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Attic Floor insulation
Well it is that time of year... and I thought I should ask before I bought anything. Right now I have a very small amount of "rock wool" insulation under my attic floor, I think around 2 inches worth. That isn't a whole lot since the joists are 6 inches or more. Only half of the attic has flooring, so it would be easy to lift up the old floor to get under it. So my options are: batts, blown in, or new spray foam. I thought batts would be better since there would be less dust (and trust me, rock wool is really dusty).. but I wanted to ask for some opinions. So what does everyone else think?
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Replies to This Discussion
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Permalink Reply by Bill Conner on -
Cellulose blown in. Lots. Safe and good insulation. But you should seal all opening first - like top of wall plates of rooms below, around chimneys and chases, over the stairs, etc. Also if you can reach, seal top of exterior wall chases. A plastic garbage bag and cellulose blown in make a good "plug". Or fiberglass in a garbage bag - about the only good use of fiberglass I know of.
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Permalink Reply by Martin on -
Sealing all that stuff may be a problem right now... I figured that with any type of blown in stuff I used, I would have to do that first. But that is one vote for cellulose so far.
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Permalink Reply by William J. White on -
Cellulose. HOWEVER: what I did in one attic was to use fiberglass around the perimeter as a sort of dam, then blew in the cellulose.That attic is unused though. No actual floor, so I could take it up to R- 56 or so.
One trick is to wire the hose into place, Blow in the insulation, then wait for the dust to settle. Rake insulation into place and reaim the hose.
Dust be damned. restoration isn't pretty
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Permalink Reply by Martin on
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Nope it isn't pretty lol. Won't cellulose break down more than the fiberglass over time? So far 2 votes for the cellulose.
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Permalink Reply by John Leeke on -
Cellulose is already broken down. If it breaks down further, into tinier pieces, it becomes better insulation.
John
www.HistoricHomeWorks.com
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Permalink Reply by Mark Landry on -
I may raise a few eyebrows with my response, but in my opinion, it is far more effective to insulate the slopes of the attic instead of the floor. The easiest way to do that is with spray foam, although cellulose blown behind a mesh barrier works well, too.
The problem with insulating the attic floor, as Bill's reply suggests, is that air sealing is critical. Just laying down a lot of cellulose or (even worse) some batts is not a very effective way to insulate attics because if you have not air-sealed, you are losing lots and lots of heat through convective means. Air sealing means closing up all the cracks and crevices where air could move. It's very tough to do on attic floors because you have chimneys, pipe chases, bath fans, light fixtures, attic stairs, etc. Without air sealing, the performance of either cellulose or fiberglass is reduced dramatically. Like, almost to the point of being useless if the gaps are large enough. For this reason, cellulose is much more effective when it is densely packed into a cavity - when it does a decent job of stopping air movement- and much less effective if it's loosely blown onto a floor.
Spray foam air seals as it insulates so spraying the underside of the rood sheathing is really effective- so effective that the codes exempt spray foam from the venting requirements. We've been using this technique for years with great success. I will say, though, that if we're working on an early house with a beautiful post and beam attic, we'll typically use cellulose dense packed into the roof slopes instead of spray foam. Why? Because cellulose is more reversible than the foam. If a future owner really wants to expose the sheathing boards and rafters, it's quite easy to remove cellulose, quite a bit more difficult with foam.
Mark Landry
www.landmarkservices.com
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Permalink Reply by Martin on
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that is interesting...never thought of that technique before. It would be pretty impossble to seal up my stairway into the attic. I will have to look into this option.
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Permalink Reply by Mark Landry on
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This technique has slowly been gaining traction through the years, although most advice you hear still is for the old way of doing things. We've been doing it for almost 15 years. Ever feel how cold your ceilings are even though there's insulation just above them? The Journal of Light Construction had a good article on this approach about a year ago. The additional benefit to insulating the slopes is that any HVAC equipment in the attic is now in conditioned space, making it work much more efficiently and reducing the likelihood of ice dams.
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Permalink Reply by Martin on
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I have a cheap asphalt shingle roof (came with the house)... won't sealing up the slopes on the attic side cook the shingles off, with no venting? I could see where if I had slate or a steel roof it wouldn't matter. I am going to check into this though, since I was thinking of finishing off the attic eventually.
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Permalink Reply by Mark Landry on
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Most people that I've talked to haven't seen any rapid wearing of asphalt roofs. But even if, for the sake of argument, this took 10% off the life of your shingles, I still think the benefits far outweigh that potential cost.
Make sure to take out any knob & tube wiring, if you have any, before insulating.
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Permalink Reply by Martin on
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yeah this is also true.
I am rewiring the whole house as we speak to elimate all old wiring. Your methods seem very sound. I am going to investigate this further. Thanks for the info!!!
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Permalink Reply by Bill Conner on
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I support Mark's suggestion. As an alternate method, consider sheathing inside with rigid foam and then dense packing cellulose behind. When sheathing, leave a 1/3 or 3/8" gap and then seal with spray foam. Likewise, you can plug the bowing holes with a cutout foam plug and spray. You can really see if you get this sealed.
This whole insulate under the roof deck is especially beneficial if there is duct work in the attic.
You do have to have a good roof and keep a watch on it, but the celluylose is very tolerant of a leak and will dry out.
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