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Community and Q&A

Spray foam over radiant barrier

Kyle Klouzal | Posted in Green Building Techniques on

I would like to combine a radiant barrier with spray foam in my attic.

The idea is to place bubble insulation with a reflective coating down on the underside of my roof (in the attic) and then add closed cell spray foam over that.

The choice in using a reflective bubble insulation is to obtain the necessary ‘air gap’ required for a properly functioning radiant barrier.

Either two layers of bubble sandwiched between a reflective coating on either side.
https://www.ecofoil.com/SSP%20Applications/Clickstop/ecofoil_sca_kilimanjaro/img/EF2220-tech-date-sheet.pdf

Or a single reflective coating sandwiched between a layer of bubble on either side.
https://www.ecofoil.com/SSP%20Applications/Clickstop/ecofoil_sca_kilimanjaro/img/EF1620-tech-date-sheet.pdf

I’m not sure which insulation product here would be most effective.

I live in a cooling dominated climate and want to keep as much heat out of the building as possible.

I’m worried here that using insulation A will act like shining a light between two parallel mirrors and in effect trap the radiant heat, where as insulation B will work to reflect the radiant heat back out more effectively, however, the datasheet for insulation B shows it’s level of emissivity to be 43% vs only 4% for insulation A.

Can anyone weigh in here on my concerns?

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Got a ZIP code? (For weather/climate data purposes, to fine tune this.)

    Is this new construction or retrofit?

    >"... then add closed cell spray foam over that."

    Closed cell foam (even HFO blown closed cell) is one of the least-green insulation materials in common use today, and in a cooling dominated climate buys next to nothing performance-wise compared to open cell foam (at less than half the polymer weight per R). Installing it between rafters pretty much wastes any R per inch advantage too- do the math:

    https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste

    https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

    The "air gap" in bubble pack isn't very big ("sub-optimal" would be a generous description) and is thermally bridged by the plastic welds forming the bubbles. If one side is in contact with the roof deck, the other in contact with spray foam, it's not doing much for you.

    Multiple layers of RB in contact with one another adds nothing to the performance- there needs to be an air gap between each successive layer. It really needs air gaps on BOTH SIDES to have an appreciable effect that's measurable without sophisticated instrumentation when the bulk R-value is at IRC 2018 code minimums or higher.

    >"I live in a cooling dominated climate and want to keep as much heat out of the building as possible."

    Then FORGET about radiant barrier, start thinking about "cool roof"- any roofing with a 3 year solar reflective index (SRI) greater than 50, will outperform any of your proposed stackups, and often comes with no up-charge. The Cool Roof Rating Council maintains a searchable product list sortable by SRI (indpendently tested) here:

    https://coolroofs.org/directory

    Then, instead of spray foam, use an insulation type with a low thermal diffusivity (a thermal -mass effect), such as fiberboard (pretty expensive per R) or cellulose (which has roughly the same diffusivity and thermal time lag as solid brick), which will lower both the peak and average cooling load. R30 of open or closed cell foam has a time lag measured in minutes, whereas the lag at R30 of 3lbs density cellulose is measured in hours. That time lag matters- after dusk the radiational cooling of the roof starts extracting heat from the roof deck, often before the peak ceiling temperature on the conditioned space side of the insulation is reached (and it's a lower peak than with R30 foam or fiberglass).

    For a short introduction to thermal diffusivity, see page 5 of this bit o' marketing hype from a fiberboard insulation company:

    https://www.passivhausaustralia.com.au/GUTEX-Product-applications.pdf

  2. Eric Habegger | | #2

    It's really not advised to combine bubble wrap and radiant barriers. The insulation value of a radiant barrier is nil and its benefits shouldn't be confused with insulation value. If a radiant barrier is functioning properly it will get very hot. It just won't radiate that heat to its environment as long as the radiant barrier surface is not touching or "conducting" heat to anything. Practically, what that means is that a radiant barrier requires an air gap between it and any insulation. I think you may be confused that bubbles combined with the RB negates that. It doesn't. It seems to be a tough nut to communicate what RBs actually do and the myths that have been created by firms touting the combination of RBs with bubble wrap. That doesn't work.

