Insulation voids and condensation

[jmtk]

I read that voids in the insulation can cause condensation within walls and floors that can lead to rot. How big of voids are we concerned about here? I built up my doors this weekend and have the inevitable gaps between the framing and insulation from cutting the insulation a tad too small. I'm talking little gaps of about 1/8" or less. Do you bother filling up these little cracks?

Thanks,

Jeanette

[GregB]

Jeannette,

I would pick up a can of spray foam insulation (low expansion) at HD and fill those voids, wait until they dry in minutes, then cut off the excess. The thing with insulation is that a 10% void gives you a 90% reduction in R value. It pays to fill those joints up. The foam has the added benefit of adhering well to your skins and current insulation.

GB

[alaska teardrop]

Jeanette - If you are insulating with the pink extruded polystyrene as you previously mentioned, you can fill the voids as Greg suggested and then use vapor barrier tape to cover the gaps. The extruded polystyrene is a good vapor barrier as is and will absorb the least amount of moisture. The goal is to close every single hole from the inside to the inner wall including around the window, wiring, bolt holes, ect. Two pics in album as example.

Fred

Link to tape: www.venturetape.com/final/construction_ ... asp?id=459

[48Rob]

Hi Jeanette,

Filling the voids with spray foam is worth the effort.

Having voids in itself does not cause condensation and resulting rot, but failure to have a well sealed vapor barrior does.

Condensation occurs when warm moist air contacts a cooler surface.
It condenses, and soaks into whatever is available.

Just filling voids isn't enough, a barrier is required to stop the vapor, which can get around cracks and small voids with ease.

Heavy sheet plastic works well, and is a very inexpensive way to insure all your hard work doesn't end up rotten.

In a heating climate/situation (anything other than a dry desert) the vapor barrier is installed on the inside, toward the living space.

In a climate in which the inside is always being cooled, and the exterior is always hot, the barrier goes on the outside.

Rob

[alaska teardrop]

Rob - Ah, a man that understands the importance of a vapor barrier.

Especially with all that beautiful wood!

More info: www.uaf.edu/coop-ext/faculty/seifert/publications.html

Go to publication EEM-00259

Fred

[jmtk]

Thanks for the vapor barrier info, Fred and Rob, however this brings up a concern for me. I'm using pretty thin skins (1/8" inside and out) and believe I need to rely on good glue adhesion between the skins and pink insulation to achieve a rigid wall. Adding a layer of plastic over the inside before skinning eliminates that bond. Now we don't live in a humid climate like Alaska or Illinois, but we certainly exhale plenty of water vapor while sleeping overnight. Of course we'd have a window or vent cracked open, but that won't get rid of all the vapor. Am I doomed to have a lightweight but rotting-from-the-inside trailer without a vapor barrier? I've looked at a lot of build albums and plans and can't remember coming across vapor barriers before.

I appreciate the info, but now you've got me a bit worried!

Jeanette

[48Rob]

Jeanette,

My apologies.

To have you worry was not my intent.

Yes, in theory, you can avoid condensation issues with proper ventilation.
The key is "proper".

Teardrops as opposed to travel trailers have less condensation issues because the cooking is done outside.
However, as you are aware, just being alive creates a huge amount of warm moist air.

The general humidity levels do have an impact, but when dealing with such a small space, they really can't be counted on.
A few degrees difference is all it takes for warm moist air to condensate on a cooler surface, and as you can fast understand, warm sleeping/exhaling bodies can quickly raise the temperature and humidity level in a tear.

All is not lost, just keep the ventilation a high priority and you will be fine.

Some teardrops do not have a vapor barrier because the builder did not know about them, did not care, or felt it wasn't needed.
Some are constructed with open framing, and thus do not need one.

With no offense or harm meant to anyone, it must be realized that the vast majority of people building teardrops are not professional home builders...

Then on the other hand, a lot of brand new RV's roll off the assembly line destined to have water issues within a year...

Anyway, there is no set in stone way/method that one must follow to build a tear.

The exchange of information from this vast pool of experts is what makes the board concept so great, everyone is really good at something, and is willing to share.
My intent was to provide information to help you decide which route may be best for you.

Rigid foam in itself makes a pretty good vapor barrier...if you seal all the voids.

Keep a window or vent partly open when occupying the tear, and "air it out" in the morning.
Remember that wood can get wet and not be damaged...as long as it can dry quickly.
(The wood hidden behind walls with no air movement stays damp...)

I think you'll be fine with your current plan, after all, you realize the importance of proper ventilation!

Rob

[alaska teardrop]

Jeanette - That's the beauty of extruded polystyrene and the tape. Seal the gaps where needed, screw to the framing where you can, and glue the inner panel to the insulation where you can.

As Rob pointed out and the link to UAF shows, the problem isn't the humidity of the outside air, but the difference in temperature between the inside and outside air. The greater the difference in temperature, the greater potential of moisture migration into the inner walls and roof.

BTW, the interior of AK is very dry! In terms of annual precipitation, it would be considered a desert if the ground didn't have ice in it. All that nice fluffy snow doesn't have much water in it. If the world keeps warming, it will become a savanna!

Fred

Guess Rob & I were typing away at the same time. Rob - you win the typing race.

[jmtk]

With no offense or harm meant to anyone, it must be realized that the vast majority of people building teardrops are not professional home builders...

Keep a window or vent partly open when occupying the tear, and "air it out" in the morning.
Remember that wood can get wet and not be damaged...as long as it can dry quickly.
(The wood hidden behind walls with no air movement stays damp...)

I think you'll be fine with your current plan, after all, you realize the importance of proper ventilation!

Rob

I definitely fall into the "not professional home builder" category

Whew, you had me going there for a while! Nonetheless, I appreciate the info. OK, I'll use some expanding foam and make sure it's well ventilated.

Jeanette

[iamcarriesue]

Has anybody tried using some "Tyvek" for their vapor barier?

http://www.tyvek.com/whatistyvek.htm

Construction
One of the most popular and widely known applications of Tyvek® is in the construction industry, where it is used to increase air and water resistance, helping to lower heating and cooling costs in buildings and providing better protection against water and moisture intrusion. The unique qualities of Tyvek® help stop air flow through wall cavities; help hold out bulk water and wind-driven rain; and allow moisture vapor to escape from inside walls. The result is a more comfortable, energy-efficient building with far fewer chances for damage from degradation effects

[alaska teardrop]

Karen - Tyvek is used on the outside of buildings to keep wind and rain out. Usually under lap siding. I don't think most builders would consider it for an interior vapor barrier. If you're going to use a wrap type barrier, a 4 to 6 mil polyethylene sheet (the plastic that Rob mentioned) would be a better choice. It still has to be totally sealed with tape and/or a special sealant.
www.tremcosealants.com/commercial/produ ... .asp?id=32

Fred

[angib]

I'm using pretty thin skins (1/8" inside and out) and believe I need to rely on good glue adhesion between the skins and pink insulation to achieve a rigid wall.

You will also be adding quite a lot of framing inside that wall - it will need framing around the edge, around the door opening, where the galley bulkhead will go and where you want to attach any heavy items (interior cabinets, hatch gas strut, etc). This is quite a lot of framing - even if it is all made of 3/4" x 3/4" (ie, '1 by 1'), that's quite enough to connect to the two skins together, provided both skins are glued to it properly. Gluing to the foam as well will increase the strength, and support the thin skins, but it's the bond to the framing that really matters.

Andrew

[angib]

If you're going to use a wrap type barrier, a 4 to 6 mil polyethylene sheet (the plastic that Rob mentioned) would be a better choice.

It would also prevent the structure either side of it from being glued together.

Having an outer skin attached to an inner skin seems more important to me than having a vapour barrier between them, but maybe I'm wrong.

Andrew

[dacrazyrn]

I duct taped around the outside of my insulation to framing

[jmtk]

Gluing to the foam as well will increase the strength, and support the thin skins, but it's the bond to the framing that really matters.

Andrew

Andrew,

That makes sense, and yes, I've got framing planned for all the weight-bearing places. I'm actually fairly amazed at how rigid those floppy 1/8" skins became after adding the framing and insulation. My doors have 1x2s around the outside edges and the window. I don't even have the inside skin on yet, but I'm really pleased with how the doors have come out so far.

Jeanette