Insulating cabin

[mreisner]

I've got a storage building type cabin like this, a little bit larger however. I'd like to insulate it and use foam board on the ceiling. Any hints on how to vent it. I'm planning on putting the insulating board over the rafters not in between them.
I know a lot of people use spray foam but I understand that there is a problem with mildew and old possibly forming underneath or else the boards under the steel roof basically just getting baked to death over the years. I'm in Michigan so we do get a lot of sun in the summer. Thanks.

 

[Hank11]

If it has venting, I'd put in a ceiling and then your choice of fiberglass or rockwool above the ceiling. A small trap door would be helpful.

 

[dcg9381]

I've got a storage building type cabin like this, a little bit larger however. I'd like to insulate it and use foam board on the ceiling. Any hints on how to vent it. I'm planning on putting the insulating board over the rafters not in between them.
I know a lot of people use spray foam but I understand that there is a problem with mildew and old possibly forming underneath or else the boards under the steel roof basically just getting baked to death over the years. I'm in Michigan so we do get a lot of sun in the summer. Thanks.

Spray foam is EXCELLENT. But two things matter:
1) Geography
2) Use

Down south, we just spray stuff and enclose it. Why? Because 95% of the time we are cooling and removing moisture.
If this is a "habitable structure" AND up north, you may want to vent it. There should be an air break between insulation and roof deck as well as moisture controls as well as moisture control.

How habitable (how much you are cooling and heating) matters. If this was my structure up north, I'd probably just enclose it, not vent it, and put and HRV / ERV in the space. But it depends on HOW you are heating it and how often.

Traditional insulation is way cheaper. And for a "sometimes" heated structure may be the way to go.

 

[The Cobbler]

I'd like to insulate it and use foam board on the ceiling. Any hints on how to vent it. I'm planning on putting the insulating board over the rafters not in between them.

I just watched a video yesterday with a you tube creator doing this . he only has r-10 in the ceiling as a result. Styrofoam burns very toxic , so it should be covered also .
to me it's a piss poor way to insulate .

 

[mreisner]

If it has venting, I'd put in a ceiling and then your choice of fiberglass or rockwool above the ceiling. A small trap door would be helpful.

It has a couple of lofts in it so I'm hoping to preserve the height as much as I can. Basically I want to keep it about 10 degrees above freezing in the winter and to moderate the temperature in the summer. I won't air condition it but I probably will have a small heater in there for the winter.

 

[mreisner]

I just watched a video yesterday with a you tube creator doing this . he only has r-10 in the ceiling as a result. Styrofoam burns very toxic , so it should be covered also .
to me it's a piss poor way to insulate .

I saw that same video, I was thinking 4 inches of fiberglass leaving a 2-in gap on the top of it and then put in the foam over or and covering it.

 

[billconner]

I assume too late but I think the ideal way to insulate at the top would be poly-iso foam on top of roof deck, and roofing on top of that. Air seals and keeps wood and anything affected by water away from condensing surfaces. You can buy pretty good poly iso used for a lot less than new. Cover it with sleepers and metal roofing and hard to beat.

 

[Hank11]

It has a couple of lofts in it so I'm hoping to preserve the height as much as I can. Basically I want to keep it about 10 degrees above freezing in the winter and to moderate the temperature in the summer. I won't air condition it but I probably will have a small heater in there for the winter.

Open cell foam on the roof deck would get you there, and keep your lofts open.

 

[BombShelter]

Greetings from Lake Huron! I'd read what code is, normally just like a home, air flow under the roof to the ridge vent.

The more I research spray foam, the less likely I'd use it again. They've pretty much outlawed it in the UK, it encapsulates the structure and doesn't let moisture out. To fix it you have to cut it out, hand scrap the beams and hope it doesn't rain during this intensive process. Meanwhile mold loves the environment.

 

[mreisner]

Greetings from Lake Huron! I'd read what code is, normally just like a home, air flow under the roof to the ridge vent.

The more I research spray foam, the less likely I'd use it again. They've pretty much outlawed it in the UK, it encapsulates the structure and doesn't let moisture out. To fix it you have to cut it out, hand scrap the beams and hope it doesn't rain during this intensive process. Meanwhile mold loves the environment.

Your near Lake Huron?

 

[dcg9381]

The more I research spray foam, the less likely I'd use it again. They've pretty much outlawed it in the UK, it encapsulates the structure and doesn't let moisture out. To fix it you have to cut it out, hand scrap the beams and hope it doesn't rain during this intensive process. Meanwhile mold loves the environment.

Did you have problems with your install?
I've used it 3 times on structures in the south and will use it again. Dramatic differences on power bills.
Northern climates are a little more tricky, as you're heating and it doesn't tend to remove moisture like our constant running of HVAC does in the south.

I think that most of the problems in the UK are "retrofit" insulation on structures that are likely older and were never designed to be enclosed, so no ERV/HRV controls are present. They're just shooting it into old structures and of course it's going to cause problems if moisture is present.

We've used spray foam up north, it's been great so far, but only with proper venting and HRV/ERVs. We use "hyrbid" insulation designs, encapsulating with spray foam, then further insulating with traditional insulation for very high R values.

 

[Mainiac Mat]

If you want a tight house and plan to heat in winter and cool in summer, the best bet is to add an air-to-air heat exchanger. But your cabin is probably too small for that. And they are normally installed in the attic, so you'd lose your loft.

If you heat with a wood stove, you're not going to have any moisture problems as it is such a dry heat.

I can't see heating while unoccupied as producing a lot of moisture (no breathing, no cooking, no showers).

For a small cabin, I think I'd fill the rafter/stud bays with ISO board and panel/rock over that.

 

[mreisner]

Basically I just want to keep it somewhat stable temperature and above freezing like about 40 degrees in the winter as I have a collection of Mission style furniture out there. Kind of like a period correct room in my house but it's not in my house as we don't have room and my wife isn't real big on that style of furniture. It's my little get away, straight out of 1905!

 

[Voi]

It doesn't appear you have the soffit overhang or peak venting to do what you want.

I have seen people use collar ties & then install insulation board over the rafters & the collar ties, thereby leaving a slight space that can be vented with gable vents. Not sure how effective this would be but probably better than nothing.

If it were me I'd probably remove the steel & insulate the roof deck, however. Or use spray foam, probably open cell in this case.

 

[dscheidt]

I saw that same video, I was thinking 4 inches of fiberglass leaving a 2-in gap on the top of it and then put in the foam over or and covering it.

You do not want air space in the insulation. It results increased convection loss, and if there is not very good air sealing, enhanced stack effect.

 

[dscheidt]

If you use foam, it needs to be thick enough that the dew point under all conditions is inside the foam layer. With out that, you will have condensation on the foam.

 

[billconner]

The problem with any foam inside is it should (code) be covered. 3/4" wood or 1/2" drywall and possibly some coatings would be accepted.