GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Tiny House with radiant barrier

bWKTwFw6i7 | Posted in Energy Efficiency and Durability on

Hello and thank you in advance for your thoughts.
I’m in the process of building a passive solar tiny house and I’m building it with a careful eye not only toward energy efficiency and insulation, but also air infiltration and heat retention. My calculations estimate 7600 BTUs of heat per day in the winter for the Portland, Or. area.
My wall section is fairly non-traditional due to weight issues. From outside to inside I’m planning for: 1/4″ cabinet-grade plywood stapled at 4″ o.c.
2×4 stud with R-19 insulation packed in
layer of rubber isolating material
mylar sheet with taped seems
1/2″ air gap
interior paneling.
http://www.tarfman.net/TinyHouse/about/wall-system.jpg
There are friends who have advised me that the impervious barrier in such a small building will collect moisture on the inside surface causing mold within the wall cavity.
However the contractor and a couple of other people have suggested that just opening a window or door for a couple of minutes each day will prevent moisture from building up dangerously.
The building is on a trailer which means all surfaces are in contact with the air and the size is 130 sq. ft.
You can see the whole project here, and I welcome your thoughts on any aspect of this endeavor.
http://www.tarfman.net/TinyHouse/index.html

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. GBA Editor
    Martin Holladay | | #1

    Aaron,
    Condensation occurs on cold, hard surfaces. To reduce the chance of condensation, you need to keep your interior surfaces warm.

    That means (1) a source of heat, and (2) a layer of insulation to separate interior surfaces from outdoor conditions.

    In your small home, condensation is likely to appear first on your windows. To minimize problems, you'll need double glazing. Triple glazing would be better, of course, especially if you end up taking your trailer to Canada or Alaska.

    Since your house is so small, it's possible that you can afford better insulation. Fiberglass batts are air-permeable and low quality. A radiant barrier doesn't do much. Instead of a radiant barrier with an air space, consider installing a continuous layer of rigid foam. That will do a lot more for you than your radiant barrier.

  2. homedesign | | #2

    Aaron,
    Not sure if you are looking for nice comments or all comments.
    I don't know your climate very well...
    I would say that your tiny house is overglazed and under-insulated.

  3. user-659915 | | #3

    I'd be concerned about racking resistance of the (yes, wildly overglazed) south wall, especially for a trailer-based structure. Seems from your post you're planning cabinet grade exterior ply as the external sheathing. Seems like an odd choice, with major durability concerns and at 1/4" doing little to help your racking issues. But then the website talks of lap siding with no sheathing. Again, racking resistance? And which? I'm confused.

    And where's the bathroom?

  4. homedesign | | #4

    James,
    I had the same thoughts about racking
    Also thermal bridging

    On the plus side... this house..... unlike many ...can be seasonally rotated

  5. homedesign | | #5

    Another plus for this project is that it is clearly a hands-on experience and a labor of love.

  6. homedesign | | #6

    Every Architect should build something with their hands as you have done.

  7. user-659915 | | #7

    "Every Architect should build something with their hands as you have done."
    Hear hear. Especially so that we learn proper respect for those that do it every day.

  8. Riversong | | #8

    I agree with others: no racking resistance, no out-thrust resistance for roof over loft, far too much glazing for either efficiency or indoor temperature regulation, poor moisture management, low quality materials.

    It's primary asset is its small size, but I doubt this will survive a trip down the road or be comfortable to live in when the sun shines.

  9. homedesign | | #9

    Aaron,
    It looks like you have paid good attention in Architecture School and to the magazines.
    If you want to win awards for your designs ..and have your work published
    you are on the right track.

    Please do not take this personally
    I am ranting about the profession...not you
    Things have changed little since I went to Architecture School.

  10. TJ Elder | | #10

    I think this project deserves credit for green credibility in having used so little new material, being built mostly from salvage. It’s odd that green evangelists would fault the effort when most improvements would mean greater consumption.

    Consider that you don’t wrap your body in R-40 worth of sweaters to go outside on a chilly day. Given the limited enclosed volume, the relative insulating value of this shell should compare to a more typical size house with super thick walls.

    I agree that the glazing area is too large a percentage of the shell, but it’s a desirable feature for such a small interior. Overheating shouldn’t be a problem with the windows open. I think John was suggesting the whole structure could rotate to face away from the sun if desired. The downside to the glazed area would be heat loss on cold nights, and thermal quilts at the inside of windows could help with that.

  11. homedesign | | #11

    There are definately good intentions here.
    The use of salvaged materials and the size is to be admired.

    I am only suggesting that it can be improved.
    Small size, recycled materials AND Building Science

  12. Riversong | | #12

    small size, recycled materials AND Building Science

    ...and basic engineering principles so that it can survive it's first road test.

    the glazing area is too large a percentage of the shell, but it’s a desirable feature for such a small interior</blockquote
    Unless this is going to be parked in the woods, the over-glazing provides no privacy - one of the most critical qualities of a tiny house, in addition to overheating, night-time heat loss and loss of structural integrity.

