GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Polyiso on interior

Cotiliar | Posted in GBA Pro Help on


Living in the Bellingham area north of Seattle it can get cold. The house was heated by propane, it was a cost that was hard to get hold of especially with some of the design of the house. (high ceiling)

I set out to improve the insulation of the home taking bits and pieces off the internet as to the techniques available these days.

I am a bit “OCD” now that the living room is for the most part done “I have some hidden inward worries”. I wonder if I am trapping moisture inside my walls having used the foilfaced polyiso just behind the sheetrock.

I gutted the walls – installed Roxul – then the polyiso – then the sheetrock.

This note to you is because some of the contradictions I seem to have come across “after” I completed this project in the living area, as top the wall drying from the interior or exterior.

Was it okay to do what I have done? The pics may help others as to whether it was right or wrong.

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.


  1. GBA Editor
    Martin Holladay | | #1

    Foil-faced polyiso on the interior of your walls won't cause any problems, as long as the walls can dry to the exterior.

    Do you have a good air barrier behind those ceiling boards?

  2. Cotiliar | | #2

    Martin - exhale - I presume things will dry from the exterior, it's typical sheathing and siding on the exterior. It's just that my wooded area with the rain and cool air air scares me with some of the stuff I have been reading lately.

    Ceiling boards-
    Wanting to have some "curb appeal" to the inside, and struggling with some of the propane energy issues I started in the living space. I have read lately that the ABC's Attic-Basement/Crawl then conditioned area of the home is the best approach towards improvement.

    I have not done anything to the attic space "yet" (exhale) my intentions were to blow in cellulose in the near future over the existing insulation.

    I know it has a VB and fiberglass bating at present.

    Thanks Martin once again for your guidance. (back into my crawl space in the coming days.


  3. GBA Editor
    Martin Holladay | | #3

    I'm not sure I understand your answer about the ceiling boards. Are you saying that the only air barrier behind the ceiling boards is a layer of polyethylene?

  4. Cotiliar | | #4

    No Martin - the ceiling boards are panel assemblies I bought at a re-store.
    They are a slat assembly. Each slat is, spaced, and secured to perpendicular strips. The srips serve as the "nailer" if you will, located on 24 inch centers. Each slatted assembly is 2'x9'.

    I secured a grid system to studs/rafters (there is still sheetrock on the entire ceiling.) I had to create the grid because the 24"oc slat panel section did not liine up with my studs/rafters.

    There is about one inch between the slat assemblies and the sheetrock on the ceiling. I never disturbed the make up of the ceiling. Drywall - VB- Insulation.

    Thanks Martin - William

  5. GBA Editor
    Martin Holladay | | #5

    Sounds good. I'm glad you have a good air barrier.

  6. Expert Member
    Dana Dorsett | | #6

    As long as there is a vented space between the existing roof insulation adding the panels in the cathedral ceilng sections aren't going to create a problem, but if you have a poly vapor barrier in there it limits what you can do for additional roof insulation.

    You can leave the existing attic-floor insulation in place, but air-seal all penetrations before adding more. Wrap R15 rock wool around any flues or chimneys (tied in place with steel wire, or wire fencing) to keep cellulose from contacting them.

    Deep roof overhangs can be almost as effective as rainscreened siding in the foggy-dew PNW. How much do you have? (About 1-foot per story of height would be good, zero would be of some concern, but you probably would have seen some sheathing damage when you gutted it if it was a serious problem.)

    At PNW type electricity & propane rates even resistance electricity is sometimes cheaper than propane. Even not-super-efficient ducted heat pumps would cut the heating bill in half or more.

    The coldest I've ever seen it near sea level in Bellingham is low positive digits Fahrenheit, which would be a common daily high temp in Winnepeg or Thunder Bay in January. Your 99th percentile temperature bin is about 20F- I'm not quite buying the " can get cold" statement, without defining "cold" ahead of time. ;-) Cold is relative! (I've lived in WA in years past, and have relatives all over the region.)

  7. Cotiliar | | #7

    Dana - I am from Hawaii (smile) just last week @ 24* "it was cold". That's my story and I am sticking with it!

