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I’m working on the design of a two story house in Climate Zone 5

gatter | Posted in GBA Pro Help on

I’m working on the design of a two story house in Climate Zone 5 for a client with a somewhat limited budget. (I’m attaching a dwg showing the schematic section)
There are a number of different conditions in the house.
The main roof over the Second Floor will be a low slope EPDM roof pitched at ¼” per foot. The ceiling of the second floor is located at the bottom of the trusses.
There is a small portion of the house that is one story and covered by a shed roof with a metal standing seam roof that meets a higher wall.
I would prefer if both these roofs were unvented. (It may be hard to vent either one.)
The first floor is a suspended slab with radiant heat over a basement space. (that’s already eating into the budget quite a bit!)
I like the idea of using dense pack cellulose with rigid foam on the outside of the sheathing for both the walls and the roofs. But I’m trying to lower the profile of the roofing over the first story portion, and the code here mandates R-20 for rigid foam which means 3”.
So going with 2” of closed cell and then dense pack cellulose would help with that situation.
As there is radiant heating in the first floor I’m thinking that, as insulation needs to be there anyway, the floor would be the thermal envelope and the basement would be unconditioned. (I would prefer if it would be conditioned space, but I’m trying to save money somewhere!)
In order to avoid having to sheetrock the ceiling of the basement I was considering using a closed cell spray foam that does not need a sheetrock covering (like Staycell One Step 255)
So, although I like the idea of the rigid foam on the outside ( as well as not really liking spray foams that much) there seem to be a few spots (the floor over the basement and the insulation at the shed roof) that are happier using the spray foam.
The siding on the first floor will be cedar and I’ve also read that cedar is not happy right up against foam sheathing so there’s the additional expense of building in an airspace or using Home Slicker or similar.
I don’t want to mix up too many different types of insulation as I think it will drive the contractor crazy.
So I’m thinking of just going with the closed cell and cellulose for the walls and roof and just the closed cell for the first floor insulation.
Anybody have any thoughts about budget implications of the rigid foam/ cellulose combination versus the closed cell spray foam/ cellulose? Or any other thoughts?
I have to admit that after several days of reading about dew points, spray foams, and cellulose I’m looking forward to finalizing this decision!

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  1. user-659915 | | #1

    If your client's budget is limited I'm not sure why you're considering spray foam solutions at all, nor for that matter a low-slope roof with parapets. A raised heel gable truss ventilated cold roof, insulated with a thick blanket of cellulose at the ceiling level, will almost certainly cost less to build, contribute fewer atmospheric pollutants in production and will be more trouble-free and incur fewer maintenance costs over its lifetime. If this is just to satisfy an esthetic preference it seems like a high price to pay.

  2. GBA Editor
    Martin Holladay | | #2

    Your suggestions and questions are all over the map. If you can narrow down your questions to just one or two alternatives, it will be easier for GBA readers to provide answers or possible solutions.

    I hope that you are at the preliminary stages of design, and that you aren't locked into anything yet. Needless to say, I don't believe that a designer should settle on a building's shape unless the designer knows how the building will be insulated. Insulation details shouldn't be determined at the last minute; they are integral to design.

    I agree with James Morgan: if the client's budget is very tight, I wouldn't consider the use of spray foam. Instead, I would come up with a design that allows the use of cellulose insulation.

    Finally, I strongly recommend that you find a way to include the basement within the home's thermal envelope. I'm guessing that the boiler or water heater will be located there -- so it's important to keep all of your equipment within the home's thermal envelope.

  3. gatter | | #3

    To simplify...

    The code in New York State/Zone 5 states that any air permeaeble insulation used in an unvented roof needs to be used in conjunction with rigid insulation of R-20 above the roof sheathing or spray foam below the roof sheathing.

    I know that both the rigid foam and the spray foam are more expensive than cellulose but I need to use either one or the other.

    I know it's also possible to vent a low slope roof (or to use a pitched roof with a truss) but It seems to me that there are many examples of insulated nonvented low slope roofs out there.

    One could argue that a low slope roof with a continuous membrane with a long life span is a simple system that presents fewer areas for concern than many pitched roofs do.

    I was just looking for some guidance on which of the two systems (cellulose with spray foam versus celluose with rigid foam) would be more economical.

  4. GBA Editor
    Martin Holladay | | #4

    In most cases, rigid foam with cellulose will be more economical than spray foam with cellulose. But the answer depends on the cost of installed spray foam in your particular area, as well as the cost of the labor to install the rigid foam.

  5. gatter | | #5

    Thank you

  6. wjrobinson | | #6

    Linda, contact Doug at

    It just may be the best call you make concerning your project.

  7. user-659915 | | #7

    It seems to me that there are many examples of insulated nonvented low slope roofs out there.

    There are indeed. There are also many examples of failures of such roofs, also of failures in the wall systems below them that are unprotected by roof overhangs.

    One could argue that a low slope roof with a continuous membrane with a long life span is a simple system that presents fewer areas for concern than many pitched roofs do.

    One could argue that but experience suggests the contrary is true. If you go this route you will certainly need to pay careful attention to critical details like the flashings of parapets, rainwater hoppers and other wall penetrations. The apparent simplicity of low-slope roofs is misleading and not to be undertaken unless you and your contractor really know what you're doing.

  8. homedesign | | #8

    Linda, I will argue that James is giving you the best advice

  9. gatter | | #9

    John, thanks for the handy chart.
    It looks to me from the chart that the unvented cathedral ceilings wtih cellulose and exterior rigid insulation do marginally better than the vented cathedral ceilings.
    That's good to know. And yes, the vented attic (with cellulose) comes out best. It's not always the case that the clients want an attic space.
    Where did the chart come from?

  10. homedesign | | #10

    Linda , Here is the link
    Concerning the clients preference ...It is better to have a vented attic and NOT use the attic space

    It sounds like the clients may have a fashion preference...that may not be as affordable, buildable or durable

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