Spray foam insulation under outdoor storage closet

[El_Viajero]

My townhouse has a very large outdoor storage closet that currently serves as a bicycle garage and tool shed. It's connected to our back deck. See pictures...

Here: https://imgur.com/a/Ws3pYHl

And here: https://imgur.com/a/LQqvPwz

The door that opens to the deck is the entrance. You can see that it's basically on a post foundation. It's a fully enclosed area at the same level as my deck. There's no crawlspace, just an open elevated area beneath.

I'm considering turning it into a mini music studio, which would involve insulating the walls and ceiling and running a chase from my supply plenum through the house foundation, under the deck area and into the floor of the storage closet-cum-studio.

Given the setup here, it seems like I could spray some closed cell foam to the underside part of the deck that the would-be-studio sits upon. The thing is, the spray foam underneath would be sorta-kinda exposed to the elements. It wouldn't be exposed to rain or anything since it's on the underside of the decking, but it would still be outside, so to speak.

Is there some reason insulating the floor this way would be a bad idea – or is this the way to go? It feels kind of unconventional, but maybe it's fine.

[lthenderson]

If you use closed cell foam, you should be fine. I live in the midwest and it is often used on the inside roofs of pole buildings to help keep it a bit cooler in the summer. It is still exposed to humidity and ambient air conditions when the doors are open and it holds up just fine.

In your situation, I would rather use a spray on closed cell foam than just about any applied insulation I can think of off the top of my head. What would concern me about other applied insulation is that it becomes a nest for mice and other outdoor critters. With the spray on foam, it is tight to the underneath side and inedible by rodents and such.

[Jon Bon]

Your insulation plan sounds fine, if you are going to condition the space it sounds like nearly any insulating material would work, especially since you are up off the ground. spray foam is gonna be like 10x the price to buy a couple rolls at the big box, food for thought.

So the hardest  part it sounds like is getting hvac to the closet. You sure that is feasible? Obviously anytime you extend the length or add bends to a run it is going to decrease efficiency. 

If your going to run your ducts outside under the deck its gonna hardly be worth it!

[SweatingInAR]

If you're running a supply plenum, you also need a return. The return could be as simple as a jump-duct to part of the house. If you don't have a return path, this closet will push air outside, and then your house will have to suck in outside air.

I think an independent system would be a better choice. Window unit, portable AC, or mini split. You can always start with a portable AC and space heater, then upgrade to a mini split if you end up using the space a lot.

Spray foam sounds perfect! Anything else isn't enough of an air barrier. Fiberglass would be useless.

You can use sheets of closed-cell foam and spray foam together to keep the cost lower, but the labor is higher.
https://www.greenbuildingadvisor.com/article/cut-and-cobble-insulation

[Jon Bon]

I would assume the closet has house wrap just like the rest of the house that functions as an air and moisture barrier.

I think we need more information on his HVAC plans, a return would be useful but not vital IMO. This is a 3rd or 4th rate space, its not a bedroom etc. As long as it has some level of conditioning that is probably fine. I guess it all depends on what your temperature delta threshold is?

The sheets of insulation and small bottles of spray foam might be your best bet in terms of cost and efficiency. Such a small job like that is not going to be attractive to a professional. Although even that will be somewhat pricy and time consuming at least when it is compared to fiberglass/cellulose etc.

[Papa bear]

Agree with above - get some rigid foam board insulation. Cut it 1/2" shy of your floor joists. Spray foam in the edges.  Much cheaper for this size job and you can get good insulation r value.


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[El_Viajero]

Thanks for all the valuable input!

It sounds like my initial plan to use spray foam on the bottom of the space should be fine, exposure to the outdoors and all. I appreciate the suggestion to use foam board and create an air seal by spraying foam round the edges, but I'm worried about moisture getting between the foam board and the floor. I live in the South, and it's like a rainforest here from April to November. And per the Green Building Advisor article linked above, there's this:

Wood framing expands and contracts with changing humidity levels, raising the possibility that attempts to seal the perimeter of the rigid foam (whether with caulk, spray foam, or tape) will fail over time. Anecdotal evidence suggest that this danger is real, especially for cut-and-cobble cathedral ceilings.

So I'm a little hesitant to go that route. I'll probably purchase a DIY spray foam kit (closed cell) and use it under the space and inside the space on the ceiling.

As for other concerns posted re: AC and running a duct... I've thought about every option. Window units are too noisy for a music studio. Ditto for portable AC units. This thing might work, but it seems kinda dinky: https://climateright.com/climateright-5000-btu-a-c-heater-2.html

I could run a duct from the supply plenum and out the foundation wall. My air handler and ductwork are in the adjacent crawlspace. This would be a straight duct run with only one 90 degree bend where it curves upward to connect to a boot that's installed in the floor. Yes, part of the run would be outdoors. That's why I'd build a protective chase around it.

SweatingInAZ rightly commented that my HVAC system would suck in air from the outdoors to make up for the air being pushed into the studio. Jumper ducts/transfer grilles are a no-go because this is a music studio and I want to minimize noise leakage from there to the living space.

