Worth insulating?

[miamicanes]

I'm remodeling the master bathroom, and at the moment its ceiling has been completely removed, exposing the steel pan deck of the concrete suspended slab that forms the roof and temporarily giving me full access to the void between the ceiling drywall and pan deck for nearly the entire second floor.

The old ceiling drywall in the bathroom had foil on the underside to form a radiant barrier. The only other insulation is minimally-useful insulation in the built-up roof above. I won't be putting in new drywall with foil on the back, because I refuse to use paper-backed drywall *anywhere* in the house. That leaves me with three real options for replacement of the insulation:

* radiant-barrier insulation from Lowe's, placed between the new DensArmor drywall and the roof deck.

* Blown-in cellulose from Home Depot or Lowe's

* Nothing at all

While I have my doubts about how useful the radiant barrier really is, or whether the radiant barrier purchased from Lowe's would be appreciably better than gluing aluminum foil to the underside of the new DensArmor drywall, I'm unlikely to skip it entirely unless someone manages to convince me that it's totally worthless.

That leaves blown-in cellulose, and the big question: if there's only 3/4" between the ceiling drywall and metal pan deck above, will blowing cellulose insulation into that 3/4" gap above the rooms shaded pink in the pic below really make enough of a difference to be worth bothering with? I'm guessing that it will provide something like "R2".

Likewise, if it would be largely useless and add minimal value over the radiant barrier already in place over the bedroom and study, would it still be worth blowing cellulose into the areas shaded other colors (where there's 3", 4", or even a foot or more of void to fill)? Realistically, I'd end up with the middle third of the roof shielded by 4 or more inches of insulation, with the two ends shielded by the radiant barrier drywall and possibly 3/4" of blown-in cellulose.

Put another way, is it worth even screwing around with 3/4" of blown-in insulation? And if it's not, is it still worth blowing it into the middle third of the house, even if it would be a complete waste to blow it into the ceiling above the study and master bedroom?

It's not shown in the pic below, but the stairs are in the area labeled "Open Below", and the master bedroom has a window like the one in the Study. The two long sides are both in direct contact with the neighbors' houses. The second pic shows the void above the right green area from inside the master bath facing towards the stairs.

 

[Nestor_Kelebay]

I don't see how you could even blow cellulose insulation into a 3/4 inch gap. Isn't the hose they use to blow the stuff in with larger than that?

If it wuz me, I'd be sceptical too. I kinda figure it's gonna cost more to try putting that 3/4 inch of insulation in that space than the insulation is ever going to save you on your A/C or heating bill.

I won't be putting in new drywall with foil on the back, because I refuse to use paper-backed drywall *anywhere* in the house.

Why the blatantly racist attitude toward paper backed drywall? I like some of the gypsum core products that Georgia Pacific is making (like Dens-Shield), but I'm wondering what makes this Dens-Armor board you're intending to install so much better that you won't even consider installing regular drywall anywhere in your house? I'm not familiar with the stuff.

 

[miamicanes]

97% of the motive for gutting and rebuilding the master bathroom is a major mold problem. Paper-backed drywall = mold food. Florida mold is like cancer... you can fight it aggressively by taking away its food & water and send it into remission, but you'll never be truly rid of it. You can burn the house to the ground, bulldoze it away, build a new house, and rest assured that your brand new house will have at least as many dormant spores waiting to bloom as the old one did.

It's made worse by the fact that we don't have a real "heating season" like you do. Up north, the months spent with the furnace on and 10-20% indoor humidity go a long way towards keeping mold growth at bay. Down here, if you run your air conditioner 24/7/365 and have a whole-house dehumidifier, you MIGHT be able to sustain 30-35% humidity. Most people are lucky to keep it below 45%. All it takes is a week or two of summer humidity without air conditioning after a hurricane to turn your home's interior into a mold farm. The sad truth is that thanks to paper-backed drywall (including greenboard), practically every bathroom in Florida is a moldy biohazard.

 

[Nestor_Kelebay]

And "Armor shield" doesn't have paper on either side, and therefore doesn't provide a food source for mold?

I'm wondering whether or not drilling small holes into the wall cavities between studs and blowing Borax dust into those wall cavities wouldn't be effective against the dreaded "Florida interior wall mold" we've all read so much about.

Boron is as effective a fungicide as copper or zinc, but less effective than arsenic as a biocide against a wide spectrum of fungii. But, unlike copper, zinc and arsenic, boron isn't toxic to mammals, and so it poses little to no health risk to people and most of their pets.

(We humans can almost eat boron and it's many compounds for breakfast without concern about how it'll affect our health. But boron and borates are highly poisonous to a wide variety of fungi, including the fungi that we call "bathroom mildew" mold and the dreaded "wood rot fungus", Serpula Lacrymans.

I expect it would be equally poisonous to the fungi that fall under the general heading called "toxic mold" that's so prevalent in America.

Log homes and very many of the wooden telephone poles in the United States are protected from the wood rot fungus (called "Serpula Lacrymans") by solid rods made from a chemical called "disodium octoborate tetrahydrate", which otherwise goes by it's commercial name as "Cobra rods" or "Impel rods". These rods look like they're made of glass, but they dissolve rapidly in water.

These rods are inserted into holes drilled into the logs of log homes or into telephone poles just above the water line. If the wood the logs or poles are made of gets wet, then the Cobra or Impel rods dissolve in that water, thereby releasing disodium octoborate tetrahydrate into the wood. Since this chemical is highly soluble in water, it migrates throughout the wet wood easily, thereby protecting the entire cross section of the log from any sort of fungal attack, including wood rot. Borates are the only chemicals that migrate throughout the entire cross section of the wood to protect the entire log from fungal attack. Other wood preservatives merely cling to the surface of the wood or are absorbed into the wood at it's surface, but don't diffuse through the wood to protect it's entire cross section like borates do.

The only reason why borates aren't used universally to protect wood from from rot is because all of the boron oxides (borates) are all highly soluble in water. So, liquid wood preservatives like Borocol and solid wood preservatives like Impel and Cobra rods will dissolve and wash away when the damage to the wood is caused by rainfall or running water. Rain fall and running water will wash away the boron based chemicals and leave the wood unprotected against attack by fungii.

But, boron based chemicals seem to be universally poisonous to a wide variety of fungii, but pose little to no harm to mammals.

I'm thinking that "dusting" the wall cavities of stud walls with Borax (the stuff you buy in a grocery store in the laundry section) would be pretty effective at killing any sort of mold or fungus that might want to feed on the damp paper on the drywall in that wall cavity.

Rio Tinto Borax : Home

Borates, Impel Rods, Cobra Rods, for natural pest control and rot protection in Log, Timber and Wood homes

That is, I'm thinking that if the dreaded Florida "toxic mold" belongs to the "fungii" family of ugly looking things, then it'd be "killable" with a boron based powder like ordinary Borax; (the stuff people use for their laundry).

That's because other boron based chemicals like Boracol and Impel rods are highly effective against the wood rot fungus and work well to protect wood from rotting. I have 6 to 8 Impel rods in each of the bottom corners of the fir window frames in my building. People sometimes ask what the "caps" on the window frames in my building are all about, and it gives me the opportunity to explain how the Impel rods under those plastic caps work, if and when the window frame wood were to ever get wet.

Borate based wood preservatives and mold fungicides have been used in Europe for many years but are only now becoming popular in North America.
That's mostly because they're both effective and "green". The wood preservatives we've used in North America till now have been effective, but not very "green".