R value of polypropelene honeycomb core with glass skins.

Discussion in 'Materials' started by Steve W, Feb 28, 2024.

  1. Steve W
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    Steve W Senior Member

    So, i have been trying to find information on this subject online and coming up empty so figured someone on here may have better research skills than i. We all know that the various different foams seem to offer R values in the 5 -6 range. Insulation foams like xps or polyiso are readily available and cheap but are not structural. Corecell, Divinycell etc have about the same R values as the insulation foams but are less available and expensive. For me the PP honeycomb is readily available, reasonably priced and easy to process when making sheet goods. However i have been unable to find a solid R value> I don't think the manufacturers are able to give this info because, unlike foam, the core itself would have no R value since it is open air all the way through, but it seems to me that once you have laminated skins on each side you now have trapped / non moving air in all the cells and i think that still air has about as good an R value as you can get other than exotics like VIP. The thermal conductivity of PP is very low so thermal bridging between the skins should be minimal, similar to the pvc foam for example. What makes the foams such good insulators is in fact trapped air. Any thoughts?
     
  2. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    Why?
    What is the purpose?
    Double pain windows have an R value of ~2 with nothing more than trapped argon.

    Did you ask the manufacturer what the R value is?
     
  3. Steve W
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    Steve W Senior Member

    Yes, i have asked Plascore, the mfg of the core but since they don't make panels they don't have that info. If they tested the core they would not get much of a value because there is zero trapped air until you laminate it to make a panel. R values are typically given per inch of thickness, i don/t know what the gap is between the panes on a window but if it were, say, 1/4" with an R value of 2 that would mean R 8 for an inch, better than any insulation foam. I don't know what the argon brings to the table. My reasoning for why this is of interest to me is that in the boatbuilding world nobody seems to care about insulation but a lot of people out there are building things like teardrop campers and the like where they have much less structural requirements but more insulation requirements. I have used a lot of PP honeycomb over the years and find it a lot easier to get and much more budget friendly than foam. I like it.
     
  4. Rumars
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    Rumars Senior Member

    It doesn't work that way with gases, the R value sinks constantly with increasing thickness, and stabilizes from one inch upwards. If you want to know more, search for R value of still air layer.
    The manufacturers have solved this problem, you can get foam filled honeycomb panels. Not to forget, you can use PU foam directly as a core if you like.
     
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  5. fallguy
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    fallguy Senior Member

    The R value of plascore will be relatively low. In the noise for them to bother with...

    The problem with plascore is the core itself will transfer heat.

    I'd say a good guess is 1.5-2 for one inch glassed both sides.
     
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  6. ondarvr
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    ondarvr Senior Member

    As fallguy says, it's so low that it isn't worth researching or advertising on their part. Plywood has an R value, but it's not considered an insulating layer.

    There is an RV manufacturer that uses this type of panel for building small trailers. They claim an R value but I can't remember what it was.
     
  7. Steve W
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    Steve W Senior Member

    Thanks guys for the responses. I think perhaps i have not done a good enough job of getting the point i am try to convey across so i will try again.
    1/ To be clear, i fully understand that the core itself, ( without any skins) is not going to have much, if any R value, simply because the cell structure is wide open to the atmosphere, so it stands to reason that the manufacturer is not going to test for, or publish an R value.
    2/ On the other hand, once you laminate skins on both sides to make a structural panel everything changes. Now you have trapped air which is a good insulator and in fact that's what foam is, and less thermal bridging between the skins.
    3/ Using say, H80 PVC foam such as Divinycell which we are all familiar with as an example which has an R value of 5 /inch i think i read somewhere. Foam is nothing more than PVC resin with a foaming agent to introduce air bubbles. In the case of structural foams the prime objective is to give an adequate bonding surface and density to meet their structural goals, while keeping the weight low. With insulation foam like XPS the goal is different as they don't care about the bonding properties but they do need to keep the price low. In both examples the R value is about the same.
    4/ A well insulated panel needs basically 2 things to be successful. maximum trapped air and minimum thermal bridging. This is where i'm having trouble, to me it seems that all things being equal a pp honeycomb panel should be better than a foam panel (with the same glass skins) as it has more trapped air and less area for thermal transfer between the skins.
    5/ PVC foam has a thermal conductivity of around 0.0374 W/M-K (watts /kelvin/meter) This means nothing to me other than Low = good insulator high= good conductor. I want a good insulator.
    As a base line, softwood like pine is about 0.12 W/M-K, so, about the same as either PVC or PP in their solid form, while aluminum is way up there at 235 W/M-K
    Polypropylene plastic (not foam) which is what the cell walls of PP honeycomb is made from is 0.11 W/M-K while solid PVC is about the same at 0.12 W/M-K.
    6/ From what i have been read, air is the insulator with all common insulation materials, the best seem to be foam simply because the air is trapped in the bubbles, it can't move, hence the high R value. Spray foam is a bit better at R 6 -7. Why? I have never seen an explanation for this but if i had to guess i would say there are 2 reasons. 1/ They are PU foam and PU solids ( this is what is creates the thermal bridging between the inside and outside ) have lower thermal conductivity than the other foams at 0.022 - 0.028 W/M-K and 2/ spray foam bonds to all the surface meaning there is no air movement at all, not even by convection.
    7/ Trapped air is only effective if it is indeed, trapped in small spaces, like in the form of bubbles in foam or sealed cells in honeycomb. This is not the case in, say, the wall cavities of a house.

    So, all the above is why i feel that a PP honeycomb panel, inch for inch, when compared to a comparable foam panel should be, at least as good, if not better as an insulator. My only problem is that i need to find where a panel manufacturer, not the core manufacturer has a published R value. I have not found this yet as they all seem to concentrate on the structural properties.
     
  8. Blueknarr
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    Blueknarr Senior Member

    R values increases with the number of bubbles between the outer surfaces.
    Foam sheets have higher r values than spray or pour foams because the size of bubbles can be kept small. Spray/pour foams often have large bubbles that limit insulation properties.

    Uninsulated walls have low r values because of the single air space allows air molecules to migrate from one surface to the other.
    Honey coam core will have a similar r value as an uninsulated wall. For the same reason. The honey coam only limits tangential air movement. While allowing unrestricted movement between outer surfaces.

    Edit

    The cell walls on honey coam will increase thermal conduction.
    The cell walls in foams lower thermal conduction by their long irregular route.
     
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  9. ondarvr
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    ondarvr Senior Member

    I wasn't confused by your question, I understood you wanted to understand the R value with a laminate on both sides. It's low, to the point of not being listed or counted. A hollow wall in your house would mean it's uninsulated, but might be .5
     
  10. Steve W
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    Steve W Senior Member

    Everything i have been able to find has commercial spray foam with a HIGHER R value than any of the sheet foams and it makes total sense to me since it is PU foam . I have not compared it to PU sheet foam though as i don't consider it to be commonly available. The solids in all of the foams are essentially a thermal bridge between the skins, not much you can do about that but at least PU has a lower thermal conductivity than any of the others. I have not used spray foams myself but have used a lot of pour foams from 2lb/ft3 buoyancy foams to 16lb/ft3 Proset epoxy foam for rudder building and not seen a noticable difference in cell size to the sheet foam, nothing like EPS, that's for sure.
    As an aside, i just found a panel manufacturer ( Fiber-Tech) who do give an R value of 5.5 per inch. This is for PP honeycomb with 17oz glass each side. This is in line with the common insulation foams like xps and polyiso or the structural foams like Divinycell and Corecell. I do plan on calling them and asking questions. About the only way to be absolutely sure would be to do test by making a box, maybe a foot cube with various sample panels for the 4 sides and fill it with ice cubes, let it sit for a few hours and the measure the temps at the center of each panel.
     
  11. Steve W
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    Steve W Senior Member

    I don't think so, a hollow wall in a house is not sealed and completely air tight. It is not glued together and the studs from cavity to cavity are peppered with holes for wiring and/or plumbing so allowing for the movement of air. If the air can move from the warmed inside and scrub off the heat to the cold outside. From what i've been able to glean in order for air to be an effective insulator it needs to be contained into small spaces where it can't move, like the air bubbles in foam or the cells in honeycomb. All common insulations are based on dead air, or at least keeping the air in a space from moving. I don't have any particular project in mind but i have been sitting at home recovering from knee replacement surgery and watching a lot of you tube and watched a bunch of people building teardrop campers or truck campers. Most are either made from plywood or out of insulation foam covered in canvas in Titebond. I have not seen many/any proper composite ones done with a structural core and glass and i understand why, H80 foam is expensive, not locally available for most people and shipping is insane. Resin and glass ships easily. In my area i can order honeycomb in half sheets today and have it tomorrow and the pricing is great so for making up flat panels i find it more practical. I would rather use foam, mainly because it is more practical to infuse but it is harder to get unless i'm placing a big order where i can justify shipping.
     
  12. ondarvr
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    ondarvr Senior Member

    I do understand, the wall comparison is because even if the wall is sealed 100% it's still not considered insulation, the R value is almost irrelevant.

    The reason a hollow insulated mug works so well is because the cavity is a vacuum, once the vacuum is compromised and the space is filled with air, the insulating value drops to comparatively nothing.

    I'll look for the RV specs, they used a double layer of honeycomb core so it created two air pockets in the wall. It still didn't have a high R value though.

    If you've been looking at RVs and DIY insulation options, Reflectix comes up frequently, and has the same basic construction as honeycomb, with skins on each side. You'll see some claim a high R value, but they're using the reflection R value, not its actual R value as an insulating layer, which is very low, and adds almost nothing in a build.
     
  13. fallguy
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    fallguy Senior Member

    I think you are confusing a couple things. An airtight compartment is not R-value. The main source of heat loss is via open drafts. A tight building envelope does not allow drafts, but also has a terrible downside. It does not allow air change, and so odors never leave the space. And so, air change is good and even more important in boats. My boat has a terrible issue with air exchange. And it lacks an exchanger. I have a plan to execute, but my boat had bigger issues to resolve last year.
     
  14. Steve W
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    Steve W Senior Member

    My understanding is that dead air spaces need to be small, less than a half inch, if you completely seal a large space, like a wall cavity in a house. You will still get heat loss because the air in the large space is still able to circulate within the cavity because of convection due to the heated inside wall and cold outside wall. If that air is contained, as in the bubbles in foam or the cells in honeycomb it can not circulate. Absolutely, even if a 16" x 8' wall cavity were completely sealed it would still not be insulated. There are still old houses around in the frozen north that have little to no insulation and are still reasonably heatable.
    As for Reflectix they do actually claim an R value of 1.1 for their product even if is used incorrectly without the air spaces. if this is the case, it looks to be about 1/4" thick so that would give it an R value of 4.4 for 1" which is higher than fiberglass batts and less than foam.
     

  15. DogCavalry
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    DogCavalry Senior Member

    Are you thinking a small HRV system @fallguy ?
     
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