Rich & Kate,
Since late Fall, I've been researching my insulation options
for my near flat roof, which has 2:12 slope built on 4x6 joists 30"OC.
I'll include a link to give you more in depth resource in the end, and I want to share with you what I've learned so far, but first I want to be sure that I understand structure of the roof above your head.
Your 1x8 planks (probably D.fir T&G) nailed across 4x8 joists 8'oc.
OK,
If that's right, then your options are limited by load bearing limits of this structure. Which any structural engineer can calculate for you for a nominal fee ($ well spent).
These are the options I explored:
(a)
Building a conventional 4:12 or 6:12 pitched roof over my existing ceiling, which would give me 2 benefits: 1_an attic space for insulating as well as maintaining my existing beautiful exposed wood ceiling. 2_Faster slope to enable drainage over comp.shingles.
This will be my most expensive option costing me north of $40,000 or more depending on your local codes and cost of structural changes necessary to bear the weight of drywall and added lumber on top.
This was my lease desirable option.
(b)
Strip off existing top layer to expose existing planks and 1" foam boards, then blow closed cell poly urethane foam over it and apply top skin of choice over it.
Blowing poly-U closed cell foam is not really a DIY operation, its best left to the pros, but there are a few companies that sell pre-packaged products for the DIY too.
Best part of this option is that you get 3.8 R value per inch of foam, which is much more than fiberglass batting.
(c)
Another option is applying another layer or two of rigid closed cell foam sheathing over existing roof, taping and staggering seams. This yields a very good result and very doable by DIY warriors.
This is my most desirable and cost effective option.
The other insulation materials like loose cellulose or fiberglass batting are not relevant to low pitch/flat roofs because there isn't adequate head-room to pack enough insulating material in to make an impact on R value.
http://energy.gov/energysaver/articles/insulation-materials
Good luck and we'll compare notes comes next Fall.
-S-
--- In DoIt_Yourself@yahoogroups.com, "mountain953346" wrote:
>
> We have 2000 sq ft roof that is composed of 1x8's over
> 4x8 on 8ft centers, then asphalt roll and tar covering the 1x8's.
>
> That's it. There is no insulation and we are loosing most
> of AC and heat through the roof.(It is a low-slope "flat" roof)
>
> We have computed the costs for doing a new inside the house drywall
> ceiling dropped down 3 1/2" with r-15 fiberglass insulation
> at about $1.25 a sq ft. minus labor..having done drywall
> before we are confident to do the work although it seems
> a lot more intensive than adding new roof...??
>
> Add roof approach is to add 2x4s on 16in centers on the roof
> and put insulation there, then cover with 3/8" CDX and asphalt
> roll roofing over that which is now on the existing roof
> (maybe 25 year old roof now so could use a replacement)...
> essentially making a new roof....we also have done DIY
> in this area. Costs of materials about the same.
>
> It seems to me that the roof approach is way less labor
> intensive than beaming the inside ceiling, juggling
> even lightweight drywall, insulating and screwing and taping
> and mudding the inside ceiling...
>
> I would be very interested in the DIY'ers thoughts on the
> approaches to this project...or a new approach we have not
> thought of to solve this upward massive energy drain. In the
> summer the ceiling inside is a virtual radiator. We live
> in San Diego away from coast 15 miles so it gets hot but
> does cool off at night (semi-desert climate).
>
> Thanks in advance for any and all thoughts and ideas.
>
> Rich and Kate.
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