    By contrast, the magnificently-designed Tumbleweed homes use a far more reasonable percentage and distribution of glazing. http://www.tumbleweedhouses.com/houses/

  13. Riversong | | #13

    WTF? My last post lost half its text.

    small size, recycled materials AND Building Science

    ...and basic engineering principles so that it can survive it's first road test.

    the glazing area is too large a percentage of the shell, but it’s a desirable feature for such a small interior

    Unless this is going to be parked in the woods, the excessive and concentrated glazing offers no privacy, one of the most essential qualities of a tiny house.

    In contrast, the magnificently designed Tumbleweed homes use a far more appropriate percentage and distribution of glazing and ventilation.

    http://www.tumbleweedhouses.com/houses/

  14. homedesign | | #14

    ...and basic engineering principles so that it can survive it's first road test.

    When I was in Architecture School we were required to take several structures classes.
    But we were never taught "the basics"
    It was also obvious that "You" can never have too much glass.
    Thank you Phillip Johnson and Mies
    At least FLW usually but not always would consider the Sun.

  15. Riversong | | #15

    At least FLW usually but not always would consider the Sun

    ...and the falling of water

  16. bWKTwFw6i7 | | #16

    Thank you all for your comments. Very good points brought up all around. I'll answer these to give you all a more clear understanding of what's going on. First of all the issue of racking. This is an obvious and understandable question. I got the idea from a professional builder when I shared how worried I was about the weight of plywood as a sheathing material. He said that thinner cabinet grade plywood has the same strength as 5/8" construction grade OSB with much less weight and cost. The material is currently nailed at 6" o.c. but I'm going to add 2" staples in between for added strength. The location for this house is Portland, Or. which is a temperate rain forest. The weather is damp and humid most of the winter with a 3 month drought in the summer. There is no privacy issue where I'm at because the yard is big, but I will have adjustable curtains. The overly large glazing area is for passive-solar heat gain. It was more window than I planned, but we got a wonderful deal on four matching windows. In the summer I can open the roof skylight and a floor hatch to create a stack effect. I'm also considering laying out trays of salt to create a thermal mass at the base of the windows to add extra heat when the sun goes down (this will also help prevent excessive moisture buildup.
    To prevent thermal bridigng, I plan to use the strips of wood inbound of the mylar to create a thermal break at the studs.
    In terms of structure, I was lucky enough to connect with a contractor who's as experienced as any engineer. We have hurricane ties at each rafter, strong 'T' brackets on the south wall between each window, and steel connectors between the studs and sole plate. To resist the bed loft cantileveer the 2x4 joists were doweled and nailed into the end beam and the whole assembly is connected to the top plate with steel ties. There's currently about 700lbs of weight being stored up there and there's no deflection at all.
    It should also be mentioned that this house was moved twice so far. Once on a 4 mile trip to the Green Building Tour, and once again just to maneuver it into final position. Both times it weathered the stresses perfectly and we didn't even have time for interior bracing.
    It should also be noted that we had a very unseasonal 18 degree freeze last week and I was only running two low-power heaters at about the same rate as comparable full-sized houses and my place is not fully insulated yet. Once I get everything in, my hope is that it will need little or no heat at all.
    Thank you so much for your compliments, it means a lot to me that so many of you folks respect this project. It has indeed been a great learning experience.
    peace

  17. homedesign | | #17

    Aaron,
    I am so glad you came back.
    Now I admire you even more.

  18. Rob | | #18

    consider http://www.google.com/products/catalog?hl=en&expIds=17259,27642,28066,28104&sugexp=ldymls&xhr=t&q=reflectix&cp=6&safe=off&um=1&ie=UTF-8&cid=2372429499870830249&ei=aXMFTcmgHY2Lswa-yqniCQ&sa=X&oi=product_catalog_result&ct=result&resnum=6&sqi=2&ved=0CFAQ8wIwBQ#
    it is approved as vapor barrier and reflect purposes.I used it on this tiny bungalow and hardly need to turn on the heat even with the 14f lows we had in Wa. state recently. No moisture issues.Windows stay dry and I am snug as a bug!!!
    https://www.youtube.com/watch?v=LO9695AMgtM

  19. Riversong | | #19

    Rob,

    The foil likely had nothing to do with your perceived comfort. Unless a radiant barrier has an adjacent air space it is a highly conductive material and offers no thermal benefit. And the claims for this product, as for most radiant barrier materials, are highly deceptive and - in fact - illegal according to FTC rules.

