Build Science 101: #9 Vapor and Vapor Control

In this episode Matt & Steve discuss the enigmatic world of vapor in building construction. While vapor and moisture are ubiquitous, they often evoke confusion and misconceptions. This episode highlights the importance of understanding vapor's behavior, emphasizing that vapor itself isn't problematic-condensation is the real concern.
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[@DrSerendip]

We have seen houses, built in the 90’s, with the poly vapor “barrier”, that caused significant problems with condensation. We remediate mold and mold issues that cause health issues and building failures. The fiberglass insulation, which is hydrophilic adsorbs the moisture and the insulation is now saturated and enough to wring out the water from the insulation. Thanks for a great series that helps not just in building but in helping to correct issues homeowners run into.

[@MrJohnboyofsj]

I've got a house build in 68 at some point was wrapped in Styrofoam on the outside has old fiberglass insulation in a 2x4 wall. The other day it was -4 C outside, the ambient air temp inside was 24 C humidity 50% and the wall surface temp was measured at 15 C at coldest in the corners. This is where I get occasional mold growth, and along the ceiling line or if any furniture is pushed up against the wall. Unfortunately the only retrofit solution I can imagine is tearing down the drywall and putting in sprayfoam to form a tighter envelope to replace that old fiberglass, increasing my wall surface temp reducing the risk of condensation and mold.

[@robertlebaa5482]

I really enjoy this show and am into the building science movement. I am not a home builder or even a remodeler per say. I have a restoration company focused on roofing, siding, and windows/doors. I would love to hear about better ways to rebuild according to building science on these restoration projects. Obviously when doing a full residing job with new windows it’s easier to see how to implement the modern products, but when doing repairs or partial replacements is it even worth “fortifying” that partial area? Anyways, please consider the restoration guys and show us how to do our jobs better.

[@markstipulkoski1389]

I live in Florida with high humidity. My attic is vented, lots of soffit ventilation and much less ridge ventilation. My attic is extremely humid but my a.c. keeps my house near 50% RH. There is no vapor membrane whatsoever in the ceiling. Just paint, drywall, and blown-in insulation between me and my 140F 90% R attic. So the problem here in the humid South is keeping the humidity out, not trying to remove it as might be the case in Boston. I wish I had a vapor membrane with a low permeability in my ceiling to help keep the attic humidity out of my living space. Opposite of getting interior humidity buildup to the cold dry exterior. The climate zone needs to enter into these discussions.

[@ralphjean-pierre309]

Community: I'm currently in the process of constructing a new home for my family and myself in Minneapolis, MN. I'm deliberating over whether to incorporate a vapor barrier before moving forward with drywall installation. Our building envelope consists of Zip sheathing 2.0, complemented by R17 EPS exterior insulation and R23 Rockwool insulation within the interior cavity. Additionally, we're considering integrating a Zehnder ERV system into the design. I'm particularly interested in the importance of including a vapor management, specifically a smart vapor management like INTELLO PLUS.

[@enriqueramos-castro2710]

you guys are thes best , can you do one video on how is the best way waterproof a flat roof on a house ?

[@speedbird300]

I think Steve is wrong about the ceiling, it’s all part of the management approach. As ping as the interior air of the house has active management like MVHR then a barrier is a good idea - there’s probably insulation over the ceiling and you don’t want lots of moisture going up and condensing in the cold roof void even if it’s vented.

[@andreycham4797]

I would agree with you too . Besides that that poly works as air barrier which all together allows attic insulation to perform much better.

[@TheDanishGuy60]

Great videos, but no background music/noise - PLEASE.

[@superfunkysungod]

This raised more questions than answers for me. The main one being do i put plastic over my insulated interior walls and ceiling, or not?

[@Dancing_Alone_wRentals]

Exactly. Say anything and add " Joe " and the discussion is closed.

[@nikojansen9648]

“Vapor can be a liability but it also is an asset” - great great building science quote.

[@stipcrane]

"Vapor permeability" is a better term. Vapor per se is not an asset to building materials.

[@pacodefrancis7235]

Great video Matt and Steve. Such an important and misunderstood topic. We all appreciate it!

