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Extra layer of XPS - gap in between ok?


jmccrack650's Avatar
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06-26-14, 01:52 PM   #1  
Extra layer of XPS - gap in between ok?

I am adding xps foam insulation to cold cellar (under front porch type) to better stabilize temperature for wine storage.
I have already added one layer of 2" XPS between the ceiling joists and 1/2 way down walls, but it is not providing enough temperature reduction in summer, so I am looking to add additional 2" layer of XPS.

For the ceiling joists, it was a lot of work to cut pieces to fit between joists since they are not at all straight. Since I already have a layer between joists, would I have any issues attaching 2nd layer to the underside of joints to allow install of full sheets, and leaving the approx 3-4" gap between the XPS layers? Would this have significant impact on R value of insulation?

 
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06-26-14, 02:38 PM   #2  
Having a gap between layers does create a potential for condensation if humid air can enter and find a cooler surface. The bigger question I have is what temperature are you hoping to reach? And what is the temp now?

Also, are any of the surfaces facing something other than cold earth?

Bud

 
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06-26-14, 02:44 PM   #3  
Currently temp is around 18C with summer heat, and went down to around 9C in winter. My goal is to bring down to about 15C and limit temp variance to around 4C over the year.

1 wall faces the interior of the basement, and has batt insulation on the basement side.

Currently I only have insulation on the 3 outer walls 1/2 way down, but I am planning on extending to the floor on all 4 walls, and maybe add second layer if needed.

The ceiling is just a pain due to the crooked joists, so hoping to avoid all that cutting again.

 
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06-26-14, 03:17 PM   #4  
Let's see, down here it's F-32 over 9 = C over 5, I know we are stubborn.
So currently 64 and you would like it at 59, hmmm.
If you are going to try to do this naturally, no heating or cooling, then the thermal mass will need to be such that the increase in summer will be about 4 and the decrease in winter the same, cantered around say 55, 51 to 59 F. Or roughly 10 to 15 C. That's going to be tough and you may not want to insulate all the way to the floor, at least for both summer and winter.

I would start with all surface areas, their insulation values, and the temperatures across them. While I think and you measure let's see if others have some input.

Bud

 
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06-26-14, 04:24 PM   #5  
My thinking with insulating all the way to the floor is that the floor (concrete) probably be the temp I want, and the most consistent since it is deepest. Also, my humidity is slightly more than I want, around 80%. Covering walls to bottom should reduce humidity (target 70%).

Room is about 10'x5' (8' ceiling).

Optimally temp would stay at around 53F all year, but I know that wont be feasible naturally.

I don't have a laser thermometer to measure temp of all walls, but last I checked floor was around 56F

 
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06-26-14, 05:18 PM   #6  
With the floor at 56 and everything above that warmer, you can't put in enough insulation to get the heat gain down to zero. I was hoping that the lower portion of the walls would be similar to the floor.

Just to throw this out to think on, you could install a small heat pump with the condenser in the basement and the exchanger in the wine room. Being very well insulated it would barely run, but would guarantee you the exact temperature you want and could extract some of that humidity.

I'll play with some numbers.

Bud

 
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06-26-14, 06:16 PM   #7  
Possible that lower portion of walls are similar temp to floor. I will have to borrow laser temp again and check. Will post results (probably next week since I am on vacation starting tomorrow).

 
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07-08-14, 05:53 AM   #8  
OK Bud, did some quick measuring.

Current temp on the floor is about 62 (warmer than I would have thought), but not sure how much the surface temp gets affected by the overall temp of the room (around 66-68).

Uninsulated bottom of walls (bottom 3.5 feet) raise in temp about 1F per foot.

The insulated potion of the wall seems to measure hotter for some reason, even when measuring the bottom of the insulation vs. top of uninsulated wall (difference in surface affects laser temp gun?).

Insulated portion of the walls (top 4' - 2" XPS) read about 66 - 69 as you go up, and roof is around 69.

Unless these temps are being affected by overall room temp, I am thinking that I need to rethink my target temp.

With these temps, do you think it is achievable to bring room temp down to 64-65 with extra insulation?
Options:
1. Add another 2" layer to ceiling and top 4' of walls
2. Insulate bottom 3.5' of walls
3. Both

Also, part of my goal is to reduce humidity as well. I assume both options above will also reduce humidity in addition to adding insulation. Humidity currently spikes to around 90% occasionally which is too high (generally around 75-80).

Thoughts?

Jeff


Last edited by jmccrack650; 07-08-14 at 06:59 AM.
 
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