Converting an attic with blown insulation into an unvented cathedralized attic
I am planning a staged renovation project where i may defer the finishing of the attic space to an undetermined date in the future. To save money now, i am planning on just blowing cellulose to achieve the required R value (20 +inches). In the event we decide to make the attic space usable, we will pull out the blown in insulation and do an unvented insulated cathedral ceiling.
Can anyone provide any tips on removing that much blown in insulation without messing up the house? it is about 800sf of floor space. I am wondering if i would just be better off now to do the unvented insulated cathedral ceiling. I would get the benefit of having usable storage space if i went that route now.
Any advice is appreciated.
thanks!
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Replies
Alok,
It will be a significant expense to install 20 inches of cellulose, and it will be a significant expense to remove the insulation later. But it's just money.
Insulation contractors remove unwanted attic insulation all the time, with large vacuum hoses. Like many construction jobs, it's all in a day's work for a contractor. Call a few contractors up and ask for estimates.
Once you hear their estimates, you may decide to turn the attic into an unvented conditioned attic.
If "...undetermined date in the future..." is likely to be sometime after 2030, go ahead and install the cellulose now. If it could be as soon as 2020 or earlier there's no way you'd be saving any money over that short a lifecyle, even if you were heating with resistance electricity at 25 cents/kwh (in which case spending the money on heat pumps would be a better investment than 20" of cellulose for 5 years.)
If you install cellulose that's only as deep as the joists you won't need to remove it if you decide to finish it off later, which saves some part of the project expense. Even if that doesn't meet code-min, it's better than lesser insulation, and doesn't cost as much as the R70-ish you'd have with 20" of cellulose.
It's not quite clear what is meant by "... required R value..." in this context. Code in the US does not exceed R49 anywhere, which is about 13-14" pf cellulose (settled depth, 14-16" initial depth.) If you have 2x12' timber joists, you'll hit about R38-R40 if you just fill up to the joist tops.
Removing the cellulose when you insulate at the roof deck is not required by code, but the R-value of that insulation cannot be applied to the code-minimums for code compliance purposes. eg: If you have R38 at the attic floor you can't just hang some R11s between the rafters and call it R49. In that scenario only the rafter insulation would count, which would be considerably sub-code. (For some reason the spray foam trades seem to promote the idea that pre-exisiting attic floor insulation must be removed.) Installing insulation in partitions within the conditioned space is done all the time for soundproofing or the heating/cooling zone isolation reasons, and does not create problems as long as the outer assembly R-values are fully up to code.
Dana,
Good answer. Thanks.
Thanks guys. Dana, i should have been more clear about 'required R'... i was shooting for R70 or so, not code minimum. But you make an excellent point... i need to look into a better balance of cost versus likelihood of future renovation. If likelihood is high, as you point out, i am better off insulating to code and not to R70.
If you do the math you'll find that the difference in actual energy use between 800 square feet of R35-ish (what you'd get with a 2x10 rafter fill) and R70 over a decade is pretty small, and the difference between R35 & R49 is smaller still.
I'm not a big advocate of insulating to less than code-min, when the anticipated lifecycle is long, but I'm also not a big advocate of wasted capital. When the 1-2 decade plan for the attic is uncertain you may be better off spending the energy conservation capital somewhere other than bringing it even to code-min. Code-minimums & higher have a very good financial rationale over the lifecycle of a house, but aren't always a good investment on a sub-decade basis. Going higher than code min (or even fully code-min) here may not make sense, even if going better than code in other parts of the house might.
There is some discussion around making subsequent versions of the IECC performance based (along the lines of the Swedish model) rather than the traditional prescriptive R value /U-factor approach. It's possible to beat a typical IRC 2012 prescriptive code-min house on energy use at sub-code R-values with a bit of design.