[@raystormont]

Vapor management is easier to understand if one knows that managing vapor is more about heat transfer than water all condensing surfaces are being heated by/during the transition of vapor to water and conversely all evaporating surfaces are being cooled by evaporation. Conditioning the air for comfort creates differences in inside versus outside temps. humidity between the inside and outside of a house will naturally attempt to equalize these differences unless steps are taken to replace these heat transfers either by adding heating/cooling and humidity control. Therefore most situations exterior surfaces will attempt to prevent water and water vapor from from moving into the structure and if carefully managed interior surfaces will allow/encourage water/water vapor to migrate inside providing a path for water in all its forms to dry/ equalize the conditioned spaces, this means that the interior wall temps. must be equalized with the interior air temp. as closely as practible to allow drying to occur w/o condensation. Ray Stormont

[@mikejarrells431]

Thanks guys. I'm a noob doing my first fixer upper remodel. I'm using your videos to educate myself so I build smarter, not harder. Right now I'm doing a crawl space encapsulation with poly (barrier?). You say avoid vapor barriers (poly), but you say air management is a higher priority than vapor management. Why not use poly as an air management layer? Will it encourage condensation & mold? Right now I'm using a paper vapor retarder (older leaky house) in zone 4. Current building theory suggests buildings in this zone need to breath (dry both ways).

[@SciaticaDrums]

The rule I go by is that in the North you want the vapor barrier to be on the inside of the home, in the South you want the vapor barrier on the outside.

[@alvaro_bf]

Matt, after a video like this, have you changed your mind about materials such on kzbin.info/www/bejne/roG7lKGfpLt1fNksi=ovMritCvS1eUgNle wich was labeled as non permeable and efficient?

[@macoveialex6823]

I love this serios. All the informations needed in one place.

[@marcodm]

Im in Montreal. I think barrier is fine for ceiling. Makes house more air tight. Winter is very dry so vapor is nice, summer is very hot and humid. AC drys it out

[@james.telfer]

In that case, why not have a warm roof (attic is conditioned space) and put the vapour barrier between the roof and outside air? Then you reduce winter heat loss and the entire building is the same humidity and more air tight. Also means attic space is usable year round for storage and access.

[@marcodm]

I could, just means I have more space to heat/AC. Its not really that high so maybe just for storage which I really don't need. I heat 5 to 6 months so it matters

[@Dancing_Alone_wRentals]

Okay, So what was the problem with the plastic sheet across the roof? tHanks all

[@gpappas5833]

I would never put plastic behind drywall. I've seen many service calls where the insulation is wet and moldy. Don't trap the moisture.

[@Dancing_Alone_wRentals]

After reading the comments.....I'm baffled and resort to facial expressions 27:33

[@bryanthomas544]

So no plastic on ceiling, and only perm vapor barrier on interior or exterior wall but not both?

[@valdius85]

Thank you for tour hard work.

[@levijones7453]

What app is Steve using on his IPad?

[@rozinant1237]

You might want to be careful endorsing a concept of no vapor retarder at the ceiling plane. Both the US and Canadian building codes require it. Both of these organizations have done huge amounts of research in this area. Vented attic spaces are intended to ventilate excess heat not moisture. You yourself say that any humidity would be equalized down to a minimal point as soon as it mingles with outside air ( in the attic). Excess heat in an attic space can be damaging to roofing materials.

[@stevenbaczekarchitect9431]

They are tied to climate zone, as I said in the video

[@ladsonbeach5438]

Steve’s final comment regarding poly on a ceiling got my attention also. Relative humidity equilibrium is the key concept here. In a warm humid climate, such as we have here in South Carolina, outdoor relative humidity will almost always exceed a comfortable indoor humidity level. I would think that some type of vapor control membrane in the ceiling would be required, especially if thermal control is taken into consideration. Great video dealing with the fundamental concepts of building envelope design.

[@ebros3]

For majority of people that can’t afford a complex roof system and goes with asphalt shingles, but still want a tight house with Zip and spray foam, haven’t heard any discuss the fact that roofing manufacturers are excluding unvented roofs from warranty coverage. Suggestions? Looking at SmartBaffle, but that will create a massive challenge to air seal everything before insulation applied.

[@greg925911]

Ok, so, I'm going to go out on a limb here, I have a 7x14x 6 trailer and I have a air gap in the ceiling with thermo duct a r12 , it's amazing just how much Condensation is build up with 2 people sleeping over night, that after 8 hrs of sleeping, that it starts to drip with the Mini split at 70° and in the low 30's, do I need to close that air gap up or have more air to be moving in that area

[@Dancing_Alone_wRentals]

Didn't they say that water will condense on the cold surface? Does that mean that the outside cold is reaching your ceiling ...cooling it to the point where inside air is condensing? If I understood them, that trailer roof does not insulate well enough. How would you close that air gap?