    Thanks for the answers regarding the foil faced Polyiso over the studs. I see it now done to the exteriors and was stressing that it might not be drying from the outside per my installation.

    Back to the crawlspace project on my next days off.

    Thanks again. - William

  8. Richard Beyer | | #8

    This installation is fine as long as you tape the seams and maintain the indoor RH (humidity). If not, mold is inevitable in time. I visited 2 homes where this was the installation system. One with fiberglass and one with open cell foam. Both homes did not have controlled humidity and ventilation systems. Both ended with sickness and asthma related issues. Fortunately for both homeowners the problem associated with their health was located inside their walls. Not everyone living with mold is as fortunate.

  9. Cotiliar | | #9

    Richard - thank you for your time and answer.
    The project was finished just this past summer. Since then, I have done continued reading. I became concerned that it may not be "okay". Just alot of conflicting information on the world wide web.

    In this project the seams were all foil taped, top plate and outlets were foam sealed as well.


    I am presently in the early stages of encapsulating my crawlspace with hopes of controlling the homes indoor R/H, which I noticed to be on the high-ish side... We have been using a portable dehum which has a been doing a great job in terms of the R/H numbers I read daily. Yet I wanted to understand a root cause of the issue - which has led me to my crawl space.

    To note - while tackling the living room in the pics you commented on - I was relieved to find no issues in the wall cavities - everything looked fine.

    Here - outside of Bellingham, WA (Sudden Valley) it is heavily wooded and shaded, combined with the PNW rains, I had an inward concer if what I did was okay in efforts to simply better insulate the home.

    Your opinion.

    I plan to move through out this house and it's interior. Do you think perhaps going with the Roxul and forgoing all the foil faced foam is better of the long haul to let the place breathe?
    I don't want to throw time an money and "create problems".

    I posted an thread "crawlspace encapsulation" just days ago if you have anything constructive regarding the endevour you input is appreciated.

    Thanks Richard - William

  10. wjrobinson | | #10

    Richard, you post everywhere that what we build is bad. Scare monger. OK, all construction, all construction can have problems easily. Water, moisture, 75 degrees and mold.

    Millions of homes have no problems. And asthma... But what no diabetes, no big blood pressure from worrying about the mold?

    Your posts are out of line with main stream home ownership. The sky is not falling. Yours is. Most are not. Mine is not.

    And what about dimensia? Foam involved? Probably right....

    Join the real world Richard. Most of us enjoy a bit of mold.... Bacteria... I love cheese....

    Speaking of cheese... Smile my man... I love yaa even though I think you have been ill effected by your own foam dilemma's.

  11. Richard Beyer | | #11


    I'm sorry you are offended by factual posts. Maybe you can explain why both of these homeowners became so sick? Maybe black mold is good? This is as real as it gets!

    It certainly blew my mind when we opened the wall and saw black mold. I'm pretty certain many homes with tight wall assemblies have some form of mold problems when moisture is not controlled. If it was not a problem explain why there's so much focus on air exchange and dehumidification today AJ. Are you one of those guys installing humidifiers on HVAC systems?

    If you do not have anything positive to say try keeping it to yourself.

  12. GBA Editor
    Martin Holladay | | #12

    Of course it is always possible for the indoor relative humidity in a home to be so high that mold growth results. This is possible but relatively rare. Most homeowners operate their bathroom exhaust fans often enough to prevent mold growth.

    There is no reason to believe that an interior layer of polyisocyanurate on exterior walls will cause health problems for homeowners.

    Your reference to the fact that "both of these homeowners became so sick" had me confused for a while, until I realized that you aren't talking about the homeowner who posted this question.

    You asked, "Maybe black mold is good?" My answer is, "Mold is usually a sign of elevated moisture levels. If you see mold, figure out why the mold is there and find a way to reduce the moisture level."

    Here is what the Centers for Disease Control has to say on this issue:

    "The term 'toxic mold' is not accurate. While certain molds are toxigenic, meaning they can produce toxins (specifically mycotoxins), the molds themselves are not toxic, or poisonous. Hazards presented by molds that may produce mycotoxins should be considered the same as other common molds which can grow in your house. There is always a little mold everywhere - in the air and on many surfaces. There are very few reports that toxigenic molds found inside homes can cause unique or rare health conditions such as pulmonary hemorrhage or memory loss. These case reports are rare, and a causal link between the presence of the toxigenic mold and these conditions has not been proven."