  20. Rob | | #20

    Thanks,
    cool in the summer, warm this winter and with board and batten hardie plank on the siding with an air gap it has been just fine FTC rules excepted.I'm also using 1/2 inch rigid foam with earthen clay plaster for the inner walls and that is probably not ftc approved either.Tiny houses skirt rules all the time as a matter of freedom.
    Rob

  21. Riversong | | #21

    Tiny or big, no house has freedom from the laws of physics.

    As we have discussed ad nauseam on this website, in most applications radiant barriers are close to worthless. They can offer some advantage under dark roofs in hot summer weather and under floors over unconditioned spaces, but most of the claims for them are pure hype.

  22. Rob | | #22

    http://www.reflectixinc.com/basepage.asp?Page=DIY+House+Wrap&PageIndex=548
    meets ASTM testing standards where they exist and maker states no claims in violation of FTC
    A quick perusal of ftc actions show none against the maker. So I guess i'm free not to worry they nor you will be knocking on my door.
    just suggested consideration
    Sure glad I did not need approval for the 80sq ft of insulated glass windows installed.

    And to conclude not perceived comfort, in the correct application and use it works just fine
    laws of physics and air gap utilized to our benefit.
    Rob

  23. Riversong | | #23

    Rob,

    I'm glad that you're happy with your radiant barrier, but you offer no data, no comparative heating costs, in fact nothing at all to substantiate your claim that "it works just fine" and that your ½" of foam and scrim of aluminum foil is an adequate thermal barrier.

    Those of us who understand thermal dynamics know that radiant barriers have limited utility in limited applications.

  24. Rob | | #24

    Thanks for the suggestion to clarify the current set-up.
    Last tiny house same basic plan and wrapped conventional tyvek $45 for 2 months PUD in Oct and Nov 2009. This iteration $18 for the same period. That without the roof finished. So while debatable the scrim vapor perm reflet is the only variable and this over a much colder 2 month period compared to degree days from last year.Been building tiny homes and cabins since 1968 .Willing to explore new materials and applications and this one is working with fine utility in this limited application. Might not work in others, thus the suggestion of consideration in light of the OP's use of very low perm solid sheet mylar space blankets.

  25. Riversong | | #25

    Rob,

    Perhaps you'd care to restate that in English.

    No one disputes that a radiant barrier, properly installed, has a measureable effect when compared to marginal or no thermal insulation. But it's never a substitute for adequate insulation.

    While you have added some data, you're still offering nothing more than anecdotal evidence which, by itself, is worthless.

  26. Rob | | #26

    The tiny bungalow is fully insulated.In English, the wrap is the only variable.
    Simple to understand. If the heating bills fail to prove it to your satisfaction I could do more testing.I am sure in your infinite wisdom you have some suggested tests and certified procedures.
    Do you live in or build tiny houses? Or just critique those that do.
    To clarify both the original poster of this thread and I have fully insulated houses.He was looking towards the effects the mylar would have and I suggested vapor perm reflect material.
    I never suggested consideration of using it alone as a insulation media.
    In English,maybe you need to examine your reading skills.
    Yikes

  27. Riversong | | #27

    I'm also using 1/2 inch rigid foam

    Fully insulated?

    Your sarcasm doesn't earn you any credibility points.

    What is the full cross-section of the thermal envelope, the area and volume of the conditioned space, the air exchange rate, the heat source and its efficiency, your HDD climate? What was your BTU/SF·DD before and after radiant barrier installation?

  28. Rob | | #28

    I'm not looking for street cred. I look at what works.
    I understand your dislike of vapor barriers and what you espouse in their place.
    This works in my application. Why on earth would I perform before and after testing when I have real world, real time comparisons? Methinks thou analyse too much.
    Good night

  29. Rob | | #29

    I understand your confusion. The 1/2 in foam is just the inner wall material plastered with ecohaus earthen clay. The studs are thermal wrapped to provide a break and the voids are filled with glass batting. I apologize for the confusion.
    Peace

  30. GBA Editor
    Martin Holladay | | #30

    Rob,
    You wrote, "The studs are thermal wrapped "

    I'm sorry, but I don't understand what you mean. What did you wrap the studs with?

  31. Rob | | #31

    Hi,
    A thin sheet of laminated rubber with fiberglass backing. Just used as a thermal break to limit transmission through the uninsulated studs.Most likely overkill with under 1300cu ft of volume.
    As more gets completed it is getting to the point that heat from the tiny fridge and the dog keep it comfortable. I defer to the expert though if they insist this application can't work. I feel quite warm, perception wise!
    Rob

  32. GBA Editor
    Martin Holladay | | #32

    Rob,
    What's the brand name of this "thin sheet of laminated rubber with fiberglass backing"? I can't visualize the product.

    Did you wrap each stud individually, all the way around each stud, or did you install strips of this rubber material to one edge of each stud?

  33. Rob | | #33

    Strips applied in a bridge like wave across each stud front and back.
    Don't have the material name.It came as a roll from a friend that works production at a large aerospace co.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |