Vapor Barrier

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Man are we ever getting close to that day.

For those of you who are new here, that day is the day when we have the Momplex full insulated, and it doesn't matter if it's -50 degrees outside, if the wind is blowing 80 MPH, if it's snowing . . . even if it's freezing rain. That day is the day we can work without -100 below boots and three layers of down. That day is the day we can work without gloves, when batteries can charge, and tools still work.

It's going to be a big day for us!

That day also is the day we get to officially start on the interior. Because today, and all the days up to it, even if we have been inside, sure don't feel like we are working inside.

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Author Notes: 
<img src=" " style="width: 470px;" alt="" /><br /><p></p><p>Now we can trim off excess.</p><p></p><p><img src="" style="width: 470px;" alt="" /></p><p></p><p>You just use a utility knife to trim it off, leaving a few inches or so to overlap the walls.</p><p></p><p><img src="" style="width: 470px; height: 400px;" alt="" class=" selected" /></p><p></p><p>I know this seems like a fairly simple task, but everything is so much harder when you are working over your head on a ladder. &nbsp;And the Momplex roof is two houses in one - about 2000 square feet. &nbsp;So it definitely was quite a task getting her done!</p><p></p><p>With the vapor barrier complete, what does that leave us with?</p><p></p><p>Yep, we get to hang drywall next!</p><p></p><p>And after drywall, we will blow in insulation and the Momplex will be fully insulated! &nbsp;That day is so close!</p>
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We've got the electrical boxes in the ceiling, and all other utilities needed to be run through the ceiling installed. Wires have been run. The ceiling itself has been blocked as needed for hanging drywall. It is FINALLY time to finish off the roof! Today is vapor barrier day. Now before I get into this process, know that ever climate and type of building requires different insulating and moisture control. Case in point - our walls are ICF and no vapor barrier is required. So check with your local codes and find out what works in your climate for controlling moisture. Where we live in Alaska, this is what happens if you don't have a vapor barrier in your ceiling or your vapor barrier is installed improperly: Moisture builds up inside your house just by living in it (showering or boiling water can put cups of water into your house!). The moist air escapes the house through the ceiling and becomes frost in your attic when it meets -40 below air in the attic. The frost builds up through the winter. When spring comes, the frost melts, and the water saturates your insulation, and drips through your ceiling into your house, creating water damage and mold problems. Vapor barrier is a good thing!
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But here's the problem. When you put the vapor barrier up with staples, the stapled holes create opportunities for the moisture to escape through the ceiling. You can go back and tape the stapled holes, but then when you put drywall up and screw it on, you've got more holes in your vapor barrier. I know this isn't a huge issue in other climate, but up here where half of the year it's winter, and on a winter day, there can be a 120 degree temperature difference between the inside and outside of your home (yes, it can be 70 degrees inside and -50 outside), you'll start seeing every single screw hole in your ceiling building up frost. So what we do is put this tar stuff up on the studs in the ceiling.
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This stuff never dries. It's like bubble gum that never dries. It is awful stuff. Do not get it on your shoes or in your hair, it will never come out.
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The tar stuff gets applied to all the ceiling studs.
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And it drips down, creating landmines of tar on the floor. I'm really careful to not let this stuff drip on my hair - having my hair washed in gasoline or the like doesn't sound like a good day at the spa.
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For the vapor barrier, we roll it out on the floor.
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And cut it to length with a utility knife.
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Then the vapor barrier is spread out in the big room.
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And brought into the room it will be installed.
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Careful not to get tar on it!
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Then you put the vapor barrier over your head.
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Climb up the ladder
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And start stapling it up. You want to pull tight here and do a neat job. We start in one corner and work our way to the other.
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It gets stapled to the studs in the roof, over the tar stuff.
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And then all the way over to the other corner.


So are you saying the the Tar, if spread throughly enough across the ceiling joist, should seal up any holes that will be created by hanging the drywall. But if there isn't enough tar coverage where screw goes, it will create a point for moisture to get through. Thanks for All your posts Love the ambition and advice.

Most people say the tar stuff is overkill because once the drywall compresses, and is finished, in theory, it should be a complete seal. But if/when this doesn't happen, you'll end up with little dots on your ceiling where each screw hole is, and then frost build up ... and then mold. We are definitely overbuilding the Momplex because we only want to do it once, and do it right. Curious - do milder climates have to be this extreme?

So the tar stuff will seal all the staple holes and the drywall screw holes?

Makes me glad I life where the big temp diff is in the summer.

Yes - when the drywall is screwed on, it - for lack of better term - "smushes" the tar across the truss and in any holes/gaps on the joint, preventing moisture leaking through screw holes. The stuff is nasty to work with, but does the job!

Can you still spray foam the attic and sealing walls, before winter, or is it to cold? If not I think you would be impressed with the results even if you do it next summer

Just curious if you looked into spray foam for the ceiling insulation? By it's nature the spray foam is it's own vapor barrier.

Thank you! About the spray foam - we definitely considered it. It's a great option and would have done the trick. But it cost $$$$$ and we would have had to hire a contractor with the kits, instead of just DIYing it with blow in insulation. We do go up after the drywall is hung and spray foam all electrical boxes, vents and any other holes - poor man's spray foam if you will!

Great post. What would you do if you pretty sure there is no vapor barrier and you have already had problems with condensation in the attic area? There must be something a DIYer could do to help with this problem.

Hi Steph, sorry to hear you are having condensation problems in your attic. If you have no vapor barrier, I would suggest that you pull out the insulation and spay foam for a tight seal. The other thing that could cause condensation in a attic in inadequate ventilation. A attic should be able to breath, and able to take moisture away. I have seen people with moisture problem just because there was no ventilation. I would check the ventilation first then look in to spray foaming second. Might just be simple as adding a vent,

Start by thoroughly sealing all the air leaks around anything that penetrates the ceiling (dryer vent, ceiling wiring boxes, especially any can lights) and then the walls (baseboards, door frames, light switches and power outlets). That goes a long way towards keeping the attic dry.

Make sure the soffit vents (under the eaves) haven't been blocked by insulation or debris. These can be added fairly easily.

Check for a main attic vent - several types are used. Sometimes an over-enthusiastic DIYer covers these over to "save energy" because they aren't thinking about moisture build-up. A cold dry attic is a good thing.

Ridge vent (long skinny one at the peak of the main roof - must be installed along with the roofing!)

Gable vent (hole with louvers in the attic gables)

Roof vent (like a stove pipe, but just the through the roof part, will have some sort of a cap on it) Roof vents have to have a chimney extension on them that is tall enough to clear any expected snow buildup. These can be added fairly easily, you just have to be careful to get the flashing installed correctly - it slips UNDER the shingle above the vent so water can run across it with no leaks.

Improving Attic Ventilation -,,194780,00.html


Adding a vapor barrier can be done, but it's a nasty job. You basically remove a section of the old insulation (lift the batts or rake the loose stuff away) then lay down the barrier and replace the insulation. Then you do the next section. :(

There is also vapor barrier paint - this goes on the ceiling inside. If you follow the directions for how much to use and apply it carefully it supposedly really helps in older houses where you have existing insulation.

There are DIY kits for spray foam. I've heard there's a steep learning curve, but for a project as big as the Momplex, it could be worth it (not saying *you* should change horses midstream, just if someone else is planning a large project, it's worth reading up before deciding).

Are you going to install a radiant barrier along with your insulation? It seems like that would make a big difference in an Alaskan winter.

Also, the last 3 images are broken for me. No one else has mentioned it, but I tried a different browser, and they didn't work there, either.

It was my job to squeeze all the black tar over everything! It was nasty. I got it all over me (but not my hair). But it was worth it to know that our house should be well sealed. We also ran a bead of it along the seams where the ceiling touched the walls (and since we did wood walls, also on all the exterior walls and where the walls touched the floors.). We also put it around each electrical box that was surrounded by vapor barrier as well. It was a nasty messy sticky job. A little tip I learned - if you get it on the floor, cover it with a piece of plastic so you don't track it everywhere (like in your vehicle, etc.).

Extreme vapor barrier tip: after drywall is complete and before blowing insulation, light up the living space with as many construction lights as you have, go into attic after dark. Any missed drywall screw holes will be obvious and you can seal with more acoustical.

yes, to my fellow commenters, thanks again for an awesome step by step process story, and for the info about the tar.
and happy thanksgiving, ana, the spirit with which you solve problems and the generosity of this whole project is really something to be grateful for. thanks to you and yours.

Are you going with cellulose or fiberglass? I know cellulose has a higher R-value but I talked to an insulation installer once who swore by blown-in fiberglass. His argument was that while cellulose has a higher initial R-value, it settles over time and the R-value drops. Fiberglass has a slightly lower R-value but it stays constant over time. He uses fiberglass but puts a greater amount down to get the needed R-value. Have you heard this theory? What are your thoughts on this?

The purpose of a vapor barrier is to hold up the migration of water vapor, which are not classically planned to retard the migration of air. This is the function of air barriers a vapor barrier on the tepid side of the cover must be joint with a venting pathway on the freezing side of the lagging the better the vapor barrier and the drier the circumstances, the less venting is necessary.