Insulating Ceiling?

[arohde]

Hello, I was hoping to get some advice regarding insulating a ceiling/attic. Specifically the question is to address a problem of too much moisture during the winter months within the attic of the garage. I live in Minnesota so winters are cold.

Let me lay out the situation

Detached 21x21 garage
Gas heater
Normally keep it at 50 degrees Fahrenheit in the winter (again Minnesota)
Walls insulated (kraft backed insulations) and sheetrocked
Ceiling insulated (no vapor barrier) and sheetrocked
No tape or mud (this is changing for the walls)
Vented Soffits
No other ventilation (this is also going to change. Likely I'll be putting a fan up there)

This is what I'm considering doing

- Putting a plywood ceiling over the current drywall ceiling (I ordered marine-grade plywood)
- Before doing the plywood, I'm going to mount 2x4 to the ceiling and they will cover the gaps on the existing ceiling drywall that normally would be covered by tape and mud.
- So there will be a grid-work of 2x4s on the ceiling over the existing sheetrock.
-Within the "voids" of this grid-work I will put 1.5 polystyrene insulation.
-I will then mount the plywood to the 2x4.

If you are asking why I'm doing the 2x4s it's for two reasons. One: It will make mounting the plywood easier. Because there is existing drywall, finding the truss joists will be slightly more difficult and when using 45 dollar a sheet plywood, I'd like to avoid putting extra holes in the plywood. Two: I will use that 1.5 inch gap as conduit to run electrical for a few more lights. This will prevent me from having to get up in the attic, or routing wire on the outside of the plywood.

In general the main reason I'm doing this is to fix the moisture problem. I realize the attic still needs better venting, which will happen. My thought is this will be helpful in creating a vapor barrier, that stupidly I didn't put down when I insulated the attic. It also has the added benefit of looking nice, and creating a simple way to add a few more lights.

Can anyone think of a unforeseen problem this might create? Also, given the added 1.5 polystyrene insulation and marine-grade plywood do you think putting a plastic vapor barrier down would still be necessary? One I would just staple up there? I'm not planning to do that but I'm curious to get thoughts. Also, for the 2x4 grid work, I'm not planning to use treated lumber. Would you encourage or discourage this? Thanks for taking the time to read all the way thru this. If anything doesn't make sense or you need clarification please let me know. Thanks!

Adam

 

[plout99]

My first concern is the load on the bottom cord of the trusses, you are going to add a substantial amount of weight in the plywood and 2x4 that they probably are not rated for.

 

[Thunderpigeon]

It sounds like it would be much cheaper to hire someone to tape the joints of your existing sheetrock. Then you can paint it with a paint that creates a vapor barrier.

If you just have soffit vents but no ridge vent I would consider adding a ridge vent before adding fans.

What sort of insulation is in your Attic?

 

[arohde]

It's blown-in insulation. Not sure on the exact brand.

That is a good point concerning the load. Are there any "rules of thumb" for this type of thing? Or a way to calculate the approximate load capacity?

 

[Roundhouse]

I agree
I’d try taping and mudding the joints and ridge vents before spending the time and money to add a lot of weight to the ceiling

 

[matt_i]

If you just have soffit vents but no ridge vent I would consider adding a ridge vent before adding fans.

This would be my first step. WHich will also help keep it cooler during 60 days of summer.

What is Gas Heater? Is that a torpedo heater or a vented combustion unit (where there's a chimney penetrating the roof)

 

[lowerthanu]

1st question is what kind of heater are you running. If it is a ventless gas heater that is going to be a major problem. Vapor barrier is necessary in MN as well.

 

[PCMusicGuy]

Blown-in insulation right on top of sheetrock ceilings with no vapor barrier used is common practice here. Is it because of your colder climate that creates an issue?

 

[arohde]

This would be my first step. WHich will also help keep it cooler during 60 days of summer.

What is Gas Heater? Is that a torpedo heater or a vented combustion unit (where there's a chimney penetrating the roof)

vented combustion. I have it going thru the side but same difference. It's all up to code.

 

[arohde]

1st question is what kind of heater are you running. If it is a ventless gas heater that is going to be a major problem. Vapor barrier is necessary in MN as well.

It's vented. Its a 50,000 btu Big Maxx vented out the side of my garage. Code here is you can go horizontal venting as long as you don't exceed a certain distance.

 

[arohde]

Blown-in insulation right on top of sheetrock ceilings with no vapor barrier used is common practice here. Is it because of your colder climate that creates an issue?

Yeah exactly. The problem is the sheetrock isn't taped and mudded so there isn't a good seal keeping the warm air in the garage. It seeps out of the garage into the attic and when the warm air meets the cold air, it creates condensation.

There are two ways to help the problem. One is to better seal the garage so less warm air escapes the garage and goes into the attic. The other is more venting in the attic so there is more cold air.

