Crawl space insulation advice

I haven't posted on a forum for a long time so apologies in advance if I give too much information, I'm just at a point with my crawl space renovation that I could use some input. I've been DIYing it and often my search queries sent me here to Reddit so I figured this would be a good place to start. I recently completely cleaned out my crawl space, leveled the dirt, and put down a vapor barrier. But with the recent heat wave and having no insulation under the home (fiberglass batts ruined by rats) my AC running for hours just to fall through the floor is pushing me to make a decision one way or the other on how to proceed. Some quick info about the house that's relevant. It was built in 1921 and had an addition put on, as far as I can tell, in the 80's or 90's. The original home was around 525 sq. ft. and the addition around 325, so the home is about 850 sq. ft. total. Both parts are vented, with the addition initially having a 6 mil vapor barrier and insulation before rats ruined it all, and the front never having a vapor barrier/insulation with the original floor boards warping from 100 years of humidity. This was addressed with vinyl plank and a subfloor of OSB/lauan to level out the front. So essentially the two portions are mostly bisected by the original rear foundation wall, with two openings for HVAC and plumbing to go through on E and W sides. I did a quick paint for reference. Foundation walls I put down a 10 mil vapor barrier extending up the foundation walls about 1 foot, secured with butyl tape, plastic fasteners, and polyurethane caulk. Replaced all flexible HVAC ducts, and insulated the pipes. Front 1 Front 2 Back 1 Back 2 Back 3 The space has no standing water that I've seen and the gutters are directed away from the home, but there is no sump pump. There are 5 vents total and all above the ground so they haven't been an entry point for water. I think that gives enough info for what I think my choices are, but I'm open to all suggestions. Should I attempt to encapsulate? I know this is viewed as the standard, but I guess I'm not sure if its worth it. As far as I know I would have to seal the rim joists and vents, put 2" polyiso foam board on the foundation walls, and wire a dehumidifier/run the excess water hose outside the crawl. I'm afraid to do this because I don't know how sealed off the space would be for humidity (if its not well sealed am I just running a dehumidifier non stop?) or how insulated it would be to retain my living space air temperatures. Energy star recommends R-25 to 30 for crawl spaces, and I have nothing in the joists with the polyiso at 2" being R-13. That and because the space is bisected I'm afraid I could get moisture buildup on whichever side the dehumidifier isn't on because the inner wall blocks most of the space. Or should I keep it vented and attempt to insulate the joists? I was hesitant to put fiberglass back under there seeing as it was musty and the rats nested with it, but I don't think I can do spray foam with the way my floor is in the front potentially trapping moisture and causing issues with multiple layers of flooring. The two sides have different measurements for joists, with 14.5" in the back and 22.5" in the front so I would need two different batt/roll sizes. I was thinking unfaced fiberglass between the joists and putting foam board over it across the joists, effectively making another moisture barrier between the vapor barrier and fiberglass which would get it to R-25. The problem with this is the cost quickly got higher than I would have liked with all the different materials. Sorry for the long post but I've been doing this all myself, and between the research and the work I just need a different set of eyes to look at this and tell me how you see it. This house has been a mess to learn on, and given that nothing has had a standard solution I've been conditioned to have to overthink things through. submitted by /u/Single-Entrance9478 [link] [comments]

Jun 26, 2025 - 23:48

I haven't posted on a forum for a long time so apologies in advance if I give too much information, I'm just at a point with my crawl space renovation that I could use some input. I've been DIYing it and often my search queries sent me here to Reddit so I figured this would be a good place to start. I recently completely cleaned out my crawl space, leveled the dirt, and put down a vapor barrier. But with the recent heat wave and having no insulation under the home (fiberglass batts ruined by rats) my AC running for hours just to fall through the floor is pushing me to make a decision one way or the other on how to proceed.

Some quick info about the house that's relevant. It was built in 1921 and had an addition put on, as far as I can tell, in the 80's or 90's. The original home was around 525 sq. ft. and the addition around 325, so the home is about 850 sq. ft. total. Both parts are vented, with the addition initially having a 6 mil vapor barrier and insulation before rats ruined it all, and the front never having a vapor barrier/insulation with the original floor boards warping from 100 years of humidity. This was addressed with vinyl plank and a subfloor of OSB/lauan to level out the front. So essentially the two portions are mostly bisected by the original rear foundation wall, with two openings for HVAC and plumbing to go through on E and W sides. I did a quick paint for reference.

Foundation walls

I put down a 10 mil vapor barrier extending up the foundation walls about 1 foot, secured with butyl tape, plastic fasteners, and polyurethane caulk. Replaced all flexible HVAC ducts, and insulated the pipes.

The space has no standing water that I've seen and the gutters are directed away from the home, but there is no sump pump. There are 5 vents total and all above the ground so they haven't been an entry point for water.

I think that gives enough info for what I think my choices are, but I'm open to all suggestions.

Should I attempt to encapsulate? I know this is viewed as the standard, but I guess I'm not sure if its worth it. As far as I know I would have to seal the rim joists and vents, put 2" polyiso foam board on the foundation walls, and wire a dehumidifier/run the excess water hose outside the crawl. I'm afraid to do this because I don't know how sealed off the space would be for humidity (if its not well sealed am I just running a dehumidifier non stop?) or how insulated it would be to retain my living space air temperatures. Energy star recommends R-25 to 30 for crawl spaces, and I have nothing in the joists with the polyiso at 2" being R-13. That and because the space is bisected I'm afraid I could get moisture buildup on whichever side the dehumidifier isn't on because the inner wall blocks most of the space.

Or should I keep it vented and attempt to insulate the joists? I was hesitant to put fiberglass back under there seeing as it was musty and the rats nested with it, but I don't think I can do spray foam with the way my floor is in the front potentially trapping moisture and causing issues with multiple layers of flooring. The two sides have different measurements for joists, with 14.5" in the back and 22.5" in the front so I would need two different batt/roll sizes. I was thinking unfaced fiberglass between the joists and putting foam board over it across the joists, effectively making another moisture barrier between the vapor barrier and fiberglass which would get it to R-25. The problem with this is the cost quickly got higher than I would have liked with all the different materials.

Sorry for the long post but I've been doing this all myself, and between the research and the work I just need a different set of eyes to look at this and tell me how you see it. This house has been a mess to learn on, and given that nothing has had a standard solution I've been conditioned to have to overthink things through.

submitted by /u/Single-Entrance9478
[link] [comments]