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New construction CA – builder used batting insulation @ roof deck

Jason916 | Posted in General Questions on

Hi folks. Lennar our builder used fiberglass batting under the roof deck instead of the standard radiant barrier roof. I found the Title 24 plans on my city website and this was approved in February 2020 so I don’t believe it was due to covid + supply chain issues….maybe just to cut costs? The whole development of 29 homes was designed this way.

My question is, is this a benefit or negative in the grand scheme of things? Our area (Nor Cal) seems to always uses the radiant barrier roof sheathing boards so this is new for us here.

Thank you.

Edit: sorry I should have said there will be blown in insulation on the attic floor and instead of radiant barrier roof sheathing they stapled fiberglass batting under the roof deck inside the attic.

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  1. exeric | | #1

    Though I'm not a professional builder I do know something about the engineering principles behind the use of radiant barriers. Radiant barriers are not used in place of attic insulation but are instead used to supplement attic insulation. This is the case where the insulation is placed on the attic floor and also where the insulation is placed at the roof deck, either above or below it.

    Even in California's very hot summer micro-climates radiant barriers work, and work well. Fiberglass insulation in the attic not so much. Blown in cellulose insulation on the attic floor is the most cost-effective way to use insulation in the attic and stops air movement heat flow through the insulation much better than fiberglass in any form. However, in lieu of supply chain problems, radiant barrier covered decking adds little cost to a build. I don't know what the builder's excuse is for not using both things together. That's what any smart builder should strive to do in a hot climate. I wouldn't assume the builder you mentioned is smart or even ethical if he isn't using either cellulose or radiant barriers up there.

    1. Jason916 | | #2

      Thank for you the reply. Sorry I just edited my original comment to say the builder is using blown in insulation for attic floor. Then they stapled up batting fiberglass insulation under the roof deck directly against the roof sheathing boards up top.

      1. exeric | | #3

        Thanks for the clarification. I still hope they are using cellulose as the blown in and not fiberglass for that. It seems like a thin strip of fiberglass batting at the roof is sort of useless. Why not just add the equivalent extra cellulose to the existing blown in at much lower expense? That's assuming they had a legitimate reason they couldn't install the radiant barrier decking. The kind of details you are describing worry me. It's as if they got blocked in one direction and did not understand that adding fiberglass insulation at the roof deck did not make any sense as compensation for that omission. It makes me wonder what other things are not understood by the builder.

        Edit: you aren't saying that they are installing radiant barrier covered batts (which is a real thing) at the roof deck are you??!

        1. Jason916 | | #5

          Sorry I don't know all the builder apologies.

          So for most new construction around here builders do the roof decking that has the layer of radiant barrier foil on the underside facing inside of attic.

          This builder use regular roof decking, I believe just standard OSB sheathing and then inside the attic attached regular fiberglass batting insulation stapled on the rafters against the roof deck. Hopefully I laid that out correct this time lol.

          I had a pre drywall meeting with builder and I asked why wasn't the roof deck with the foil on underside used, why did you guys use fiberglass batting? He said due to Covid supply chains but upon my research and pulling the Title 24 plans, this was all approved back in February-March 2020 before Covid really took off and supply chains got disrupted I believe?

          So really I think Lennar just went the cheapest route possible to build these evident with the selection to not use radiant barrier OSB roof decking. 🤔

  2. Moderate | | #4

    All things considered, I would offer that this is a net negative over the lifespan of the home. A simple and affordable solution was bypassed, likely in the interest of profit. Had radiant barrier sheathing been installed, with the appropriate air gap between it and the insulation, your AC would run less during the summer cooling season and the radiant barrier would continue to reflect energy until the house was no more . They likely used this assembly because it allowed them to meet Title 24 and (at current prices) to save $1.85 per sheet of sheathing. One downside to a performance based approach to energy code compliance (Title 24), is that there are few minimum standards. Simple and effective, yet inexpensive products like radiant barrier sheathing are not required if the standard can be met by an alternate means. That being said, as has been advocated here, the falling cost of solar has provided new perspectives on how to address energy usage and energy detailing, net zero is net zero, whether that is achieved through radiant barrier and 12 solar panels or no radiant barrier and 14 solar panels.

    1. Jason916 | | #6

      Thank you, great info. This is EXACTLY what I am thinking, they used fiberglass batting in lieu of the radiant barrier sheathing to save money.

      The house I am buying is part of a 3 phase development and they are basically the same models at each phase give or take a few minor changes along the way. I pulled the title 24 documents for the Phase 2 builds from 2016 and they did use radiant barrier sheathing to meet the requirements laid out in those Tital 24 documents. The builder then was Cal Pacific Homes back in 2016.....which Lennar bought out in 2018 I want to say.

      Now Lennar is building Phase 3 which I am buying in, and the Title 24 docs show no radiant barrier sheathing, just insulation batting. I believe it is due to cost savings as Lennar is doing a ton of questionable cost saving on the job things but that is for another post...

    2. Jason916 | | #7

      Well we did get no radiant barrier + 9 solar panels. 9 was the total panels Lennar had designed and we were not allowed to change that. We are a family of 5...will most likely be adding more panels in the future.

  3. Expert Member
    Akos | | #8

    People have this theory that radiant barriers are magic and reflect all heat. The reality is that for an uninsulated or barely insulated attic, they do make a big difference. Once you have code min amounts of insulation on the attic floor, they pretty much do squat.

    This is all assuming the air handler and ducts are not in the attic. I really hope they are not, this is a much bigger energy penalty than the lack of any type of insulation or radiant barrier.

    If the ducting is up there, the insulation against the roof deck helps much more than any radiant barrier would.

    1. Jason916 | | #9

      Left those details out in my post, sorry about that.

      This is in Northern California, handler and all ducts run inside the attic space. There are O Hagan roof attic Vents throughout roof deck, maybe 8 or 9 total.

    2. Jason916 | | #10

      This was going to be one of my follow up questions, should I rip out the fiberglass batting at the roof deck and install a radiant barrier? I found a DIY barrier from a company called "atticfoil" but not too sure.

    3. exeric | | #11

      Akos, I seldom disagree with you, but I have to here. I have radiant barriers up against the rafters and they make a big difference. For quite a while I had no air conditioning in my home. I just had R-40 insulation in the attic floor and the radiant barrier. I installed a whole house fan, opened the windows in the evening and used that to blow out any hot from both the house and attic. It was remarkably effective. Believe me when I say that even in weather that got up to 110F during the day I was always able to keep the house below 85f. That would not have been possible without the radiant barrier keeping the attic from cooking during the day.

      Remember, this was at a time when I did not have any AC. Going without AC is something unheard of in these parts but I was able to with the help of an RB.

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