a fair bit of the way already, change could be very expensive.

    1 1/2 story places with complicated roofline you'll be looking at spray foam for the roof, best accept this and the cost associated with it. There is really very little changing the roof structural elements will do for reducing cost or making the insulation job easier. An in-between could be to design a vented roof above the great room to be built with I-joists/trusses and SPF elsewhere.

    There is one very ugly roof detail, above your entrance you have a roof that pitches down towards a wall. This is a detail that is questionable even when done right and will always create issues. I would design it out.

    A couple of the rooms look like addons to a new build. Not sure how these came about. I would aim for a more integrated look to the house instead of all these shed roofs. The attempt to pitch up the bottom of the main roof to match the shed slope is a detail that only adds complication to your roof build.

    You are also mixing shed dormers on a gabled roof. Generally looks more intentional if they they match, I would put doghouse dormers.

    PS. The back elevation needs a window on the main floor bellow the gable. I think this is into your en-suite bath. It will look much better with windows and make the bath feel extra spacious.

    1. Deleted | | #20

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      1. Expert Member
        AKOS TOTH | | #22

        Screened porch can be under a shed roof. Something that is part of the house (ie master), well, it should be part of the house not an add on. Increase the house footprint or go to 2 full stories. Either option is less expensive than random add on bits. This is just my personal opinion.

        At this stage, your goal is to not make the design an energy pig. Thermal bridging concerns are best left to energy nerds building passive houses. Aim to have code min insulation and above code air sealed building. Sounds like a low threshold but anything more than that will add significant cost.

        Looking at the design, I think you are suffering from the first "custom designed home with everything I dreamed of" syndrome. Details cost money, each time you say detail, you are adding at least $10k to $20k to your budget, it doesn't take a lot of "details" to blow your budget. Simple houses are inexpensive and can be made very energy efficient.

        P.S. I spent a lot of time working and biking in rural France and I can tell you nothing in this style exists there. French country style is a North American invention.

        1. insaneirish | | #23

          > Details cost money, each time you say detail, you are adding at least $10k to $20k to your budget, it doesn't take a lot of "details" to blow your budget. Simple houses are inexpensive and can be made very energy efficient.

          My house was originally built in 1916 and is--by and large--simple. But it has two details that I have come to loathe: (1) a dormer on the back side of the house to accommodate a stairwell into the attic and (2) a first floor bay on one side of the house.

          These details seem innocuous enough. Common to the Nth degree on any new house, of course. And here, they're even tastefully done. But just as was said, they disproportionately eat into budget and time. I think of the speed at which my framers simultaneously demo'ed and rebuilt portions of my envelope. Then I think about the dormer and how it seemingly took forever to detail right and how that speedy framing crew was slowed to a crawl as we figured it out.

          It reminds me of Norm Abram (This Old House, The New Yankee Workshop) talking about his own house being built on The Modern Craftsman Podcast (https://themoderncraftsmanpodcast.libsyn.com/79-norm-abram). He remarks how, since he was paying time and material to a trusted builder, every time he'd see another corner on his house, he'd think about how expensive it was. It's funny but it's completely true.

        2. JWalker1511 | | #24

          >>Looking at the design, I think you are suffering from the first "custom designed home with everything I dreamed of" syndrome. Details cost money, each time you say detail, you are adding at least $10k to $20k to your budget, it doesn't take a lot of "details" to blow your budget. Simple houses are inexpensive and can be made very energy efficient.

          Maybe so, but like I said if the budget demands cuts then I can make changes or put certain things off till next phase. We haven't even gotten any cost numbers yet so it's still in planning stages. I may not get everything I want but I am going to start from a place of trying to get everything I want and then cut things from there. If I wanted a simple rectangle house then I could have bought any of the ranches, capes, colonials, and boring run of the mill old leaky houses on the market and be housed but unsatisfied with my living space. I want something with cathedral ceilings, having grown up in one it is the type of house I enjoy living in, which is exceedingly hard to find. Hence building my own. And I figure if I'm going to the trouble and expense of building my own then it's not going to be a boring run of the mill house, and it's going to be efficient, or there's no point in doing it, for me.

          And yes I know it doesn't look like its been plucked out of France, especially since it's not 300 years old and American construction style is vastly different than anything in Europe. But I like the American version better than any other housing styles to be found here. Rich people can afford to make a really authentic looking fancy french house. There are elements you can incorporate to give it that vibe though, like the flared eaves and timber. To me that is a compromise between full fledged going for that style for megabucks vs a plain house with zero creative or stylistic elements which I would be unhappy with. Why design a house without inserting any of your style or creativity into it?? Nobody NEEDS it but if needing things was the only criteria then I wouldn't be doing this at all. I want to love the house I see when I drive in my yard every day. I wish I had simpler taste! I'm jealous of people who like modern minimalist style, but it's just not me!

