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oceandreamer
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Posts:2
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| 06/07/2009 5:15 PM |
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Hi all,
I'm a property manager in Oakland CA. I'm really enjoying reading through the posts on this site and appreciate all the great information.
Two of the older buildings I manage- two story, 4 units each, built in 1927- are basically uninsulated (there's some cellulose in the "attic" (crawlspace) and some fiberglass batts in the basement, but that's it).
The buildings are stucco exterior with single pane aluminum windows that were installed in the 1950s. The roofs are flat with a silver reflective coating. The buildings get hot during the summer and often don't cool down (on their own anyway) until the early morning hours. The tenants buy portable AC units and fans to get through the grueling (LOL) Northern CA summers and it just seems like such a waste of energy.
A friend was visiting from New Zealand and she talked about the various ways that people shade their dwellings in NZ-- from elaborate motorized screens to makeshift "double" roofs. It got me wondering how much such a covering would help in my situation.
Would something like a fabric canopy over the roof be of any benefit? Something to keep the roof in the shade? The building footprint is about 35' x 45' and I've got secure points to attach, for example, cables from which to hang white or otherwise reflective material.
With the uninsulated windows and walls, I'm guessing that the building will get hot anyway, but maybe it wouldn't get quite as hot and would cool down quicker.
Thanks for any comments and suggestions.
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smartwall
Registered Users
Basic Member
Posts:129
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| 06/08/2009 6:50 AM |
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| If there is a crawlspace in the attic , then reinsulate. It has to be a lot cheaper than a sun shade system. |
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oceandreamer
Registered Users
New Member
Posts:2
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| 06/08/2009 10:39 AM |
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Thanks for the reply, smartwall. Do you mean replacing the cellulose
in the attic with another material, or adding more cellulose, or...?
There's a pretty heavy layer in there now.
A sun shade system could be rigged up fairly
inexpensively-- I was thinking about stringing cable and attaching
sheets of UV-protected material- but I'm just wondering if it would be
a waste of time. I'm sure it would help reduce the heat on the roof
itself, but because of the way the rest of the building envelope is
constructed and uninsulated maybe it won't really make a difference. |
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ICFconstruction
Registered Users
Advanced Member
Posts:638
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| 06/19/2009 8:37 PM |
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Here in MN we put 18" or more of insulation in the attic.
In Argentina a lot of houses have a second sheet metal roof on steel trusses above the other. |
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Brad Kvanbek - ICFconstruction.net |
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smartwall
Registered Users
Basic Member
Posts:129
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| 06/24/2009 6:53 AM |
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| No add to it |
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techlady
Registered Users
New Member
Posts:1
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| 06/25/2009 8:41 PM |
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| What do people think of a radiant barrier, such as Low-E, as a solution here? It could keep some heat out of the attic, and, if you place it over the cellulose, reflect any heat in the attic away from the next floor down. |
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Dana1
Registered Users
Advanced Member
Posts:702
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| 06/26/2009 3:14 PM |
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Posted By techlady on 06/25/2009 8:41 PM
What do people think of a radiant barrier, such as Low-E, as a solution here? It could keep some heat out of the attic, and, if you place it over the cellulose, reflect any heat in the attic away from the next floor down.
It depends on how much cellulose is in there in the first place (and how much you might be able to blow in there.)
If you have less than 5.5" of cellulose in there (~R19), then a radiant barrier will give you roughly the "R-equivalent" in cooling of having 11" of cellulose (with negligible improvement in heating-season efficiency.)
If there's space to just blow over to achieve a total depth of 12-15" (R40-R50) that will be more effective than radiant barrier, with significant improvement in the heating season as well.
The performance (and therefore the value) of a radiant barrier is highest when the insulation R values are low. As yo add R-value, the performance of the radiant barrier goes down, since it reduces the total heat by a fixed percentage, and the higher the R-value, the lower the initial heat flux. But the cost never changes. To figure out whether it's cost effective for you to add radiant barrier, based on your R-value & climate, the Oak Ridge National Labs did the math for you already:
http://www.ornl.gov/sci/roofs+walls/radiant/rb_tables.html#table4
Find the R-value & city most similar to yours. The number in the box is the MOST you should pay for radiant barrier (installed) in order for it to at least break-even over 25 years. If you look at the R19 column, you'll note that the maximum cost-effective price per square foot is less than the cost of the material for most cities listed.
They have a similar calc for the cost effectiveness of more insulation:
http://www.ornl.gov/sci/roofs+walls/radiant/rb_tables.html#table6
Cellulose is cheap stuff, and doing open-blow cellulose in attics isn't rocket science as a DIY project. If you have skimpy attic levels it's almost a no-brainer. 25lb bags cost ~$10-11 in big box stores, and that'll cover 40square feet at a settle R-value of ~R19, so that's $025-0.28/ft^2 for the materila. Most of the box-store chains that carry offer a "free rental" of a blower for a coupla days if you buy 10 bags or more.
If you have 4" of insulation or less in the attic it's cost-effective everywhere to add another R19 on top of it. If you already have ~5.5" it's still cost effective to add another R19 as a DIY project in most places- everywhere except Florida & the Gulf Coast and the coastal southern California.
And once you're at R30 or more, adding radiant barrier isn't cost effective anywhere.
So the short answer is: Boost the R-value to R30+ if you can, and forget about radiant barrier. If there really isn't space to go R30+ (rare, but it happens), radiant barrier can still provide some relief at very low R values.
There have been literally hundreds of studies on radiant barriers over
the past 30 years at the FSEC and Texas A & M to back up these
guidelines, and the ORNL cost-effectiveness tables are conservative. You might calculate a different price point depending on what you believe energy price inflation will look like over the next quarter-century. But R-value generally trumps radiant barrier, no matter what the price of energy.
The roof is only one part of the solar-gain equation (albeit an important one, particularly on low-angle & flat roofs in southern US latitudes.). Glazing on the E,S, and W sides are another huge factor to control, and exterior shades (awnings or shutters) beat the performance of any low-E coating or window film, etc. (But reflective window film is cheap, and DOES make a difference.)
Another huge factor in older sun-belt studwall buildings is radiant transfer through the siding. Again, exterior shades are more effective than radiant barriers, but light colored paints with infra-red emissivites between 0.3 and 0.6 can make a difference. (But again, not as much difference as blowing insulation into the stud-bay cavites. The cost of retro-insulating walls is much higher than attics per unit R-value, and the solar gains lower, so as a cooling strategy it may be harder to rationalize strictly in dollar terms.)
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