The good news: I've already got a fresh air damper that supplies my home with small amounts of filtered outdoor air when my HVAC system runs. The additional required makeup air, in theory, will be provided via this damper (so, filtered air) and not via undesirable infiltration. Right? The environment inside the studio will be all positive pressure. Yes, I'll lose conditioned air this way, rendering all of my HVAC less efficient. But it's a really small space and I don't anticipate it making an enormous difference in my energy usage.

My HVAC system is already oversized due to energy upgrades I've performed in this home. If anything, adding supply air to another room will allow my heat pump to actually use that extra tonnage for something.

[Dancin'Dog]

What's the size, about 8'x8'?  Spray foam seems like it would be way too expensive for such a small space.  Why not pack it with fiberglass batts and seal it with plywood?  $200 and an afternoon, done.

[lthenderson]

What's the size, about 8'x8'?  Spray foam seems like it would be way too expensive for such a small space.  Why not pack it with fiberglass batts and seal it with plywood?  $200 and an afternoon, done.

Actually you can buy closed cell foam kits for $40 that will do around 15 square feet, 1 inch thick. So theoretically you could do 64 square feet cheaper than $200. What I really like about them is they are essentially disposable hoses, nozzles, etc. so you don't have to bother with cleaning up.

[Jon Bon]

@El_Viajero

Ok you this is going to be some sort of music room? As in storing instruments out there?

If this is the case I really don't think what type of insulation matters if you are going to pull HVAC from the main system. In order for this to work I think you 100% need an independent system.

This room is not going to be connected to the ambient air of the rest of the house is it not? So as a result it is going to have all kinds of strange temperature and humidity swings. This would be no issue for a person, but for a $10,000 guitar for instance, this would be a terrible idea!

Tell me you have thought about this? And its just a space to play instruments and not to store them? If you are going to keep valuable instruments in there I would say a mini-split or a halfway decent window/wall AC unit that can keep it consistent humidity and temp.

A 5000 BTU wall unit is going to have zero issue with that small of a space.

[SweatingInAR]

I agree that the noise can be bothersome from portables and window units. That climateright unit looks interesting... a ducted window unit.

Another option is a ducted mini split, you could put enough baffles in the ductwork to have sound blocking.

I've been in several recording studios that were converted rooms or garages, and the best solution is to just turn off the AC when you're recording. Takes are typically less than 10 minutes each, and it's rare that you would record for longer than an hour or two without a break. The vibration and sound of your existing compressor (which looks right next to the studio!!) will interfere with recording.

Will you be storing any wood instruments out there? If they are just electronic or brass there isn't much of a need to control the temperature and humidity when you aren't in the room. Wood is another story.

Jon's point is an interesting one. If you just blow your conditioned air through the room (while the house AC is running), the temperature in your studio is uncontrolled. It would be cold when the AC is blowing, hot when it stops. Hopefully you will be able to insulate it well enough to buffer that temperature. It'll take some effort to balance it out though.

[Dancin'Dog]

The simplest & least expensive solution would be to simply cut a vent hole right through the adjoining wall.  I guess you'd really need an upper and a lower one so they would circulate. 


You could cover or disguise them so they don't look bad.

[El_Viajero]

@El_Viajero
This room is not going to be connected to the ambient air of the rest of the house is it not? So as a result it is going to have all kinds of strange temperature and humidity swings. This would be no issue for a person, but for a $10,000 guitar for instance, this would be a terrible idea!

Tell me you have thought about this? And its just a space to play instruments and not to store them? If you are going to keep valuable instruments in there I would say a mini-split or a halfway decent window/wall AC unit that can keep it consistent humidity and temp.

A 5000 BTU wall unit is going to have zero issue with that small of a space.

Good points! My thinking is that window units are too loud for a music space and mini splits are overkill. My main system also has a ducted whole-house dehumidifier. I suspect it will help keep this little room sufficiently dry as well. Yes, I would be keeping wood instruments in there, and the effect of temp/humidity swings definitely has occurred to me. I figure it will be ok given that I have the whole house dehumidifier. I can also put a little humidifier in there during winter if needed. Any reason this will be a serious problem?

Also: I don't own a $10K guitar. This is the MMM forum, yo! Let's just say I don't own anything that I'd cry about if it got stolen. I'd be pissed for a day and then I'd just go get another one.

The vibration and sound of your existing compressor (which looks right next to the studio!!) will interfere with recording.

Ha! Yeah, that has occurred to me as well. I'm hoping that the added insulation and the addition of flooring + door soundproofing will keep just as much of the heat pump noise out as it keeps musical noise in. In any event, it's something I can't really control. I might just have to live with it until I replace my heat pump with one of the quieter ones they make nowadays.

The simplest & least expensive solution would be to simply cut a vent hole right through the adjoining wall.  I guess you'd really need an upper and a lower one so they would circulate. 

Nice thought, but what you can't see from the photo is that my chimney is right on the other side of that wall. So it's a no go.