Wall insulation and vapor barriers in hot, humid climate

[JivaGo]

I can see this is a UK based forum. Maybe someone can help anyway, or suggest a high traffic US based DIY forum?

I have a small wooden house on cinder blocks in Gainesville, FL (North Central Florida: considered a hot humid climate, but it can get cold in winter sometimes). It's a common post war type building often referred to as a "cracker" house (although technically a cracker house is a very specific style and construction that this is not). It has 2x4 wall studs (designed by a crazy person) and an unsheathed metal roof.

The walls have a type of scalloped board as siding (and sheathing... just one layer total), and thin paneling with plaster in the interior. There is zero insulation in the house, but for an insane mountain of some kind of fluffy stuff in the attic.

I have some insect damage, so I pulled all the siding off the front of the house and screwed up pressure treated plywood. Now I want to unscrew that, replace all the studs (there a reason, too long to explain), insulate, and then replace the windows.

General plan: Replace wall studs section by section, add fiberglass batt insulation, have exterior grade OSB on the *interior*, and then pressure treated plywood as the initial exterior wall. As the budget permits, then a layer of rigid foam over the plywood and a final layer of Hardi board (cement fiber siding, looks like wood siding). Then plasterboard in the interior etc.

So getting to my question (finally, sorry):

-How do I avoid trapping moisture in the wall during the long hot humid summer? There will be lots of heat and moisture outside and lots of cold air (AC) inside the house. I'm a displaced Northerner, where I think the vapor barrier was behind the plasterboard. Down here, I think it's opposite, underneath the exterior.

So with my plan of OSB, fiberglass batts, and plywood, do I need an additional vapor barrier? Both the OSB and the plywood are effectively vapor barriers, should I use a house wrap *under* the plywood to act as a drainage plane to wick out moisture? Or would the house wrap/drainage plane be after the plywood when I finally get to adding the foam board and the Hardi board? Of course the foam board is a vapor barrier as well.

Pardon my verboseness and hopefully the core of what I'm asking is clear.

Thanks,

John

 

[bernardgreen]

There is an alternative to vapour barriers and that is breathing walls. The architect Walter Segal designed houses with breathing walls. I DIY built one and lived in it for over 30 years and never had any problems with damp or condensation trapped in the walls.

http://www.selfbuild-central.co.uk/first-ideas/examples/11a-b-and-c-allerton-park-leeds/

http://www.mvuf.org.uk/epicentre/breathing-walls/

 

[AronSearle]

For US sources of information you could try http://www.awc.org/helpoutreach/faq/faqFiles/Wood_Repair.php

Yes, you have to worry about what we call reverse condensation with AC. I am not wholly familiar with your climate or practices over there, but if it is always colder on the inside, then reversing the wall will probably work, may also depend on internal humidity.

Be careful with architects and the like putting forward breathing wall construction. The principle itself has some sound merits, but such people also insist on mixing it with other poor details (like insulated panels in a frame, or direct render with no cavity, both of which have lead to a great number of failures). Avoiding condensation in walls is climate specific, so just because breathing walls work in X or Y, doesn't mean they will work in Z.

I can never understand the American practice of direct rendering/cladding with no cavity, it has been shown to cause failures on a significant number of buildings. You also need to remember that as you start increasing the insulation, you reduce heat loss through the wall, sometimes this heat loss is what keeps a wall dry if it has no cavity. Maybe Florida has little rain and you don't need to worry about moisture penetration?

Anyway......

The approach in this country is to use more vapour resistant materials on the warm side, for breathing walls you could consider using materials of the same breath-ability throughout the construction (so OSB both sides).

Generally, if all the materials are of the same breath-ability then you can avoid condensation, but this tactic may be a bit risky with high levels of insulation as you will then get large temperature differentials.

You could also look at things like intello membranes, these are membranes that will change from being breathable to non breathable depending upon the environment http://www.ecologicalbuildingsystems.com/products/airtight-membranes/intello-plus/

In the UK there are programmes like build-desk (google it), that allow you to run a condensation analysis. I haven't used it for ages, so can't remember if it only works in the UK, or if there is a US version, normally these programmes ask you to select a country/region as this will affect the answer.

 

[tony1851]

DIY Chatroom is a US forum with some excellent info. on vapour barriers.
Their use and position varies widely in US depending on location, so that would be the best site to look at.

 

[r.bartlett]

make sure the ac isn't too large. US houses size ac small to ensure the system runs longer to remove moisture as much as heat.