  3. Eric Habegger | | #3

    To make it crystal clear why either of the two examples of eco**** the OP mentioned shouldn't be used I'll explain: The first example has a reflective coating on each side separated by bubbles embedded in plastic.. That plastic will have some conduction of heat that is greater than air inside of the plastic. So the radiant barrier is WORSE than a radiant barrier without bubbles because the conduction of heat through the plastic overwhelms the air insulating value. Also the insulation sprayed onto one RB surface completely short circuits any "remaining" RB effect and renders it nil.

    The second example is just a variation with the bubbles on either side of 1 dual sided radiant barrier. Again, the plastic surrounding the bubbles overwhelms the RB value of the RB alone. Again, spraying insulation against the plastic enclosing the bubbles just short circuits any remaining value the RB might have had.

    The important thing to remember is that you have to thread the needle that RBs DO work while simultaneously recognizing that bubble wrap RBs are a scam. It is surprisingly hard for many people to hold those two ideas simultaneously. Those two ideas aren't mutually exclusive.

  4. Kyle Klouzal | | #4

    The zip code in question is 85122. Temps usually go above 115F for a couple weeks out of every year.

    I want to clarify that I understand the bubbles do not provide a true air gap and that the bubble wrap material creates a small thermal bridge, however, while exploring the idea of if it would be beneficial to combine a radiant barrier and spray foam (or another type of insulation) so as to provide protection against conduction and radiation, I realized a radiant barrier needs an air gap otherwise there would be no potential for heat energy to radiate and thus the barrier would have nothing to block and thus came here to ask for expert opinions.

    This is definitely not an optimal approach but this is a 950sqft 60 old home and I want to take every step toward energy efficiency no matter how small the gains may be.

    As Dana pointed out I think a product like this will be on an order of magnitude more effective: https://foursevenfive.com/gutex-thermosafe-wd/ which is something I was previously unaware of.

    1. Expert Member
      Dana Dorsett | | #5

      Basically the benefits of radiant barriers become almost vanishingly small if the roof is insulated to the current code minimums with fiber insulation.

      How deep are the rafters? What type of roofing?

      You are in US climate zone 2B, where it's fairly moisture-safe to insulated unvented roofs with cellulose tight to the roof deck. See Table 3 in this document, in particular the "2B Phoenix" row, and "Cellulose" column, which simulated the risk at R30 ( ~8") of cellulose with both a high SRI (light metal) and standard roof:

      https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf

      At R30 and higher the low thermal diffusivity benefit of 2.8-3lbs density cellulose is pretty substantial.

      If this isn't going to be a finished ceiling, using a PERFORATED aluminized fabric type radiant barrier to hold the cellulose in place would improve the roof deck resilience, since those products are vapor retarders running about 5 perms, not true vapor barriers, and at the same time would reduce the amount of heat being radiated toward the attic floor.

      The current IRC code min for zone 2 roofs is R38, which is only about R3 less what you'd get filling a 2x12 rafter bay with cellulose. A 2x10 bay would be a bit under R35, which is still pretty good. If the rafters are smaller than that it's possible to build out truss type assemblies to add on to the rafters with half-inch OSB gussets (of arbitrary depth) on 2x3 or 2x4 rails, which would also reduce the thermal bridging dramatically compared to full dimensional lumber rafters.

      1. Kyle Klouzal | | #6

        I've got a regular asphalt shingle roof. When it comes time to replace the roof I would like to go for a light colored metal roof (99% of homes out here are dark asphalt shingles). The rafters appear to be 2x6's however I will need to measure to be sure. The attic is small and cramped and as such will never have a finished ceiling, just a place to contain HVAC, electrical, etc..

        Currently there is a large vent on the north and south side of the attic wall. The house sits north-south on the lot. I'm wondering if after turning the attic area into part of the conditioned space if I need to cover these vents up. There does not appear to be any soffit vents.

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