  13. wjrobinson | | #13

    Richard, I don't agree with most of your posts.You focus on the few homes that have issues.that is now your new line of work its an income source for you.but the facts are that although you find problems here and there with all kinds of construction much of what you state is just not true the majority of the homes similarly constructed

    Yes, some have issues out of 100 million in the USA.

    A builder here has used rigid foam always now for 4 successful decades. It can be done.

    I am the one posting about the positive attributes of inner rigid. You are the negative.... Asthma dude. Sky is falling typr.... Glass is empty one.... Lions and coyotes and fischer cats oh my....

    Love yaa Richard. Keep posting death and more... It's entertaining at least. Can I feel the love back.... Please... Pretty please?

    If this reads as taunting publishing at...Taunton Publishing... Then the shoe may fit and I do apologize for that aspect. It's difficult for me to read some of what is posted here. Most of you make great sense and I learn every day here and so appreciate the site.

    Happy turkey day... For you too Richard....

  14. Richard Beyer | | #14

    Martin the next time your in CT look me up. I'll introduce you to this homeowner and allow you to argue with him over what made him sick. I'm just the messenger and reporting what I witnessed.

    Maybe mold is a hoax to rob people of their money? Maybe you have an answer as to why there's a mold exclusion in all of our insurance policies? I now wonder what all of those mold remediation companies do in their down time since mold is such a rare occurrence in homes?

    Within that same link you posted Martin it states the following;

    "How common is mold, including Stachybotrys chartarum (also known by its synonym Stachybotrys atra) in buildings? Molds are very common in buildings and homes and will grow anywhere indoors where there is moisture. The most common indoor molds are Cladosporium, Penicillium, Aspergillus, and Alternaria. We do not have precise information about how often Stachybotrys chartarum is found in buildings and homes. While it is less common than other mold species, it is not rare."

    This is what the American Lung Association has to say about mold;

    Exposure to mold can trigger allergic reactions and asthma symptoms in people who are allergic to mold. Researchers are investigating whether damp indoor environments and mold may actually cause upper and lower respiratory problems.4 Furthermore, anyone -- with or without allergies -- may experience irritation of the eyes, skin, nose, throat and lungs when exposed to airborne mold particles.3

    Mold has also been linked to:

    Worsening of asthma
    Nasal congestion
    Sore throat

    An uncommon disease known as hypersensitivity pneumonitis has been associated with exposure to indoor mold in people who have weakened immune systems.1, 3, 5 This disease creates flu-like symptoms that may recur.

    Nice as always to hear from Dr. AJ Builder... Love Ya to! ;)

  15. wjrobinson | | #15

    Richard. For weeks you miss the point of my posts and Martin's. We both agree with the fact that some homes can have various moisture issues.

    But you post like we are all, most homes, are going to sicken. The percentage of homes with built in moisture problems is not high if materials are used properly and homes are manager properly and the owners aren't deficient in healthy genes.

    If someone has health issues they may need a special environment. We agree.

  16. wjrobinson | | #16

    I'm not a doctor. I am a builder with decades of builds behind me that have 100% happy healthy inhabitants.

    In my book experience is everything. You are in the business of finding problems. I am building homes less the problems and will continue to do so.

  17. Richard Beyer | | #17


    I think the problem here is how you are interpreting my posts. I'm not saying ALL homes will have trouble. I'm saying if you do not control your environment your likelihood of having trouble is high when you build for energy efficiency. I also provide examples of people I come across who are having trouble and actual issue's I personally find in homes. I do not charge for this either. These issues just have a way of finding me.

    Recently I was in a home where the ionizer was malfunctioning in a doctors home. The air cleaner was emitting high levels of ozone. It was clear to me as I entered the home. My purpose of being there was to make a granite countertop template. In the end I was thanked for averting a potential long term lung problem. The doctor actually thought the odor was a sign of clean air and thought this was normal. His wife's burning eyes and tight chest wasn't even a tip off because he was unaffected.