[@TomDAwn1]

maybe not a vapor issue per se, but what is your opinion on the following - older house near Memphis, TN about 125 yrs. no A/C but they run dehumidifiers (but not all the time) in the summer. They're having mold problems. Is my thinking flawed - if moist air wants to move to drier air like warm goes to cold could dehumidifiers be making the RH problem in the house worse? There is an attic fan. Would air movement maybe be a better option than only running dehumidifiers occasionally? Thank you

[@billsnyder6945]

I watched these videos, and no idea the right answer for remodeling a bathroom in an 80s house in Colorado Zone 5, a very arid climate, but still not sure what is the right thing to do is. The outside wall had poly, guessing 6 mil. Parts of the ceiling to vented roof has some too. Do I remove it, keep it - fixing any holes, or replace it with something else, and what would that be?

[@tlangdon12]

Vapour is your friend! While water is in vapour form, it is easy to move and get it out of the house before it becomes condensation.

[@stipcrane]

I like your twist. H2O is the entity, vapor is Dr Jekyll, the healer; condensation is Mr. Hyde the killer.

[@bols82]

What about below deck of a tub, I’d like to put mineral wool the one exterior wall opposite of spout side. Balloon framed. Kerdi board is going above deck. Thanks

[@rikmorel4870]

I'm currently building a small RV camper made with 2x3 wall stud construction and 2x4 studs for the roof, plywood sheathing with a Soprema SOPRASEAL® STICK VP WRB, Rockwool insulation in the walls and pink insulation in the ceiling. The outer skin will be RV aluminum. I'm about to add poly on the inside walls as an air barrier and my thought is that the Soprema wrb will allow any moisture that penetrates in the wall to exit via that membrane. Thoughts? The outside roof material is EPDM rubber so what about putting poly on the inside roof? is this even needed?

[@jonerlandson1956]

what's the order?... water... air... insulation and vapor?

[@nikdoznamej7698]

depends on the climate zone. in hot humid climates the humidity is on the outside, while in cold climates in winter the humidity is on the inside

[@mojodojo5533]

Do I get some kind of professional credits watching this? If not WHY!!

[@jesinbeverly]

I’ve noticed that interiors need to be at least primed in order to get a certificate of occupancy. Is this because paint is considered a control element?

[@rozinant1237]

Latex paints have a perm rating. It’s possible they meet the standard of vapor retarder in your area.

[@stipcrane]

Latex paint has a perm rating of about 10. Vapor retarder primer has a perm rating of 1. It used to be part of Washington's energy code, but it wasn't ever inspected in earnest and builders quit doing it. I only do the ceilings under attics now. We've had occasional problems with mold/mildew growing on the underside of the roof sheathing on vented roofs. It could be simply horrible air sealing around recessed can lights and bath fans. Whether mold is caused by air flow or permeation, I don't take chances.

[@speedbird300]

Ping->long. Also familiar with barrier that stops liquid but vents vapour, as per roof membrane, but not selective directional vapour, be interested to hear more about that and related detaiiling.

[@charlesviner1565]

👍

[@gastonignacioarias1894]

SALUDOS DESDE ARGENTINA . EXELENTE APORTE A LOS AMANTES DE LAS CONSTRUCCIONES . GRACIAS POR EL APORTE

[@Vladthelad129]

As somebody located in a very humid and cold Nordic environment, I see it being near impossible to leave out the usage of damp proofing membranes as you are inviting mold creation in the construction. We would avoid using even smart barriers since it is not guarenteed that temperatures get high enough during summer moths for a period long enough for them to release the accumulated dampness in the constructions. There have been some recent successful experiements with super-tight prefabricate housing that allows for them to be left out, however we do not know yet how these will perform in the next few years.

[@inventor1214]

Do you put the membrane on the outside or inside of the insulation?

[@Knappconstruction]

First!

[@shawnshurtz9147]

You should probably use it to ask something profound, not be a tool.

[@dbarrington1]

your just mad he beat you... lol@@shawnshurtz9147

[@ridethetalk]

@buildshow I'm considering using SIPs with colorbond steel skins - these are likely going to be a vapour barrier i.e. impermeable - is this a bad idea?

[@stipcrane]

Probably depends where and how you use them. Alaska had a bad experience with SIPS used as cathedral ceiling/roof structures. Humid air rises, and in a vaulted ceiling will collect at the highest point and want to escape (or equalize, whatever your terminology). Few if any were able to seal that joint between the panels 100%. Vapor created lots of condensation in the cold temps. The OSB rotted quite quickly, and the roofing would fail as well soon after.

