Insulation voids and condensation

[alaska teardrop]

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

Of course Andrew, you are correct. In Jeanett's case, she can tape just the gaps and glue to the extruded polystyrene and glue/screw to the frame. Thereby, accomplishing both objectives that she raised.

In Karen's case she didn't mention which type of insulation that she planned to use.

The polyethylene sheet should be used as a vapor barrier for the following:

Expanded polystyrene board

Expanded polystyrene bead

Fiberglass batt

Cellulose

Rock Wool

Perlite

Vermiculite

I personally wouldn't recommend any of them for use in a small trailer because of the water absorption and/or moisture damage factors. Also, creating a truely effective vapor barrier in a trailer with a wrap would be very difficult and time consuming.

Foamed in place urethane is a good choice if it's closed in, all the air pockets are filled and the inside of the wood framing is covered with a vapor barrier.

Urethane Isocyanurate (R-max) with the foil face inward is an effective vapor barrier. However, the edges and every cut in the foil needs to be taped or otherwise sealed. Also, it's not very structural and tends to break when bending.

I have used the foil bubble type insulation, but I don't think that I will again. I'm not sure of the R-value claims in cold weather when installed in a trailer in the way that I used it. Maybe, someone else is more knowledgable and has an opinion about it's use in a trailer.

Reference: www.uaf.edu/coop-ext/publications/freep ... -01454.pdf

Tracy wrote: I duct taped around the outside of my insulation to framing.

Duct tape isn't considered a vapor barrier because moisture effects it's adhesion and probably permeable by water vapor. The Venture tape is made for the purpose. It will stick and stay on insulation, wood, steel, aluminum and polyethylene sheet.

I really should claim no expertise other than paying close attention to and study of insulation and vapor barriers for building homes in the arctic.

Hope all this helps.

Fred

[asianflava]

I'm actually fairly amazed at how rigid those floppy 1/8" skins became after adding the framing and insulation.

I was actually able to stand on mine when it was propped up on each end with 2X4s. Mind you, I couldn't dance around or anything but then again, I'm not (as doug says) a svelte guy.

Keep going, getting the body shell together is the most exciting part.

[Artificer]

This didn't go through this morning... lets try again.

If you use extruded polystyrene (pink or blue) or isocyanurate (white or tan with facing foil/fiberglass) and fill all gaps with Great Stuff or other expanding foam, I don't think you'll have a problem. The foam itself is a great vapor barrier. I actually tend to cut my foam a little bit small (1/4" on each edge) to allow me to shoot some of the expanding foam in.

I believe the tape is redundant, and not necessary. On house construction its easier/faster/cheaper to tape the joints. Who would want to go around every crack with the foam. 30 seconds with tape and a knife, and the sheet is sealed. On the other hand, you can take the time to ensure a good foam seal on your trailer. It has the added benefit of bonding the foam to the framing.

Rob: I've always wondered: in a heating area like Wisconsin, why do contractors use foam and seal/tape all joints on the OUTSIDE of the house? I know about the vapor barrier on the inside, but the outside taping seams to be a moisture trap.

[alaska teardrop]

I believe the tape is redundant, and not necessary. On house construction its easier/faster/cheaper to tape the joints. Who would want to go around every crack with the foam. 30 seconds with tape and a knife, and the sheet is sealed. On the other hand, you can take the time to ensure a good foam seal on your trailer. It has the added benefit of bonding the foam to the framing.

Artificer - True if you are using the insulation board as an inside sheathing (enclosing the wood or metal framing). However, if you only insulate between the framing (in house terms: between the studs-rafters-floor joists) you haven't created a vapor barrier. The framing is still exposed to the moisture from inside and it's dooming damage.

Fred

[48Rob]

Artificer wrote:

Rob: I've always wondered: in a heating area like Wisconsin, why do contractors use foam and seal/tape all joints on the OUTSIDE of the house? I know about the vapor barrier on the inside, but the outside taping seams to be a moisture trap.

You are not the first to have that concern.
Under ideal circumstances the second layer can act as a vapor barrier, in theory trapping moisture in the cavity.
Studies have shown however, that unless you have a very poor, or nonexistant vapor barrier inside, that condensation rarely forms on the cold side (in the cavity next to the exterior wall).
It amounts to keeping as much warm moist air in (the living space) and as much cold outside air out (of the cavity), as possible.
When those goals are met, the cavity performs well as a dead air space.

There are still so many studies, and theories floating around that the questions of "how, when, and why" are hard to define with a "one size fits all" answer.

Some advocate no barrier, some a breathable barrier, some suggest air infiltration control will negate the effects...

My answer? Seal the exterior well, use a vapor barrier or retarder, and remember ventilation is the key.
Control the humidity level, and there is no issue.



As Fred pointed out, all the interior sealing won't stop water vapor from being transmitted through wood framing, but, if the warm moist air is properly contained inside the structure, and there are no exterior leaks, there will be no excess moisture inside the walls to do harm.

There will always be some moisture inside a wall.
The air always has some moisture in it.
As long as the air inside the wall, and the air outside the wall are the same general temperature, the moisture won't condense, and be deposited inside the cavity.

As one might imagine, those who are successful in getting a perfect seal, are those who will soon have very stale air, venting is a must!

[GPW]

How about recycling some bubble wrap as the insulation /vapor barrier combo ...need it thin , pop the bubbles...??? Would be cheap and seal with tape ... Silly idea huh ???

[spyderhead]

I am thinking of adding - having an auto upholstery shop - add a cloth headliner over my wooden ceiling. Would that make the condensation problem better . . . or worse?

[Gary T]

I was thinking of using ice and water shield, a product used in roofing, as vapour barrier.The hotter it gets the more it sticks. It seems like it would be a great vapour barrier as well as making a water proofer

http://www.graceathome.com/pages/roofin ... ?WT.srch=1

Any thought?

[alaska teardrop]

Spyder wrote:

I am thinking of adding - having an auto upholstery shop - add a cloth headliner over my wooden ceiling. Would that make the condensation problem better . . . or worse?

Spyder - Depends on what it's made of, I suppose.

Gary wrote:

I was thinking of using ice and water shield, a product used in roofing, as vapour barrier.The hotter it gets the more it sticks. It seems like it would be a great vapour barrier as well as making a water proofer

Gary - Also known as Bitchithane. It is definately a vapor barrier. I have seen it used to wrap window & door openings with the inside edges taped with Venture tape (Canadian product by the way) on the inside. My concern would be what kind of vaporous fumes that it might release into your trailer!

Fred