Garage Insualtion Help

[billyh7]

I have a Detached Garage 24x48 that Im looking to make a nice man cave we can hang out in year round. It has soffit vents and 2 roof vents.
I closed up all the vents with foam board and spray foam thinking I would keep hot and cold air out. All the reading I've done I might have made a mistake. Some people say add baffle vents and let it vent and others say close it all off so Im really confused and need help. Whats the best way to approach this?

New York Zone 4

One Section has a cathedral ceiling roof and the rest has a ceiling. There is a little loft up top for storage.

Roof is 2x8 so I have 7.25 inches of room.

I was thinking doing R13 in all the wall stud bays.
Doing 1 inch roam board in the roof and spray foam the edges to air seal and then adding R19 faced pointing to living area on to the foam board.

Can I add fiberglass onto foam boards with out having condensation issues?
Is that enough insulation for the roof?

Thanks
Bill

 

[AC-WC]

No offense but yes, you goofed. Open the sofffits back up, then put in rafter vents all the way to the ridge. Then vapor barrier fiberglass (paper lined insulation). Or insulation and plastic vapor barrier.
You must have some kind of vapor barrier on the heated side of the room. Otherwise you are likely to get mold growth between the insulation and ceiling covering.

 

[AdamMopar]

With this type of roof system and vaulted ceiling I would highly recommend spray foaming the full roof deck and walls. Create a fully sealed envelope, no venting required.

Doing it the traditional way is going to be much more of a pain and inefficient. If you go that route you will need your soffit and ridge vents.

 

[billyh7]

Could I do just the roof deck in Spray foam 2 inches and then batt over it ? Do the walls in r13 fiberglass?

Thinking that would be cheaper?

Bill

 

[AdamMopar]

You could. It would not be my choice, but people do it. 2 inches of closed cell foam on your walls and 3 inches on your ceiling would be plenty for a shop/man cave. My house in Iowa is 3 inches on walls and 4 inches on the roof deck.

If you decide to go your route make sure it is 2 inches minimum otherwise you could moisture between the spray foam and bats. What you are describing is called flash and bat. In the end I am not sure you will end up cheaper or better, but it is a common process.

 

[billyh7]

What results do you get with spray foam? In cold weather months if its 20 degrees outside what is the temp inside the garage with no heater turned on? Same in summer when its 90 out what's the temp inside?

 

[C-S-H]

Very nice building! I really like it. A few concepts to think about:

1. Are you going to condition this building full-time? Run a dehumidifier full-time with a little supplemental heat in winter? Only condition it on occasion? Your wall and cathedral ceiling layups will be difference for each of these cases. For example, in the last case you would want vapor semi-permeable ceiling and wall assemblies so the building can take care of itself with the help of some sunshine.

2. You are in Zone 4, so four seasons. Moisture drive is from inside-out in winter, and potential condensation surface is on cold exterior surfaces. Moisture drive in summer is from outside-in, and potential condensation surface is near the inside of the assembly. Correctly designed vapor semi-permeable assemblies will allow water vapor to pass through at a slow rate. The dew point will be a step-function from inside to outside across the insulated cavity. There is no condensation when this is done right in Zone 4. If you put a vapor impermeable layer in your cathedral ceiling assembly, then you are into at least dehumidification. Water vapor rises. If you put vapor barrier in the wall layup as well, then you are into HVAC 24/7/365.

3. If you put a vapor impermeable layer in your wall or cathedral ceiling layup, then you must build-in or provide for prevention of condensation. The vapor barrier partitions the assembly dew points. For example, right now you have a vapor impermeable layer above your roof deck sheathing; the shingles. Your provision against condensation on the roof deck sheathing is outside air ventilation of the entire building. If you build a ventilated gap under the roof deck sheathing, then EPS board (vapor impermeable, or semi impermeable), then batt insulation, drywall, latex paint, you will need to get the ratio of R-values correct for EPS/batt or you will get condensation. Total R-value lower than 38, too, and a ton of work.

4. If you open-cell spray foam against the roof deck sheathing, you will get condensation and rot. If you open-cell spray foam the walls you will have an unservicable wall assembly that I dislike. If you use closed cell spray foam against the roof deck sheathing you will have made a rot sandwich, and the roof will rot when it leaks. If you closed cell spray foam the walls, you have an unservicable assembly that I do not like.

The requirement for cathedral ceiling assembly takes away the cheap and highly effective latex paint/drywall/blown-in insulated ceiling option. Unless you are willing to tear off the roofing and do exterior insulation/ventilated roof deck, you are limited to lower performance options for cathedral ceiling assemblies.

If it were my garage and I had your goals, I would:
1. Use soffit vents/rim joist holes/ridge vent
2. Attach 3" ISO to bottom of rafters, foil side down, tape seams
3. Air seal, put mineral wool in walls/drywall/latex paint
4. Dehumidifier low / window A/C unit up high / mini-split / woodstove

Hope you enjoyed reading this lengthy response and get something out of it.

 

[AdamMopar]

I don't have anything in my shop or garage to measure temperatures currently, but in general if it is insulated with any method you will see benefit out of that.

I'd really suggest reading up on building science and spray foam. Lots of misinformation out there and on this forum also.

If you like watching videos Spray Jones on YouTube has great videos on it and will tell you everything you need to know.

It is expensive, but I never will insulate a building with traditional methods again. There is no comparison.