    Most all of my business over the past 30 years is in existing homes. I have the opportunity to see things the average new home builder will never experience. That's what makes me the crazy guy. ;)

    Maybe the Building Science Group on LinkedIn would interest you. Almost every other headline is doom and gloom relating to moisture problems in homes, fires, IAQ trouble and modern building practices with a whole lot of crazy! Maybe they are crazy to? ;)

    Here's the most recent topic.... "HVAC Humidifier Contamination in Unsold Homes"
    and a nice article Mr. Butler posted relating to molds and other illnesses caused by moisture and contaminated water sources via humidification systems attached to the HVAC pipeline.

    But mold and health issues are rare! Professionals working in the field say otherwise.

  18. Cotiliar | | #18

    Gentleman - wow - truce?

    I am a member of a Corvette forum - a number of us can tune a Vette "to out run the word of God".
    We cautiously give advice to "younger" members - (the power of a Corvette can kill you as it is). But us old timers typically want to provide answers to questions (usually posed by newbies) to help them with their mechanical issues - and typically close with "be careful".

    The nature of this post/thread was to help a guy like me to "tune his walls".... Yes; it's not talk of rich or lean, trouble codes, Michelin or Bridgestone, Calloway or Hennesey. But of air flow , to promote/discourage dryness/moisture either from the outside or the inside.

    I posted the first picture in this thread with thoughts of the DIY'er (like me). A clear depiction of what I did. Was it right or was it wrong? In a true way - I wanted the other DIY'ers to see if they too could simply do the same, should do the same, or not do the same.

    I am certain the last half a dozen replies to this thread it has left them "scratching their heads"
    I was personally scratching my head when I came to this site, posed a question and looking for a solid answer.

    I take nothing away from anyone that has replied.... "Time will witness what the old folks say". (experience) Perhaps crankshaft end play, a distributor too far advanced or retarded, or a bad HUD is easier to define and correct. Yep, even the guy/gal in Mile High - Denver has issues due to altitude, and cooler air.

    To all of this - I saw no mold in my walls before my demo - I did what I did - I have elevated R/H levels in my living space and I am trying to rectify this condition through crawl space encapsulation.

    To all the other DIYer's with thoughts of doing what I did? Well if you own a Corvette and have issues send me a note - I will do the best to help you and tell ya to be careful.

    Martin - if you read this I enjoyed your post on "Cooking a Turkey" I have a sister that is a 4 Star Chef. (really) I sent that to her because I bore them with my emails and my energies of my house projects, telling her "cooking a turkey is like building a house" (casually)(smile).

    Her reply was she read the thread half way through. She say's out the window with "spherical" she typically prepares 20 turkey's at a time, all sectioned because the white meat and dark meat all cook at different rates.

    Man- How did I end up on this Web Site (smile)
    Polyiso - EXP- XPS, Corvettes, Turkey's, Mold, Dementia,

    Thanks everyone and have a safe and Happy Thanksgiving

  19. Richard Beyer | | #19


    Your so kind! At least we found one thing in common. I tooooo Loved My Datsun and also believe Corvette's are JUNK! My father has a Vette sitting in his garage with 600 miles on it parked next to his Mercedes and BMW. He never drives the Vette! I always wondered why.

    Now that we are smiling again, do not inhale to much CO2 on Turkey day! ;)

  20. Cotiliar | | #20

    For the record the Datsun 240 Z (Z car)was/is a great car - however; prone to mold growth beneath the non breathable poly headliner. '70 was the first and worst year for this, the following years were less prone to this do to a more breathable/perforated poly headliner.

    The tires despite having great "air circulation" (rrrr) would also develop sweat between their inner walls - this condensation would lead to a slime build in the interior cavity of the tire and the bias ply would tire would deteriorate from the outside-inward do to a lack of the "wicking affect".

    The dual side draft carbs were somewhat tough to balance, the rich/lean scenario would produce a vapor lending itself to toxic emissions inside the cabin - many would air seal the firewall with grommet plugs to stop the emissions - but in turn slow the dissipation rate from the cabin, creating even further an
    in hospitable environment for the occupants.
    The internal combustion combustion power plant was best satisfied by a leaded gasoline product which we all know now depletes the ozone. In turn can cause cancer and other ill health affects.

