3" or 1" - sub floor query

[Shambhala Tinbolle]

hello everyone,

I'm starting to gather the materials for the sub floor on my bus conversion.
would this : 3" rigid foam Insulation 30 PSI
be acceptable to use? It's commercial grade.

I'm also looking at getting the underside of the bus spray foamed.
if i go that route, would 1" rigid foam Insulation 30 PSI suffice under my 5/8" plywood???

Thought I'd get some input from those of you who have completed the sub floor stage.

with gratitude,

[Brewerbob]

Originally Posted by Shambhala Tinbolle

hello everyone,

I'm starting to gather the materials for the sub floor on my bus conversion.
would this : 3" rigid foam Insulation 30 PSI
be acceptable to use? It's commercial grade.

I'm also looking at getting the underside of the bus spray foamed.
if i go that route, would 1" rigid foam Insulation 30 PSI suffice under my 5/8" plywood???

Thought I'd get some input from those of you who have completed the sub floor stage.

with gratitude,

Any particular reason you are looking at spraying the underside and not just spraying from the topside? Much easier to spray from the top.

This guy has a lot of well done videos.

https://www.youtube.com/watch?v=vEyskuN4b_U

[Shambhala Tinbolle]

hi BrewerBob,
I'm spraying the top too. Just want to focus on the rigid foam choice for now.

[Greatestrr]

I think 3" is way overkill if your infact going to spray the under side. You could put firring/ sleepers up to spray foam that cavity. Then put your 1" rigid over that then your plywood. To answer your question 1" should be more than enough.

[Brewerbob]

Originally Posted by Greatestrr

I think 3" is way overkill if your infact going to spray the under side. You could put firring/ sleepers up to spray foam that cavity. Then put your 1" rigid over that then your plywood. To answer your question 1" should be more than enough.

I think the guy in the video said 2" ~ 2 1/2" of spray foam was enough. Anything beyond that was diminishing returns.

[PNW_Steve]

How much is enough??

Before you can answer that question you need to look at the environment you will be using the bus in.

If you are planning on traveling in more extreme conditions such as Phoenix in August or Fairbanks in February then I would suggest installing as much insulation as practical. If you plan on chasing comfortable weather then you may be plenty comfortable with considerably less.

I believe the "beyond X number of inches" statement is very dependent on the delta.

If you want to take a proper approach to insulation, heat and A/C sizing take a look at one of the online heat/cooling load calculators. With climate data, target temp and some detail on the space you can play with different R values, AC & heater sizing.

How to Perform a Heat-Loss Calculation — Part 1 | GreenBuildingAdvisor.com

[Brewerbob]

Originally Posted by PNW_Steve

How much is enough??

Before you can answer that question you need to look at the environment you will be using the bus in.

If you are planning on traveling in more extreme conditions such as Phoenix in August or Fairbanks in February then I would suggest installing as much insulation as practical. If you plan on chasing comfortable weather then you may be plenty comfortable with considerably less.

Yes and no. It isn't Phoenix or Fairbanks. The insulation doesn't care if you are trying to keep heat out or trying to keep it it. It's the delta from outside to inside that is important. Granted there is generally more of a delta on the cold side than the hot side. 70° -85° = -15° ... some of us may very well see -15°. The Canooks and MN folks in particular. Not sure you will find an ambient temp of 70° + 80° = 155° and if you do then there are going to be bigger problems.

I believe the "beyond X number of inches" statement is very dependent on the delta.

The guy in the video that suggested 2 ~ 2 1/2 inches was an estimator in the industry. I commented on wanting to use 3 1/2" because that's what a 2x4 on end measures. He talked me out of it (along with the cost). He also said the only time they ever installed 3" was when a customer demanded it. Something that needs to be considered is the product used for insulation. Closed cell foam is only so closed, it conducts heat at some level, and it costs so much per inch. Diminishing returns is an economics term. Your R value is increasing at a decreasing rate. The difference between 3 feet thick and 5 feet thick isn't that much. The trick is to find where the x and y axis cross... while not breaking the bank. You could line your entire bus in space shuttle tiles. Those are only 3 or 4 inches thick. Might cost you more than a couple of coach RVs tho.