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Velocipedic 01-11-2018 11:26 PM

Wood as Thermal Break
 
I'm looking to install wooden ceiling paneling on the bus, and I'd like to avoid losing headroom (I know, East Coast CB, I have a lot to begin with... :whistling:)

Basically, What I'm thinking of doing is screwing 2"x2"s to the aft side of each horizontal rib, spray foaming the ceiling between the ribs, and then installing wood paneling (1/4" thick at minimum) on top of all of it, attached to the 2"x2"s so there's no metal touching metal. As I plan it, the ribs will have minimal foam over them in order to maintain the luxurious headroom I have.

Will 1/4" thick pine paneling possibly resting on the ribs be enough thermal break, or will the wood conduct heat/cold unbearably? I could go an easier route with FRP, but I don't like the aesthetic.

Opinions/thoughts/comments?

gobygoby 01-12-2018 02:44 AM

This is a hard question to answer because it is so subjective. It really depends on your goals.

I used styrofoam and spray foam between the ribs then 1/2" furring strips on top of the ribs, then poly panel or 1/4" whiteboard material attached to the furring strips. This arrangement is good enough so that my heaters and air conditioners can keep up and I can be comfortable.

I started in Texas, but I live in Utah now and when it snows, the snow can pile up pretty high on top of my bus. After a few days when the snow starts to melt, I can actually see small indentations in the snow pack on top of the bus corresponding to the ribs and the skylight. So the heat transfer is faster through my roof ribs than through the rest of the roof, but not by a ton.

I have 5" of Styrofoam in the lower walls and 3" in the upper walls and roof, plus 2" in the floor. I did a 39" roof raise so headroom wasn't a huge concern. I was more concerned with value. Our plan is to live in the bus for 4 years total (1.5 years already done). Insulation is a big fixed cost up front but lowers the monthly cost each month over time, so finding the right balance was important for us.

My gas bill is about $30/month in the winter and air conditioning is about $50/month (electric) during the summer. Utah springs and falls are very comfortable so those numbers drop considerably for those months. And I'm always making little improvements here and there in the insulation.

Twigg 01-12-2018 07:44 AM

The wood will make a decent thermal break.

Avoid metal to metal, at all costs. Some heat will transfer through the wood, but you will have reduced the problem about as much as is practical.

jazty 01-12-2018 10:42 AM

If you'll be in cooler climates (hovering around or below freezing) then you'll likely get condensation and eventually mold growing on the section of the 1/4" wood panelling that touches the metal rib. How do I know? Experience :) I have a few small areas around the rear door where oak veneer plywood is directly over top of the steel door frame.

The FRP won't mold, but it will do no better in regards to condensation.

The bare minimum that I would do is 1/2" rigid foam between the ribs and the plywood.

Velocipedic 01-12-2018 01:30 PM

Spray Foam the Ribs?
 
Thanks for the responses so far, all! I do plan on possibly being in freezing areas.... yannow, any Air Force Base is possible.

Jatzy: Please forgive my ignorance, but how compressible is the DIY spray foam? If I spray .5" on a rib (1.5" elsewhere) will I be able to apply my ceiling and not worry about condensation? I believe this will alleviate the thermal bridging issues as well....

I know that applying a barrier directly to the metal will just retain moisture between them, so that's right out.

Twigg 01-12-2018 05:10 PM

Quote:

Originally Posted by Velocipedic (Post 244799)
Thanks for the responses so far, all! I do plan on possibly being in freezing areas.... yannow, any Air Force Base is possible.

Jatzy: Please forgive my ignorance, but how compressible is the DIY spray foam? If I spray .5" on a rib (1.5" elsewhere) will I be able to apply my ceiling and not worry about condensation? I believe this will alleviate the thermal bridging issues as well....

I know that applying a barrier directly to the metal will just retain moisture between them, so that's right out.

It has good enough compression strength for what you need, but getting it even would be a challenge.

The other issue with your proposal is that the metal screws will still go directly from the inside of the bus to the ribs, so expect icicles from them when it's cold outside.

The suggestion to use wood is so that you can screw or bond the wood to the ribs, then screw the ceiling into the wood. That breaks the direct path from outside to inside.

Velocipedic 01-12-2018 05:52 PM

Thanks for the info on the foam.

Panels will be screwed into the 2x2 which is on the side of the rib. Therefore there will be no metal touching the screws.

Twigg 01-12-2018 06:09 PM

Quote:

Originally Posted by Velocipedic (Post 244890)
Thanks for the info on the foam.

Panels will be screwed into the 2x2 which is on the side of the rib. Therefore there will be no metal touching the screws.

Sounds like a plan :)

rffffffff 01-12-2018 06:23 PM

I think it sounds fine... I screwed 1/4" cedar directly to ribs because of low ceiling height and have lived with what we call snow buddies growing from the screws during the colder days... the pieces that are touching the ribs with no screws seem totally okay.

Ultimately I'm not sure that anything I have seen is good enough for real permanence... I've been told that rvs have vents on each panel to let the air out from behind the insulation but you would need a lot of headroom to make that work on a skoolie because there's rarely enough air movement behind the insulation.

I call this our mistake skoolie because I'm learning a lot but our next one is getting a roof raise and a better ceiling system than this one!

Robin97396 01-12-2018 06:34 PM

Quote:

Originally Posted by rffffffff (Post 244905)
I think it sounds fine... I screwed 1/4" cedar directly to ribs because of low ceiling height

I almost did the same thing on my ceiling, but decided to put a 1/2" foam panel between the rib and the wood to stop moisture penetration.

joeblack5 01-12-2018 09:14 PM

From a thermal bridging standpoint gobygoby has the best approach because whatever bridging is there is only on a small surface area. Not sure if he used insulation between his wood and the ribs to improve even further.

Later J

Juicifer 01-16-2018 08:23 PM

Just asked myself this question a couple days ago as Im at the same point in my build.

Found this resource: https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

And discovered extruded polystyrene foam is about 5 times less of a thermal conductor than wood.

Based on that I put 1/2 inch extruded polystyrene foam strips between my ribs and furring strips (which are 1x3s running lengthwise attached under the ribs so theyre 3/4 thick) and the foam compressed to 1/8th of an inch. Ill lose about 1 1/8 to headspace and will have close to 2 1/2 inches of closed cell foam.

I thought running them lengthwise makes the most sense because there is minimal wood touching metal at all. Still doing a 1/4 inch Luan ceiling. My headspace will go from 77 to 75 inches in the end. Anyways youve got so much headspace, pick the best option.

joeblack5 01-16-2018 10:01 PM

Juicifer,Excellent. Looks to me you have maxed out your design. Are you going to spray foam after you put the furring strips up? closed cell 2pcf is a pretty darn good glue.
In our small bus (Elfie) that has a screwed aluminum ceiling I would like to peel the ceiling back but leave it attached on one side. Then spray a little in between the roof skin and the inner skin. Put a couple of screws back and then foam again. In that way every thing gets bonded together. ( In the aircraft industry people are obsessed with structural strength)
I would imagine you could do that very easy with tongue and groove as well.

later J


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