          1. user-2310254 | | #25

            Actually, a well-built modern, minimalist house is an expensive build. You can disguise a lot of poor workmanship with traditional design elements. In my last custom build, I really would have liked to included trimless doors and flush bases. But those details are not easy or cheap.

            While we also wanted to express our style, we had to set limits. On that project, we designed the second floor as a single bedroom, bath, TV-watching "masters" suite, for example. That space usage wasn't what most people would do, but it worked for us (and didn't really cost extra). We also splurged a bit on the kitchen while economizing in other areas.

            I agree that you should build the house you want. But your original question suggests your current plan is creating budget issues, which means you need to find cost-savings. I suspect you could saving a lot for opting for conventional 2x6 construction instead of a post and beam structure with SIPs. Maybe this small change would allow you to have the design you want on the budget you have.

          2. JWalker1511 | | #26

            Steve - Ah I assumed trim was more expensive than no trim. My mistake.

            The whole house won't be timber, just the great room, and some decorative beams elsewhere. It may save money to have decorative trusses that aren't load bearing in the great room as well, if I am going to need to frame it out with 2x6 anyway. I just fell in love with the look of timber frames and it was the basis for my entire design idea.

            That was really what my initial question was intended to be - what is a cheaper construction method than SIPS, as that was the initial plan, considering labor + materials, while keeping the performance. I didn't think SIPS were THAT much more expensive once reduced labor was factored in but that doesn't seem to be the case. If I could know the most cost effective way to build the shell then I can figure out the rest of what I can do from there. I wasn't really expecting a full critique on my entire house plan haha, although I appreciate the educated opinions here I also am determined to make it work! :-) I feel like I've already cut a lot from the original plan - SIPs construction, 2 car garage and upstairs bonus room and storage, reduced kitchen wing, removed a bedroom and dormer, screen porch, doing the painting myself, etc. I think I can find other ways to cut costs by installing some of the flooring and trim as I can afford to and putting in the fireplace insert later. And there's always the lottery! haha

          3. Expert Member
            AKOS TOTH | | #27

            My reply was not meant to be a critique of your personal style. Taste is a subjective, no two people will never agree on what is right.

            Reading your comments it felt like you were engaged in a bit of double think when it came to energy efficiency, design and cost. There is no way to make a complicated design cheap to build and in a similar way it is very expensive to have a complicated design that is energy efficient. Combine both as requirement and you definitely will have a very expensive project.

            You are doing the right things by being flexible for the layout and compromising on certain elements to stay within budget.

            I'm not a designer, I can generally look at a house and feel that something is wrong. A good architect can tell you what that something is, this is why they are well worth the money they charge.

            A cross gable is a very traditional building shape. Can be adapted to many styles, layouts and uses. Definitely be a great design choice and with a bit of timber work would be a fantastic house.

            Sticking random blobs onto a cross gable and calling it French country style is just good marketing. Very close to calling concrete pavers cultured stone.

            Good design is not expensive, does take a bit of care early on and also means trusting the professionals to do their work. Having a coherent structure sometimes means taking a step back and forgoing some of those "details" that don't belong.

  9. user-2310254 | | #18

    Jessica,

    The French Country style involves a rambling look that is supposed to suggest the home has been added onto over the generations. Was this your choice or a community requirement? Where I'm planning to build, it is one of the styles mandated in the build guidelines. But maybe you have more flexibility.

    Did you set a target budget with the architect before starting the design process? Has your builder ballparked this plan? I'm assuming $300 a square foot for my next build (not including land). I will be delighted if it costs less. But I'm not counting on it.

    1. JWalker1511 | | #21

      Oh I haven't heard of that description before but that makes sense, and seems like my house will fit the bill haha. No it's not a requirement, there is a square footage requirement only, but it is just the style I like. I ended up combining elements of my two favorite house styles, Mountain and French country.

      I did set a budget but haven't gotten any pricing yet except for the SIPS. So that's why I'm trying to figure it out myself. We designed what I wanted and once I figure out how much it will cost then will make changes from there if needed. I'm hoping it won't cost $300/sf but this being the absolute worst time ever to build a house, it's just my luck! I figure a few more things will probably end up in phase 2 for later.

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