[@gypsytoday]

Uggh, HeLP! i am under construction (12×32) in Alaska. 2x6 walls with: tyvek/ 7/16 plywood/ R21 fiberglass batts. DIY build. I have 6 mil poly almost everywhere. Had to do some wiring rework where I didn't have poly up for approx 2 weeks. Sub freezing temps hit and i noticed condensation behind insulation where no poly was. How worried should I be that this has the ability to dry/or be absorbed by plywood?

[@rozinant1237]

The whole reason you use Tyvek is because it has the ability to breathe out any trapped moisture in the wall. When the building envelope heats up again it should be released.

[@CMbassin]

I have a similar problem here in VT. Trying to redo my interior in winter and if I insulate a wall and go to bed and come back to work on it the next day, it’s wet on the backside of the plywood. I think it’s very important in a cold climate to install 6mil poly and be very detail oriented when installing it. I caulked it on and sealed around any penetrations that may allow air/vapor ridden air into the wall. Going to leave one wall with just the VB on it and peel it back in a couple months to see what I find. If not moisture or mold then I’ll know especially after drywall is installed over it we’re good. This whole thing has made me re think the holes in exterior walls we make when we hang pictures and things

[@CMbassin]

I bought a home here in VT. There was no WRB on the OSB sheathing from the beginning. Rot under the windows and deck let mice and bugs in. I replaced the sheathing, siding, doors and windows and deck. The sheathing is Zip system caulked on at the perimeter, nailed and taped. Now I’m trying to re do the inside and I have the drywall down. I can’t insulate the house more than one wall at a time without the exterior sheathing condensating on the back side if I don’t install 6mil poly as I go and it’s got to be caulked on and rolled. I’ve decided to install an hrv system to help deal with the vapor in the air but trying to live here and do the project in winter, by myself has been really difficult just because of vapor and not being able to close the walls up in a hurry.

[@kk-pz8gn]

what does hrv system stand for? thanks

[@CMbassin]

@@kk-pz8gn heat recovery ventilator. ERV is Energy instead of heat and more common down south.

[@nannesoar]

nice background music

[@Mcv2023]

What about tyvec on outside does that breathe

[@rozinant1237]

Yes, it breathes water vapor and blocks water droplets. That’s the reason you are paying big bucks for it.

[@deanpeterson3495]

This is how it was explained to me in regard to vapor barrier in the ceiling: Here in MN with the cold winters and the far higher RH inside the house there is a great deal of latent heat energy in the water vapor. By just letting it out the ceiling and out the roof vents you are increasing energy loss. Air exchangers can recoup much of the latent heat instead of letting it escape up and out. I suppose a retarder would perform fine but poly in the ceiling with a properly functioning attic are the rule here.

[@walterhiegel3020]

So lets assume you are correct. My question to you would be how does the moisture inside the house get into the attic so that its excellent ventilation properties can remove that moisture? I am just trying to understand. I am not a builder just a dumb homeowner with an engineering degree who lives 30 miles from the gulf coast where the humidity is normally above 60% and usually higher.

[@rozinant1237]

@@walterhiegel3020Be very careful! What works in MN does not work in Florida. Consult builders in your local area, to see what works best in your locale.

[@coolestrock2008]

Yet again, i would like to point out that scientifically, water vapor is air. You *cannot* have a membrane that is an air barrier but not a vapor barrier. Steve and Matt, i love that you are getting rid of the word barrier for vapor. But that same logic should apply for air as well. because vapor is present in air. its not a separate thing.

[@stevenbaczekarchitect9431]

So what do you say to Tyvek - It is basically vapor open, but is an air barrier? Or SIga Majvest?

[@Jaker788]

You can absolutely transfer moisture from air into separate air through some type of barrier. Just look at ERVs, they use a vapor permeable heat exchanger to allow some movement of vapor, but there is no air mixing whatsoever. The air barriers that allow some vapor permeation are as far as you can measure and as far as it matters beyond tiny amounts of potential diffusion, air tight. But they allow a small amount of moisture in the air content to diffuse through the barrier to let things dry out. It's a very small amount compared to nothing, but it's a lot more than vapor barriers.

[@Living_Despite]

Native tribes also practiced enslaving rival tribes so maybe some things were meant to end. It wasnt genocide, it was Conquerism. Y'all lost. Get over it.