Foam vs Fiberglass

[Anonymous]

Hi all,
I'm getting ready to insulate the walls and ceiling in a small cabin that I have. Right now, the exterior sheathing is up, but the interior is bare - no drywall or anything else. This is in the coastal hills of California, in sight of the ocean, so we get a lot of damp fog, but the temperature never gets down to freezing. Usually lows are in the high '30s in winter, and highs get to 100 in the summer. The building is 2x6 framed, with a cathedral ceiling, and heating will be with a propane fireplace. The roof is sheathed and has soffit vents and a ridge vent to help prevent hot roof problems in the summer, and will be metal standing seam. The walls will get 15 lb felt and Hardi on the exterior.
I've had one estimate that suggested spraying foam in the wall cavities to seal, but to use fiberglass batts in the ceiling to allow moisture to escape, while a second said to just use fiberglass through out. None have mentioned any type of vapor barrier. I appear to be in a mixed climate that is not heating or cooling as defined in the books. What would be the best approach to insulating this place and keeping it from getting damp when not in use?
Thanks, Ched

[dhill]

Damp from the exterior should be taken care of by the exterior tyvek or vapor barrier that's in place over the exterior tyvek. I'm not sure about inside damp; why would the inside become damp? Are you planning on using an evaporative cooler inside for air conditioning? I don't see that interior moisture will be a problem and if it is, you can probably alleviate it by using a dehumidifier. So, what I see as what your question should be is how much r-value are you going to need in order to properly insulate the house for temperature purposes. To my knowledge, that's the purpose of the insulation. I priced foam insulation and the r-value for sprayed foam is about r-25 in a 2x6 wall. I think one of the advantages of spray foam is that it seals all air cracks. The disadvantage is that it is extremely expensive here, about 1.65 sq.ft. Batts are much less. You should be able to get r-19 value with batts which IMO is generally sufficient. You can find out what the recommended r-value is for your area by checking a chart provided by most insulation dealers. I'd recommend you make a few phone calls around your area and talk to some hvac guys for their recommendations.
Dusty

[Anonymous]

Thanks Dusty,
I agree that the outside damp should be taken care of by the housewrap, but what I've noticed at other weekend cabins in this foggy area is that after being closed up for a week or so, it feels damp inside. Once the place is occupied for an hour or two, everything warms up and is fine. I think the dampness is caused by air infiltration. I'm leaning toward using foam in the walls to seal the place, combined with a south facing picture window that should keep the inside warmer than the outside so that any moisture won't condense. I don't know if I need to foam the ceiling or not, and I don't know if I need a vapor or moisture barrier in the wall or ceiling, in addition to the house wrap. We've got no power up there, so AC and dehumidifiers are probably out.
Thanks again, Ched

[dhill]

Well, I've known of people who put plastic under the sheetrock on the ceiling. That can eliminate some air infiltration. I don't think you want a completely air tight home though, even to eliiminate humidity. Where's all the experts? Somebody around here surely knows the answer to this.

[danh]

Airtight only works in the city because city houses have electrically-driven mechanical ventilation.

VDRs aren't air infiltration barriers, but they can be taped up so that they meet that function. I'd only do this in the walls though: leave the ceiling sufficiently leaky so that stack effect vents interior moisture for you, and use a small passive vent for make-up.

This is an a-typical installation. It may be a good idea to talk to a historical expert - this is how the older homes were built :)

[rabadger]

Make sure you have plenty of ventilation for the attic. Use Vapor barrier, and have A/C sized and installed to keep the humidity down in the summer. I would think it should be sized like any other hot climate. When the load calculations are done you will have to furnish the height above sea level.

If this place is small look out for running out of combustion air for the heater. It will not take long when closed up. You not not want to create a down draft problem and fill the room with products of combustion.

NO POWER :!: OOOPS NEVER MIND. FORGET I WROTE ANYTHING.

[danh]

Make sure you have plenty of ventilation for the attic. Use Vapor barrier, and have A/C sized and installed to keep the humidity down in the summer.
I was tempted to make the same recommendations, but the design challenge for this project is lack of electricity.

[Anonymous]

Ops :!: I screwed up :!: I missed the part about no power. :cry: