Fiberglass insulation on the roof

[MXD]

So I have a 7x16 Car Mate that I converted to a poor man's toy hauler. I recently installed a Mach 3 Power Saver and it does a great job but at the races this past weekend in the 90° beating sun, the interior would only cool to 78. It was still plenty comfortable in there but as a little experiment, I backed my trailer up into the shade for the afternoon and the temp dropped to 71 within 30-40 minutes. I currently have zero insulation. I used 2x4's to brace the roof for the weight of the AC so I have enough depth to roll out some fiberglass insulation in the bays and then finish it off with some 1/8 inch luan or something similar. Is there any reason I shouldn't use fiberglass if I have the room? Here is what the roof looks like and below that is a pic of the AC installed. I'm also going to coat the roof in Henry's.

[troubleScottie]

The big question is R factor. A great deal of the insulating power of fiberglass is the loft or depth of the material. Taking insulation for 6 inches of space and reducing it to 4 greatly reduces the insulating value. Also if it gets wet, it compresses and again it insulating performance reduced. Typically it does not dry out well. If it settles due to motion/vibration, you would again lose R value.

That is why people use rigid insulation like: Formular 250 2" 4x8 sheet R-10. It is R-5 per inch of thickness. It does not absorb water easily, can be cut to fit, etc. You might be able to do two sheets, so R-20 or 1 2" and 1 1 1/2" sheet, so R-17.5. The equivalent fiberglass for 2x4 construction is typically R-13 to R-15.

[tony.latham]

I think it'd be fine as long as you put a moisture barrier between it and the living space. (Between your luan and the fiberglass.) Otherwise it may attract condensation.

Tony

[MXD]

Scottie, my roof is one piece aluminum so I'm not worried about leaks. So far I haven't seen any sign of condensation on the inside of the trailer. What other sources of moisture could there be? My 2 concerns about the foam board is the temp of the roof and sealing tightly against all surfaces. The fiberglass can fill spaces more easily. As far as the temp, I've heard that the max temp for that foam board is in the 160 degree range. I spend quite a few nights sleeping in this trailer so I'm a bit concerned with the foam outgassing from overheating. Maybe I'm over thinking it.

[MXD]

I think it'd be fine as long as you put a moisture barrier between it and the living space. (Between your luan and the fiberglass.) Otherwise it may attract condensation.

Tony

Excellent, thank you.

[onehoser]

you might find roxul (rock wool) is nicer to work with than fiberglass, it's great for sound proofing, doesn't absorb water and doesn't itch like f/g

[QueticoBill]

If your predominantly in a cooling climate, I'd be inclined to not install a moisture barrier. It could become the condensation surface for any humid air that gets inside the skin. In a heating climate, maybe the moisture barrier on back of interior skin helps, since the inside air is the one with humidity and can condense on the inside of the exterior skin.

[pchast]

Your normal respiration provides a Lot of moisture in the air.
Personally I'd go with something that does not hold water like
fiber glass does.

[tony.latham]

Condensation happens when warm, moist air is cooled by a cold surface. If you have an impermeable barrier at your ceiling, below the insulation––where it will be warm––the moisture will never find a cold surface to condense on. It can't get there. If there is not barrier, it will travel through the fiberglass and find the cold aluminum and condense.

Now... that's just my opinion.

T

[MXD]

The trailer is only used from late spring to early fall so I don't ever heat it.

[QueticoBill]

Condensation happens when warm, moist air is cooled by a cold surface. If you have an impermeable barrier at your ceiling, below the insulation––where it will be warm––the moisture will never find a cold surface to condense on. It can't get there. If there is not barrier, it will travel through the fiberglass and find the cold aluminum and condense.

Now... that's just my opinion.

T

That's true in Idaho, where you heat more than cool and there is more moisture in the interior air, but in climates where there is more moisture in the exterior air, it's reversed.

In this case, with a barrier on the outside - the aluminum skin - there's no way for the cavity to ever dry out. Based on less than constant use, and not used in a heating climate, I'd avoid the added inside side barrier.

[tony.latham]

Bill:

"- there's no way for the cavity to ever dry out." Just out of curiosity, with your hypothesis, how would the moisture ever get in the insulation since it's sealed above and below?

Just a friendly question.

T

[McDave]

My biggest concern would be the potential for airborne glass fibres all the time. Unless those batts are hermetically sealed, then the potential exists for the fibres to shake out during transit and cover the interior and embed in clothes, blankets, carpet etc. If they ever settled, the next transit would re contaminate. You would need to have a barrier to seal the glass in under the luan. I don't know that would happen, but I'm pretty sure you don't want to suck that in while eating and sleeping.

McDave

[QueticoBill]

Bill:

"- there's no way for the cavity to ever dry out." Just out of curiosity, with your hypothesis, how would the moisture ever get in the insulation since it's sealed above and below?

Just a friendly question.

T

I've never seen a wall or roof cavity that didn't leak at some point in its life. I just think it's prudent planning to allow for this. Its a little moot in this case as the glue in the plywood is usually a pretty good air barrier.

[QueticoBill]

My biggest concern would be the potential for airborne glass fibres all the time. Unless those batts are hermetically sealed, then the potential exists for the fibres to shake out during transit and cover the interior and embed in clothes, blankets, carpet etc. If they ever settled, the next transit would re contaminate. You would need to have a barrier to seal the glass in under the luan. I don't know that would happen, but I'm pretty sure you don't want to suck that in while eating and sleeping.

McDave

Worth considering. I'd rather breath fibre glass than what XPS outgasses, but we all have different demons. And actually, I'd prefer to blow in cellulose into the cavity. Better air sealing than fibreglass and more resistant to radiant transfer.

Whatever material, it is best of it is in contact with the ply, more than the aluminum, so the ply is not a condensing surface. And if you do use foam, considering installing with an intentional gap - like a 1/4-3/8" - between pieces, and foam in a can to seal really tight. It will be a much better air seal than trying to butt two pieces of XPS - and easier to install.