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I have installed a screen porch on my deck. I need to close the spacing down between the deck boards due to stink bug and other critters get in through the space/ What material can I use to close up the spaces?
how big are the gaps?
If the deck isn't waterproof, then I would get underneath it and install either 1/8th hardware cloth or window screen, this will allow moisture to pass and the wood to expand and contract with moisture, while keeping out the critters.
If the deck area is now waterproof, then I'd overlay it with plywood to create a barrier to the critters.
Backer rod is a foam coiled tube that is used behind caulk. In your case just jamb it between the boards. It comes in different thicknesses, in 10 foot rolls I think
The space is still unheated so the boards need room the expand and contract. Foam between boards holds water and water causes wood to rot, even treated wood. The best bet would be to get under the porch and staple screen material to the bottom of the boards. Fiberglass screening would be better than aluminum.