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Community and Q&A

Least expensive R-50 to R-60 walls and unvented roofs?

wjrobinson | Posted in Green Building Techniques on

As I work on building homes more and more green and sustainably designed, I have tried sprayed open-cell foam with an unvented roof and liked it very much. And as I go to conventions and ask questions of oh so many in the industry and read for months on these internet sites, I am leaning more toward products that use less oil and nonrenewables. Also very into buying as local as I can.

So…. reading here and at Building Science Corp. … there is all talk of foam, 1/3 of the assembly to the outside, sealed like the dickens, then go with whatever one likes for the inside 2/3s of the R-value and all will be fine rot- wise… (condensation destruction.)

I am aiming toward super high R — R-40-60. I really favor cellulose. Seems to be the greenest insulation by far. It seems to have properties that (with controlled moisture present in a home) aid in not allowing the wood frame to wet to the point of rot or mold starting.

I also am leaning toward building in permeance through the insulation that is not so low that mechanical air exchange is needed. What I really desire is that no air leaks directly into the home. Permeance through dense-packed cellulose to me is OK. I would like to move away from Advantech products someday but for now I think they are amazing in their ability to last long in the presence of moisture. They also now make the roof and wall system that (when taped) seems to be the cat’s meow and a perfect permeance.

So the discussion I am looking for is… with the above products and no foam…. who thinks all will work just fine. with no moisture issues? And propose to me the least expensive R 50-60 walls and unvented roofs you think will work moisture-wise and R-wise.

Thanks all… I hope the discussion is long and lively.

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  1. GBA Editor
    Martin Holladay | | #1

    You wrote, "I also am leaning toward building in permeance through the insulation that is not so low that mechanical air exchange is needed." I think you are confusing two issues.

    The permeance of different elements of a wall assembly or a roof assembly matter because they determine how easily the assemblies will dry out if they get wet (for example, due to wind-driven rain -- the most common method for water to enter walls).

    When you refer to "mechanical air exchange," I assume you are talking about a ventilation system (for example, an HRV or an ERV.) But whether or not you need an HRV or an ERV is dictated mostly by the air-leakage rate (that is, the results of your blower-door test), not by the permeance of the materials used in your wall and roof assemblies.

    If you're a green builder, you want to make your house as tight as possible. You're aiming for a low air-leakage rate (as determined by your blower-door test). So, if you're a green builder, your house needs a mechanical ventilation system.

  2. Doug McEvers | | #2

    You would do well to get familiar with the superinsulation movement of the late 1970's and 1980's. Superinsulation with some modern tweaking is a fine way to build, I like vented roofs.

  3. Riversong | | #3

    Bill (I take it you're not the Bill Robinson who's a moderator at JLC-Online and contributed to my leaving that site),

    You're certainly moving in the right direction for truly "green" or sustainable building, but are still caught up in the current trend to make homes into hermetically-sealed boxes.

    This "building science" trend is based on the illusion that a sealed box will never leak and hence requires no significant drying potential. As we all know, the wood-framed structures that have lasted hundreds of years did so because they breathed. Wood doesn't mind getting periodically wet as long as it can dry in a reasonable timeframe, and it has a pretty good moisture buffering capacity. The more processed the wood is, the more vulnerable it is to moisture (hence plywood's advantage over OSB and sawn lumber's superiority over both).

    The moisture studies that deliberately included normal expected wall or roof leakage into the assembly (MEWS and FSEC) have demonstrated that tightly-sealed and non-vented structures simply don't dry out before mold and rot take hold. And external insulation maintains the warm conditions conducive to mold and decay organisms, greatly accelerating the demise of wooden materials.

    I have to chuckle when you refer to R-40-60 insulation levels as "super high", since I've been building to those levels for more than 20 years in New England and built my first double-wall house way down in Tennessee in 1982.

    I would agree that, of the conventional insulation materials, cellulose is the greenest. A big leap beyond that is straw bales and clay-lime plasters. I can assure you that a double-frame wall system with nothing but dense-pack cellulose and a well-vented roof with loose-fill cellulose perform very well in climate zone 6 and 7. Depending on local wind-bracing requirements, it's possible to build a 12" thick double-walled house with less forest resources and far smaller ecological footprint than a conventional 2x6 house, and that uses less than half the energy of an IECC home.

  4. wjrobinson | | #4

    Robert, no I am not the moderator you speak of. I am building more and more green and also trying like all get out to keep costs down while also trying to get to net zero. I have read many many of your posts and between you and the cell guys I have met at conventions... I do like cellulose for all of it's advantages especially how green a product it is and how well it behaves and how benign it is to humans.

