[garye]

Hi. I’m building an indoor sport court in Minnesota that has 12 foot concrete foundation walls on two sides. It is framed with a 2×8 exterior wall and a 2×4 interior wall, with about 1.5 inches between the two walls. I just finishing applying 2 inches of spray foam on the wall. Next, I plan to put up 3.5 inch (15R) mineral wool batts, and then finish it with CertainTeed MemBrain.

My question is… should I place the 3.5 inch mineral wool batts on the 2×4 wall that is closest to the inside and where the drywall will be hung? Or, should I place the batts all the way against the spray foam on the 2×8 wall?

 

[Marson]

If it were mine, I would want to fill that whole cavity with insulation. For one, I believe it is in MN energy code that the vapor/air barrier must be tight to the insulation. But I don't think you'd want a big gap between your batt and the spray foam. For one thing, I would think over time the batt would be prone to falling out from between the studs.


This really would be a good situation to use dense pack cellulose and fill the whole cavity.

 

[Bud9051]

Good morning Garye.
First question, is that spray foam open or closed cell and what is its advertised r-value?
With rigid foam on the exterior there will be a dew point somewhere in the middle. Sounds like you are ok, just checking.

I personally don't like open spaces so I would do a complete fill but to add more r-value we need that dew point evaluation.

Note, empty spaces suffer from air circulation which increases heat loss.

Are you covering the concrete with anything?

Bud

 

[garye]

Good morning Garye.
First question, is that spray foam open or closed cell and what is its advertised r-value?
With rigid foam on the exterior there will be a dew point somewhere in the middle. Sounds like you are ok, just checking.

I personally don't like open spaces so I would do a complete fill but to add more r-value we need that dew point evaluation.

Note, empty spaces suffer from air circulation which increases heat loss.

Are you covering the concrete with anything?

Bud

I used close cell spray foam that is approx. 7r per inch. I don't think i can add more batt insulation to the wall without adding more spray foam, due t the dew point evaluation you mentioned.

 

[Bud9051]

If you put it all the way towards the outside be sure to add a piece behind each stud to reduce the thermal bridging.

Here's the chart used to guide the ratio of rigid to fluffy insulation. Not sure of your climate zone but sure you know.
https://www.finehomebuilding.com/2018/01/11/choosing-right-thickness-exterior-foam

Bud

 

[Bud9051]

Here are a couple of references:
https://www.finehomebuilding.com/2018/01/11/choosing-right-thickness-exterior-foam

https://www.efficiencyvermont.com/M...nking-the-rules-on-minimum-foam-thickness.pdf

Bud

 

[garye]

Here are a couple of references:
https://www.finehomebuilding.com/2018/01/11/choosing-right-thickness-exterior-foam

https://www.efficiencyvermont.com/M...nking-the-rules-on-minimum-foam-thickness.pdf

Thanks for sharing the articles. They were more focused on exterior insulation versus interior spray foam. In my case, do you recommend I put the batt insulation right next to the spray foam, or do I leave an air gap between the spray foam so that the batt insulation is right up against the drywall?

According to the instructions for a hybrid wall, I'm limited to a 50% ratio of spray foam to batts so I can't increase the amount of insulation in the wall. (See https://1drv.ms/b/s!AoQ6bkrOeqV-0w5e5zrpA6UjGCvL for a reference).

 

[Bud9051]

Need to iron out some of the details.

Your exterior wall is 2x8 and will have 2" of closed cell on the interior of the exterior sheathing. That leaves you with a 5.25" void to the inside of that wall.

Then you will have a 1.5" gap then another wall of 2x4".

Your climate zone is either 6 or 7 (not specified) but if we use your 50/50 ration then you are limited to r-14 towards the inside of that foam.

Also not mentioned is what will be covering the lower concrete wall area.

Still waking up and it is snowing here and I'm grumpy.

Just thinking out loud, but your foam is either too thin or your wall is too thick, assuming you avoid the empty space you are contemplating. In terms of best practices, leaving an empty gap will promote convection inside the wall cavity and unknown how that will affect moisture issues. It will affect insulation performance but would still be a good wall.

Back later.
Bud
Note also, rigid foam on the exterior vs interior of sheathing is basically the same in terms of dew point.

Could not post at 5 am so saved and will put this up now.