Insulating a roof
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Last year we moved into a log home in Tennessee (zone 4A) and have now been through one winter and a summer here. The home has a timber framed loft, which has proven to be difficult to cool in the summer and heat in the winter. The ceiling of the loft is 1″ tongue & groove poplar, which also serves as the roof deck, on top of the poplar is ½” of blue board, then metal roofing. Using a DOE R-value calculator I get an R-value of about 2 – not good.
Traditional batts and blown insulation are out, as there is no place to put it. We are starting a remodel on the home, where among other things we are adding an addition and some dormers to the existing house, in the course of this we will be replacing the metal roof. So this gives us an opportunity to improve the insulation on the roof. I have been looking at options, such as EPS panels, poly-iso panels and SIPs. I have been concerned with the thicknesses required to get into the R-38 range, looking at the data on the Owens Corning site with their EPS.
Thermocore polyethylene SEPs may provide a path forward, but I am worried about cost and logistics (probably need to get the metal off to make accurate measurements, then there is a long lead time for mfg plus skilled labor to install in our rural area). Today I ran across Raycore, which seems like a plausible solution to our problem, however I can’t be the first to struggle with a problem like this. Are there others that have some experiences to shareable?
Also, there are lots of manufacturers claims as to thermal efficiency, are there any independent test reports available?
Thanks
Steve
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Replies
Steve,
There really are three possibilities here: (1) you can install one or more layers of rigid foam above the roof sheathing; (2) you can install nailbase; or (3) you can install SIPs.
In most cases, installing one or more layers of rigid foam above the roof sheathing is the least expensive option.
If you haven't read it yet, I urge you to read How to Install Rigid Foam On Top of Roof Sheathing.
Steve,
You didn't indicate how old the house is. If it is older it is likely the rafters may be undersized and should be brought up to current code. It is also likely the roof is not straight. With a older not straight roof, you most likely want a site built solution. Perhaps over frame the existing roof with engineered lumber where you can get a 14" or 16" joist depth. Room fro insulation and structure for the roof.
The home is about 12 years old, the fellow that built it bought poplar lumber from a local saw mill and put it together himself. The loft is framed with alternating 2"x8" and 4"x8" (full 2" / 4" / 8" dimension) timber trusses set on 3' centers. I should have mentioned that the home is T-shaped, the main loft forms the top of the "T" has an 11/12 roof pitch, the stem of the "T" has a 12/12 pitch.
Given that we have no building codes where we live, and the homes log walls will likely set a limit on thermal efficiency of the structure as a whole, I am thinking that pushing for a modest improvement in roof R-value (maybe R-20) will make heating & cooling much more manageable, then investing in a geothermal heat pump would be the most financially efficient way to go.
Another problem to tackle at some time is that the lower floor is not insulated. The floor structure is 4x8 timbers set in 48" centers, with 2x6 dimension lumber laid across it as underlayment, with a ¼" plywood "skin" forming the floors top surface. We do have a reasonably sized crawl space, and most of the HVAC ducting is located under the house. Access is through a hatch in a closet, there is no access through the foundation, which is large blocks of limestone masonry set on a poured foundation.
To answer the prior question, the roof does appear to be quite straight, but I am tending toward a site built solution as well, the dimensions on this place tend to be a bit "informal", and we won't know the precise dimensions of the roof until we get the existing metal off.
Steve. Note that you can buy reclaim EPS for significantly less (if you opt to do the outside insulation). Check out Insulation Depot or similar companies for pricing.
Is your crawlspace conditioned? See Martin's article (https://www.greenbuildingadvisor.com/blogs/dept/musings/building-unvented-crawl-space) on why you might want to include this type of project in your remodel.
Why not have the building modeled in an energy program? It will help you figure out where your best bang for the buck is. It seems you have lots of items you want to upgrade, get professional help to determine where the money is best spent.
Also just because they don't enforce a building code where you are, doesn't mean you can choose one and follow it. The code is there to set a minimum for safety and efficiency.
Interesting article on crawl spaces, ours has plastic on the floor, though I doubt it's sealed. We do have a few vents through the foundation. The photos show some nasty places, the sagging insulation looks like my mom's place, I can see where venting just brings in humid air to be dehumidifies on any below dew point surface it can find. Getting an energy model / audit would be great, but we are in a rural area and I haven't had much luck finding one.
Found another problem yesterday, was drilling a hole through one of the gable walls, found it was insulated with ½" of blue board - that was it other that 3 ½ inches of dead air, though convection will reduce the value of the air's insulation.
Right now Menards has 3" sheets of poly-iso for $33.37 (R-17.4) and 4" sheets for $44.49 (R-23.6), looks like we will need about 2,500 square feet plus whatever overage is customary for fitting.
As to the code, I try to follow it when it makes sense, but I question the value of R-49 insulation in a conditioned attic when the house walls are more like R-6 to R-8