    I really am not for new home smell (VOCs) new car smells... particle board and so forth.

    I also feel we should be able to build net zero homes without breaking the bank which I see at the present cost of foam and GEO and SIPS... well they just seem too expensive to be logical choices.

    Cost, healthy air, non mechanical air quality, net zero energy, thermal mass, solar advantages active and passive.... my direction.

    I am not sold on this HRV talk... maybe a tiny tiny set up.... maybe., Too me with a well insulated home... door and window opening is my air exchange system.

    And Martin, I learn much from you... but... I do believe that air leaking through cellulose ever so much... way less than you are thinking is still letting quality O2 get in and CO2 get out. And if the home is built with no toxins inside, the air will be good air inside with no HRV. I am not talking about a home with toxic insecticides sprayed at the rim joist... Who ever mentioned that area of concern got my attention and I have been against synthetic carpeting wall to wall for a long time along with all the other non natural elements we put in homes. And there is furniture to be had or made from just wood, no particle board.

    Why put toxins in a home and then put in an expensive detoxin system called an HRV? I don't get it? Just start with a home toxin free and keep it that way no? I say yes... IMO.

    Not a fan of HRV.

    Not a fan of venting a roof along with my paid for heated air.

    Not a fan of closed cell foam and the potentials for rot, that I have had to deal with already.

    Am a fan of keeping the building cost down, keeping the building nontoxic and minimally mechanical, and getting the energy needs to zero. Solar is said to be so close to $1/KW by next Spring. NG is very inexpensive, wood for a real well insulated home becomes a much smaller workload....

    Anyway... keep helping me with my direction.... You all have my attention and deepest appreciation for your insights and knowledge.

  5. GBA Editor
    Martin Holladay | | #5

    If you pay attention to air sealing, it's possible to vent your roof without losing any conditioned air from your home. Establish a good, tight air barrier at the ceiling plane, and your roof venting shouldn't be sucking any conditioned air from your house.

    You're right to be concerned with "air leaking through cellulose" -- which is why you need to do a good job addressing air leaks. Build tight and ventilate right.

    If you want to avoid the expense of an HRV, you can install an exhaust-only ventilation system. But any new home needs to include a mechanical ventilation system. Building a home without "toxic" materials isn't enough ... you still need a little fresh outdoor air.

  6. Riversong | | #6


    You're definitely way ahead of most "green" builders in your inclination and approach, but you're still stuck in some fundamental misconceptions.

    As Martin suggests, there need be no energy penalty to a well-vented roof if it's properly isolated from the conditioned space. There are also several substantial advantages, including lower summer roofing temperatures, reduction in summer radiant gain, reduction or elimination of ice dams, and increased roof system durability because of dramatically improved drying ability.

    It's also impossible to approach zero energy if the "ventilation system" is opening windows, which is nothing but an energy throw-away in cold weather.

    But what you're reaching for is perhaps a low-tech variation of the Dynamic Insulation system developed in the UK by the Environmental Building Partnership Ltd. This high-tech approach uses air-permeable walls as the ventilation and heat recovery system, coupled with a mechanical exhaust to maintain negative pressure. It's been built and tested and proven to work, but requires careful calibration of air flow rates.

    The system is based on creating a constant steady air-flow through a filtering assembly within the walls and ceilings, which not only cleans the air but also transfers outgoing conductive heat loss to the incoming fresh air (just like an HRV). If the energy recapture closely matches the conductive heat loss through the thermal envelope, then a building can be dramatically more thermally efficient.

    It may be possible to couple passive stack-effect or wood-stove exhaust flow with a slightly air-permeable cellulose-filled wall to create a similar effect, but careful engineering, calibration and testing would be required to make sure it really works as intended. Winter-time infiltration cannot create condensation - only exfiltration can cause that. And this is one of the reasons I design and build homes with lots of cellulose, vapor-open assemblies, and tight-but-not-too-tight envelopes with passive air exchange systems, sometimes relying on the woodstove as the primary negative pressure generator.

  7. wjrobinson | | #7

    Robert... exactly what I desire to build. The wall as an HRV is what doctoral candidate years ago talked of at a NYSEG sppncered event I attended. Joe L spoke there too... my favorite frothing building scientist for enthusiam alone.

    Are you designing 2000 to 3000 foot homes with natural materials that entail net zero energy demand? I would also eventually try to seek the end of grid connection. Today the monthly connection is near $20 and I am betting it will start to move up as fast a possible till some regulation comes in to cap it.