I'm planning to vent. But right now I'm just wondering about the ceiling and options to better seal it. the standard way is sheetrock, mud and vapor barrier. Vapor barrier isn't a easy option unless I remove all the insulation so that's why I'm getting creative.

 

[arohde]

My first concern is the load on the bottom cord of the trusses, you are going to add a substantial amount of weight in the plywood and 2x4 that they probably are not rated for.

I calculated it and it's approximately an extra .7 lbs per square foot. Or 303.18 lbs total. This wait will all be evenly distributed and the plywood will stiffen up the structure. It's a little different but a good snowfall could put as much as 4,500 extra pounds on the roof. Almost 15 times greater than the load I'm adding with the plywood.

I'd be curious what the "just use the joists as an engine hoists" guys have to say about this....

 

[matt_i]

What is your attic insulated with, fiberglass? Personally I think its a poor choice unless its "bagged" becaause "open" can't trap air. And then its not ideal because it can crush down.

The sheet insulation is super expensive compared (by R-value) to what you can get with loose-fill cellulose. But it is your money and not mine.

It would be a perfect time of year to climb up and saw in a new ridge vent. Shingles are still "tough"...they get pretty squishy come summertime and much easier to damage when walked upon. GAF Cobra Snow Country ridge vent would be my choice going back, you'll need some bundle(s) of cap shiingles to match as well.

Are you going to mud & tape the drywall before you start the plywood ceiling because that's going to be looser than the drywall. So many gaps and a harder material to "fit" when there's the inevitable "out of square or parallel" condition. Caulk all of the plywood and paint it? That sounds like a lot of tubes.

 

[jkuro]

Tape, mud and paint existing sheetrock. Run new wiring in attic for lighting. Your attic insulation will now be compressed and messed up from you moving it around. Add another layer of insulation to finish. While in attic, make sure you don't cover your soffit vents with insulation and check for some kind of ridge vent.

 

[yeldogt]

How do you know you have a moisture problem ? when do you notice it ?

The goal is always to seal and seperate .... mositure or not. Venting is taking air you paid to condition and move it to the outside.

Your fix is overkill in terms of lumber.

warm air does have the ability to hold more moisture -- the probelm some have with not heating enough is they never drive the moisture in the shop/slab out of the building.

 

[arohde]

How do you know you have a moisture problem ? when do you notice it ?

The goal is always to seal and seperate .... mositure or not. Venting is taking air you paid to condition and move it to the outside.

Your fix is overkill in terms of lumber.

warm air does have the ability to hold more moisture -- the probelm some have with not heating enough is they never drive the moisture in the shop/slab out of the building.

I'll have to take a look at what the moisture level is in the shop. Agreed it's overkill with lumber but it's also solving 3 - disputably 4 others problems. It's going to look really nice, will seal the shop, adds more insulation (didn't count this one cause I really don't need more insulation), will create a conduit to run more electrical (this is a big one), and saves my neck and sanity from trying to tape an mud the ceiling.

Question for ya - going back to the moisture in the shop - I have a 3hp dust collection system that vents out the side of the garage. I would think if I ran this for a few minutes, it would remove enough air (and moisture) that it wouldn't be a problem (assuming I have moisure in the shop in the first place). But I had a home inspector tel me once, in regards to fans in bathrooms that they will not remove moisture, only methane (smell). I always questioned that. Granted a fan isn't going to be a 3hp but I still have always wondered it that was true of not. Because everyone installs fans in bathrooms more to fix moisture problems instead of getting **** smell out

 

[matt_i]

Question for ya - going back to the moisture in the shop - I have a 3hp dust collection system that vents out the side of the garage. I would think if I ran this for a few minutes, it would remove enough air (and moisture) that it wouldn't be a problem (assuming I have moisure in the shop in the first place). But I had a home inspector tel me once, in regards to fans in bathrooms that they will not remove moisture, only methane (smell). I always questioned that. Granted a fan isn't going to be a 3hp but I still have always wondered it that was true of not. Because everyone installs fans in bathrooms more to fix moisture problems instead of getting **** smell out

The difference in the bathroom/shower scenario is there is a source actively inducing moisture into the room. So venting it is always going to be replaced by makeup air with a lower amount of humidity.

To remove moisture in the shop you should a) vent the attic cavity so it can flow via natural convection, b) use ceiling fans inside the shop, c) dehumidifiers to concentrate it out of the air and d) keep it closed and sealed as much as possible. Bringing in makeup air from outside is 50/50. 50% of the time it can help and the other 50% of the time its going to make it worse, and you have to ask who is going to be the logical controller/decision maker to sort out when to run and when not to run.

 

[arohde]

Wanted to give an update on this. I finally have finished the insulation. I think I'm just going to keep it like this for now, and forego the plywood. It did work out well to create a "conduit" for installing more lights. I think it looks kinda cool too. Thanks everyone for the suggestions!