Subfloor suggestions

[Rucker]

Originally Posted by musigenesis

I stand corrected lol. R∞ would be pretty dang toasty in wintertime.

I'd have to argue about R∞. Most of us use bluetooth to connect our phones to the speakers and that signal is going to heat up the metal portion of the bus.

:-')

Now I'm truly wasting everyone's time.

[Happy Feet]

Originally Posted by musigenesis

Polyiso gets used (and mentioned on this forum) all the time. One of its disadvantages is that its R-value drops precipitously in colder temperatures, making it most suitable for hot climate insulation. Or it can be used to advantage in a layered situation with XPS/EPS against the skin or floor of the bus and polyiso on the inside.

Just 1/2" of polyiso on the floor would be much worse than 2" of XPS. Even in warm weather (above 75°F) it would be R-3.5 vs. R-10, and that R-3.5 would drop the colder it gets.

Linking to the earlier discussion, I thought, would make it clear that "here" meant this thread. Apologies that was insufficient.

Studying the R-value curves, "drops precipitously" appears to mischaracterize the data. R values actually rise with falling mid-range temperatures, same as with X/EPS. Only at an inflection point in the mid-30s F do the values begin to decline, and not as "a knee in the curve," but sloping roughly 0.2-0.4 of R value per decade (10 deg F) ref Fig. 3 on that page (2" thick samples).

For those like us who will rarely go below 25 degrees, polyiso remains the better choice. (But not the anomalous "polyiso B," which will hopefully be easy to identify in the marketplace by lower price, etc.)

What I'd really like to know is, which of the four manufacturers makes the top-performing "polyiso D." Maybe a phone call can get the Building Science guys to say.

Lastly, no argument that 2" of either insulation would be warmer than 1/2" if ignoring all other heat loss, which is why I referenced the earlier post. If 90% of cabin heat is escaping above the floor, even super-insulating (R-50 or higher) the floor won't make much difference.

[musigenesis]

Originally Posted by Happy Feet

Studying the R-value curves, "drops precipitously" appears to mischaracterize the data.

I was referencing this source, which gives an R-value of 2.0 at 15°F (a drop which is definitely "precipitous"). The data from your source (which seems perfectly reasonable) shows less of an R-value loss but also doesn't go below 25°F. It appears that your source gives the net effectiveness of polyiso in practice, where the outside is at 25°F and the inside is 75°F (the temperature difference of 50°F mentioned in the article), meaning the inner part is warmer and thus more effective than the outer part.

That article also mentions what I suggested a way of using polyiso: "Use a hybrid insulation approach – install cold temperature-tolerant insulation over top of the polyisocyanurate insulation to increase the mean temperature of the polyisocyanurate."

Originally Posted by Happy Feet

Lastly, no argument that 2" of either insulation would be warmer than 1/2" if ignoring all other heat loss, which is why I referenced the earlier post. If 90% of cabin heat is escaping above the floor, even super-insulating (R-50 or higher) the floor won't make much difference.

I mean, it's true that if you don't insulate your walls or ceiling (or the front of the bus) it's not going to make a lot of difference what you use on your floor. I would certainly not advocate insulating only the floor.

[Happy Feet]

Originally Posted by musigenesis

I was referencing this source, which gives an R-value of 2.0 at 15°F (a drop which is definitely "precipitous"). The data from your source (which seems perfectly reasonable) shows less of an R-value loss...

Actually found that OC page first but their data seemed so out-of-line, I went looking for figures that were maybe a little more realistic. OC is one of the two titans in the multi-billion dollar X/EPS market (Dow/DuPont being the other) so slamming a competing product -- even using false data -- might be expected in today's business climate, unfortunately.

OTOH, the Building Science people have no such bias, AFAICT, even going so far as to mask the manufacturers' identities. So I'm placing my trust in their figures (unless a link to funding by the polyiso industry is found haha).

In any case, the two data sets are so divergent, there's definitely no way to reconcile. Extrapolating even the lowest part of the performance envelope (of BScienc data) to 15° (which is easy since linear) yields R-3.5+, approaching double OC's claimed value. The best-performing samples would be R-5.4, strikingly higher than R-2 and comparable to the X/EPS product. (All of this assumes, of course, that R degradation in that range remains constant.)

So my strategy will be to identify that top performing polyiso board and go with it. If not possible to ID though...we may just use the known XPS material.

Originally Posted by musigenesis

I mean, it's true that if you don't insulate your walls or ceiling (or the front of the bus) it's not going to make a lot of difference what you use on your floor. I would certainly not advocate insulating only the floor.

Key parts of DeMac's post (that I referenced) addressed this:

Originally Posted by DeMac

For insulating a steel bus with windows on both sides, there are other values which are more impactful...

Both heat and cold conduct through bus window glass...

The dense steel construction conducts the outside heat and cold more efficiently than most building materials...

Testing reports a 40-60% loss in R-value from not taping fiberglass seams. Just like a winter coat...

Which brings us to Convection. School buses have hundreds of hidden perforations, gaps, & holes that breath, suck, blow the temperament air right out. I could create an entire thread on seam sealer, fire calk, foam sealant, adheasives, patches and gap stopping. As far as I can tell, school bus manufacturers have zero concern for insulating a school bus.

Insulate all of the exposed steel & remove or cover as many windows as you can live without. Big gains.

Heat rises. The floor, in the absence of radiant heating, would be expected to be coldest. A balanced approach to insulating makes the most sense, we may agree, spending resources (time and money) evenly over the entire 'bus thermal system' proportional to heat losses/gains in each area.

[TheHubbardBus]

Originally Posted by musigenesis

That article also mentions what I suggested a way of using polyiso: "Use a hybrid insulation approach – install cold temperature-tolerant insulation over top of the polyisocyanurate insulation to increase the mean temperature of the polyisocyanurate."

Yep. That's what we did. I'm not displeased with the choice, but it was as much accident as intentional. Couldn't find 1.5" of anything in either polyiso or XPS. We went with 1" XPS against the bus sheet metal then .5" of polyiso inside that. The original plan was to use XPS-only, saving the polyiso for the walls & ceiling. But if 1.5" polyiso had been available and no XPS, I probably would have used it alone with zero regrets. In our case 1.5" was the depth of our sleepers (floor 'studs'), and hence the depth we needed to fill.

I imagine with enough number-juggling, you could get a pretty good idea of what the optimum combination would be (ratio of XPS to polyiso) to give the best thermal performance across an anticipated range of floor temp. Or if just using one or the other is best. I just don't think the very small amount of juice that might yield is worth the squeeze. Within 1 or so r-value/inch @ room temp (advertised), they're effectively the same as far as I'm concerned. If I chose one over the other it would be based on other factors if possible.

FWIW, we also covered our sleepers and all the cavity-fill with a final (continuous) layer of .5" poly-iso. So excepting the sleepers, we're at 1" XPS + 1" polyiso across our floor, w/ XPS on the cold side.

[musigenesis]

Originally Posted by Happy Feet

Heat rises. The floor, in the absence of radiant heating, would be expected to be coldest. A balanced approach to insulating makes the most sense, we may agree, spending resources (time and money) evenly over the entire 'bus thermal system' proportional to heat losses/gains in each area.

Heat does not rise - hot air rises, because it is less dense than colder air. Heat flows from hot to cold irrespective of gravity, at a rate proportional to the temperature difference. In a small enclosed space like a bus (and absent circulation from a fan or even the movement of the inhabitants), the air temperature might be a few degrees warmer at the ceiling than at the floor.

If you take the extreme example of 75°F at the ceiling and 65°F at the floor (which I don't think you'd really find in a bus, especially not with a heat source near the floor), when it's 25°F outside this means a temperature differential of 50° through the roof and 40° through the floor. So you would be losing 25% more heat through the ceiling than through the floor (actually a tiny bit more than 25% since the ceiling has more surface area).

Moreover, this difference becomes less the colder it gets outside, and it becomes less with the simple expedient of a fan to circulate air inside (if it's ever anywhere close to 25% in the first place). So the difference is really not enough to justify any major difference in the thickness of the insulation on any surface of the bus.