    Need to read up on your breathing walls... I may have read about them already in the past. Will have to draft up the plans to solidify assemblies materials labor and costs.

    You teach close by in VT... seminars... do you have plans and house tours?

    Martin, thanks for the insights.... I do believe that a exteriorly vented roof will work as you and Robert say and not hurt my heating costs along with aid summer build up. I just like to keep thinking farther ahead to integrating all good and eliminating systems of past that are kind of good. Here's and idea some have worked on.... Solar panels combining solar electric, solar heating solar electric increased efficiency via the cooling of panels via the heating of air for heat use.... and... just spend some experimenting time to also... (drum roll) be the exterior superior location venting space.

    Anyone like that idea? All "gains and no losses"... forgive the pun...

    Green building as it comes to be .... needs to incorporate assemblies that are lowest in cost not just green and definitely not just greenly remarketed green.

    simple, natural, local, efficient, independent, harmonious with community, man, nature and machine.

    More ideas all keep em coming...

    little sidenote.... Lake George is 70 degrees in some points today this last day of May 2010. That temp is normally something I would find somewhere on the lake 4-6 weeks from now. Who knows what our fossil fuels are really influencing... but it has been the warmest winter and spring here in my lifetime. And don't we all wish that deep drillling well heads had all there back up systems installed before the drill ship starts drilling instead of after it burns, sinks and kills 11?

  8. Mike O'Brien | | #8

    Hello, ADKJAC--

    Good point in that sidenote, perhaps we should be designing and building houses for a world that is 5 to 7 degrees warmer and experiences severe weather events more frequently. The we might be discussing things like reflective roofing, overhangs and low shading coefficients.

  9. Riversong | | #9

    Did I pass into an alternative universe?

    How did the original poster Bill Robinson morph into ADKJAC UPSTATE NY?

  10. Tom Marsh | | #10

    I would much rather use and HRV system that provides controlled and consistent fresh air at fixed points. Infiltration can be variable depending on temperature and wind and can't be increased to remove excess humidity. An HRV system can use a humidistat to control fan speed for more ventilation when required and less when not. My perspective is from the cold long winter climate in Alaska. But the mechanical ventilation is also great in the summer as it provides fresh air while it can filter out pollen, dust, smoke particulates from forest fires and mosquitoes.
    Also I didn't see any mention that dense pack cellulose will not allow much, if any, air movement, especially passively.
    The Cold Climate Housing Research Corp. , , has great info on many areas and an excellent resource.

  11. adkjac | | #11

    Neat trick the morphing ehh Robert?

    Tom... thanks for the website posting... I do love the idea of quality air and moisture being in a range that is comfortable... A friend here has built homes for decades that are walk in cooler tight and insulated as such. He has found his homes to be in this area able to stay in a very acceptable range of humidity without any HRV hooked to the outside. And as to pollen... it is outside a home not inside a home. Radiant homes have been built for eons with no HRV. If a home has no built in pollutants and does not circulate air... the air tends to have way less in it... ie dust and pollen... Still air lets whatever fall out of it. With all that I mention.. I am willing to put in HRV... but as some here even agree, we don't need to size HRVs nearly as large as is the new trend. The air in a natural home is good air IMO.

    Anyway... just remember this posting is only to do with the climate in Lake George NY and surrounding Adirondacks area. Moisture is moderate and not a huge issue... air quality as far as I am concerned is good. I am a firm believer in living in your enviroment and not filtering air free of nature. Cleaning too much is giving people sickness... immunal disorders... we need exposure to the enviroment. The only people that need HEPA filtered air are the unfortunate children of over zealous over cleaning rubber glove wearing dirt scared parents.... IMO.

  12. Riversong | | #12

    Neat trick the morphing ehh Robert?

    No. It's deceptive and misleading. Who are you, really?

  13. wjrobinson | | #13

    No deception... just errors in getting online over two winters with time to read and interact... Don't worry, I am not following your situation with t*x protesting. I just about support every post you make... and am glad your voice is on the web Robert.

    adkjac is a screen name... I ski winters at Mad River in your neck of the woods and have been out to Yestermorrow... may have even met you... not sure.... who I met there... Part zimers... Also fly and have been to your sailplane port... nice set up there.

    peace... keep up the great work Robert and worry not.

  14. PDaMuGz8Tb | | #14

    I like R-40 walls and R-60 Roofs
    I think all new homes should have mechanical Ventilation.

  15. jbmoyer | | #15

    The multiple personalities of AJ....

    Favoring cellulose, and then morphing into a foam fanatic.

    I better not say anything more than that, or else GBA will sensor my comment.

    What happened to this website?

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