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Comment by lhorie | Hacker News Reader

Not everyone has the time and energy to build their entire house, but I found that one relatively easy thing that helps a lot is attic insulation and a bit of DYI. It turns out that contractors often do really shoddy work when it comes to insulation because it's exceptionally easy to skimp on materials to save money: they buy the cheap fiberglass stuff and they use too little of it.

You can buy 15-20 bags of cellulose insulation and rent the blower machine from home depot, and you can do the whole attic in a few hours yourself. Including a rental van, that cost me around $600.

This alone made a noticeable difference in my house in Toronto (where it can easily get to 32°C/90°F in sun in the summer)

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I’d asking recommend people try to air seal as well as they possibly can from the attic side too. And install fire proof covers over all recessed cans.

Halogen light and others that create heat are so very bad. I’ve found scorched and charred timber several times and they have appeared to be installed correctly. LEDs are just so much better.

My mom paid to re-insulate her attic a few years ago. I think she paid $3,000 (in a rural area), but says it's dropped the temperature in her house by 10F, and when I've visited in the summer I've noticed. It's a remarkable win.

Also in hotter areas good to have ridge vents installed. Its basically make small slot in the peak of the roof with a cap above it. This allows hot air to circulate out of the attic.

This is next up on my todo list.

Any particular advice/pain points for re-insulating an attic with cellulose?

It's actually surprisingly simple to do.

If you've never done it or seen it done before, my only real advice is break up the cellulose into fluff before you put it into the blower machine, and put little bits gradually and slowly rather than stuffing it to the top. My wife thought she could shove a whole bag inside and have the mixing blade break it up, but it clogged the machine instead, and I had to waste a couple of hours unclogging the tube.

In terms of applying the insulation, you just want it to be thick, fluffy and somewhat even.

Other than that, just captain obvious stuff:

- you need two people (one in the attic and one feeding the blower machine)

- start from the edges towards the manhole

- use a respirator mask

- stay hydrated and take breaks if you need to (the attic is surprisingly hot)

Thanks for taking the time to respond.

The worst mistake I've ever made in an attic was doing cable runs (TV to the bedrooms) on a summer day in Texas. Sure, we started when it was cool at 7am... but we finished around noon.

So, to add to that: do it on a nice day in spring or fall.

Wear a respirator and goggles and do it during the coolest period of the year; even so, bring some water with you. Put down as much as you can afford to (as thick as possible, but evenly thick). When my contractor did mine, he put in 1-2 feet thick.

Also, because I live in Phoenix, we installed some ventilators on the roof to help exhaust hot air in the summer (in addition to the peak eave "vents" at the ends of the house). We wanted to do an exhaust fan, but there wasn't any way to get one into the attic. So far, the ventilators have worked fine.

Did you remove the old stuff first? Or do you just layer over it?

Just layer it on top. The rule of thumb is: the more you have, the better, regardless of which material you use.

This is a bad rule of thumb in some climates. Your technique of piling on insulation might create a dew point at a place that isn't ventilated, and thus cause a buildup of mold.

AFAIK, cellulose insulation is chemically treated to be mold-resistant and fire-resistant. Also, remember a rule of thumb is just that: by definition it can't cover 100% of edge cases. If you have a water leak in your roof, you obviously have bigger problems to address first...

It doesn't matter much if your insulation is mold-resistant. Presumably it's resting against wooden beams or other things that would be friendly to mold, or the water can drip down once it condenses to something that's friendlier to mold.

You wouldn't get water from a leak, you'd get it from thin air via condensation. Ever had fog on your car window? That's water on the inside of your window, and not because the roof of your car has a hole in it that's letting water through. Water condenses out of the air.

Making sure condensation happens in the right places is one of the main jobs of well designed insulation. What you saw as shoddy work from a contractor may have been a standard insulation design that's intentionally skimpy on the internal insulation, because the condensation should happen in an outside layer.

By piling on insulation you may have moved that dew point to the inside of the house or past a layer of insulation meant to deal with it.

The rule of thumb is that you can't just add more to one layer of your overall insulation (which is everything from your outside weather layer to the paint on your inside wall) without considering the system as a whole.

Search for "hygrothermal analysis" or "dew point" in relation to outer wall insulation. You'll find resources like [1] and [2]. The former has an example where more interior without corresponding changes to the exterior makes things worse.

I'm no expert on insulation, just a fellow home DIY-er. I've had enough experience with it to be very paranoid if I were in your shoes. It's worth bringing in a professional to evaluate this home improvement job of yours,

You may have created conditions for mold to thrive in your attic, or for water to seep into structural layers of your roof. By the time you notice those sorts of problems without having to open up a wall it's often too late.

1. https://www.jlconline.com/how-to/insulation/avoiding-wet-wal...

2. http://www.superhomes.org.uk/resources/interstitial-condensa...

I did have a mold issue in the bathroom shortly after buying my house (a few years before adding insulation). Had to redo the ceiling paint job, which was annoying to be sure, but that was about the extent of the damage, and we never got a mold problem again afterwards.

I also did have someone come over to quote me on insulation work when I was planning it (since the mold thing was a concern). They said my setup was fine as far as mold risk was concerned, and that it was clear that the two layers that were there were insufficient.

If in doubt, you can always ask for a professional opinion before attempting any DYI.

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