    The doors, trunk lid and rear wheel wells in this purely mechanical machine were hollow the noise levels could or would cause hearing loss to the occupant, the factories petroleum based sound absorber's were insufficient and some turned to a foil faced membrane to dampen the sound. The petroleum sound dampening patches in these doors would also deteriorate in different ways in different climate zones "4C" the Goldie Locks Zone usually experienced rust behind these patches.

    The heat from one's "ocole" (ass in Hawaiian) would produce moisture into the seats hay like cushion fibers, these fibers would retain moisture in turn rot and mold. The later years moved to a closed cell type foam less friendly to the environment, but better at preventing moisture saturation and mold.

    AJ - I need to go for a drive after all that scare stuff I just went over. But it's raining.... I do need a form of transportation.(smile)

    Happy Thanksgiving - William

  21. wjrobinson | | #21

    William, nice post, stick around, we definitely are in need of a mediator.

    Richard,, I do not want to go to the doom site you mentioned. I have come here to learn how to build super insulated and I love the Maine-iacs PGH pretty good house movement. Also I do rely on my experience and Martin's for decades and am very glad I may have helped drag Dana over here from another site he was regularly posting on.

    As to ozone... and whatever... off topic... not interesting. I know nuts and avoid them. Yes I am one too... by the way... Richard... you like to forget the second o... when you post too.....

    As to homes that can handle improper levels of humidity for example... I know of a design that can and has been around for decades... check our Bruce Brownell... he builds with exterior rigid foam and no infill insulation.

    Read up on Bruce, great guy... a neighbor and fellow designer builder. And he doesn't obsess over sick homes sick people... etc... that's your thing Richard... enjoy... I'd rather obsess over getting a bit more glide out of my next hang glider. First one had a 2-1 glide ratio... they now are close to 20-1.

    As to corvettes... have to mention this.. the worst car ever produced. My Datsun 240Z was my favorite toy years ago and I even got to use it for transportation. Would love to have one again.... find me one William

    May all your turkeys taste great... all.

    ozone.... you're killing me Richard

  22. wjrobinson | | #22

    I had a '71 Z Will I am... your ride looks great by the way... I so wanted to Scarab my Z.... You probably know all about that... putting a V8 in it... someone used to sell a kit... there is a great old car mag. to do with the Scarab.

    I so loved going up and down the gears in my hay wagon... and the manual choke was the best thing in the cab... I loved pulling choke to start up... feathering it back... the big straight six under the hood... and the reverse opening bonnet.... the side covers.. for the brake master and the battery... the sheet metal was so thin... you could dent it by looking wrong at it.... she spun a lot of tires for me... would go the whole oil change miles without need a drop of oil.... I used Castrol... had to search far and wide for it... nice clear oil... was told only to change with Castrol...

    Silver... not the best paint... but oh did I love that auto... drove it between here and Daytona Beach where I attended my flight university... Arab sons driving Ferrari... learning to fly the college Lear. Harley Chopper parked on the sand... while surfing... that's the way to go to college.

    Raining here... have to go renovate a boiler I installed twenty five years ago... that's my only related to GBA... thought... for now. Will... great Z post... didn't have any of those issues... but neat to see the list.

  23. Cotiliar | | #23

    AJ - I am glad you didn't have any of those issues with your Zcar.
    I heard "silver paint"
    "Choppers and Florida"

    Scarab - if you wanted a Vette - you should have bought one (smile)
    Silver paint (not the best) in longevity terms I would have to agree - UV rays and motling...
    Florida - Children s Miami. (a nice day in my careeer)
    Surfing - from Hawaii - a wonderful part of my life.
    College - Waianae HI school of rough waves and hard knocks.

    Second Generation spray painter/ Aviation.
    Retired from painting jet liners with Boeing
    Choppers - Keystone Helicopters Westchester Pa.
    Bluegrass Army Depot - Richmond KY.

    GBA - please forgive this banter - consider it an introduction of a newbie.
    I am MOST certain there is a wealth of knowledge here. I do value the wisdom.

    Off to work - a border to protect Blaine/Bellingham WA

    Salute and thank you to you all.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |