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I have a raised home in New Orleans, where the weather is hot and humid 2/3 of the year, but it can get quite cold for brief periods in the winter.

The house is about 100 years old and there is some old fiberglass insulation installed between the beams under the house. But in most places the batts do not fill the entire space between the beams. There is still plenty of room for some more insulation. I have started to fill the remaining space with some more fiberglass insulation, and I am also stapling Tyvek sheets to the lower edges of the beams, so that the Tyvek will completely cover the underside of the house, and help support the additional fiberglass.

But before continuing with this labor intensive project, I would like to have an opinion: does it make sense to add the extra insulation that way, and is the Tyvek cover going to serve any insulation purpose (other than helping with the support)?

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Tyvek will not add any insulative value.

Aside from helping keep the insulation in place and preventing it from falling down, it will act as a draft stop and might be worth your time to install it.

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It may no longer be the case, or perhaps in this one, but Tyvek cannot handle long term UV exposure, and would figure that the underside of the home will at least get some exposure, even if it is indirect. After a period of time the Tyvek becomes very weak, like notebook paper and may not be able to handle a stiff breeze or wind. I used Tyvek on my home and evidently, did not cover it in the time specified and when I did a chimney addition I found this. I wasn't pleased at all.

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    When I built my home, Tyvek had a 30 day exposure warrantee, now I believe it is 90 days to allow it to be exposed before covering, to keep its warranty. At the time I was certain it was less than 30 days but who knows? It still does not last with exposure.
    – Jack
    Oct 22, 2023 at 16:18
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    I did not know about the effect of UV exposure on Tyvek. I am hoping that the underside of my house is sheltered enough from light that it won't be a serious problem but in any case I plan to check regularly and fix whatever problems as they occur.
    – Valerio
    Oct 23, 2023 at 11:03

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