Insulating Roofs

[SLOBuds]

We have a cabin that we'd like to insulate significantly more than it is now. I'll start with a question about the roof, then post separately about the walls.

The roof has no insulation at all. It is a tongue and groove wood which is nicely exposed on the inside of the cabin, then covered with tar paper and shingles on the exterior.

I want to get that changed from no insulation to a very high insulative value.

With no experience at all in home construction, the way I picture this happening is to remove tar paper and shingles from the roof, add framing on top - with enough space to lay down some type of insulation material - then add sheeting over the frame and insulation, then re-shingle.

Is this the correct procedure? If so, what are details of the framing and what additionally should I consider for the project.

Thanks,
Martin

[Pat]

Martin, If the current roof is sound you'd be ahead to insulate on the inside if at all acceptable. If insulatioin thickness is an issue use high R-value rigid foam sheets. You can texture directly over the insulaltion or for something more like the original, cover with bead board or whatever you prefer the appearance of. Do some analysis no matter what you do to ensure the added weight doesn't exceed the safe limits of the roof structure to support.

It isn't cheap but there is a product that would make doing the job on the outside fairly easy. It is called insulated ventilated nail deck and it is available from various sources. You would probablly want to strip the shingles and tar paper to rid yourself of the weight. The product could then be laid out on the existing roof deck and attached to it. YO then shingle over it.

The product is a layer of rigid insulating foam spaced out with foam plastic spacers from a layer of OSB. You can specify the thickness of foam (controls the R-value) and the thickness of the OSB (you need just enough to nail to, probably about 1/2 inch.) You then use continuous venting at the eaves and a continuous ridge vent.

Solar heating of thl roof will start convection currents which will draw fresh air in at the eaves and expell it at the ridge. The rush of fresh air through this structure sweeps out a lot of heat from the roof deck which makes shingles last way longer. If you manage to lower the temp of the underside of the roof deck by 50% that is as good as doubling the R-value of the roof insulation. The convection currents are free. If yoiu specify foil backed insulation sheets (with the foil toward the shingles) that will reflect 97% of the radiant energy coming down from the roof deck.

I used this stuff in my 48x36 ft garage shop roof. It works. Before I knew it was commercially available I built essentially the same thing for my mom's sun porch roof and it worked like a charm making the room inhabitable more of the time and only a tad hotter than the outside shade temp.

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[SLOBuds]

Thanks. I'll look up the exterior materials because I like the aesthetics of the exposed wood.

Martin

[Pat]

Martin, This may be of interest to you...

http://www.certainteed.com/NR/rdonly...ltWarranty.pdf

http://www.atlasroofing.com/download...e_brochure.pdf

There is plenty more info available via Google search but these two, especially Atlas, gives a real good idea about the product. You can "simulate it" with foam sheets, spacers, and OSB but it wasn't a bargain in the case of my garage/shop to try to build my own.

Hope this helps.

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[SLOBuds]

How much do they cost?

Also, do you know how I should go about verifying that the current structure is able to hold the weight of the added material?

Thanks,
Martin

[Pat]

Martin, I'm sure materials costs have changed significantly since I bought mine over 3 years ago but a few well placed phone calls will get price and delivery info.

Which ever way you choose to add insulation will result in increaed roof loads given your cathedral ceiling where the boards you are looking at inside as your ceiling are the boards to which the roofing materials are attached. If it were me I'd be checking with a good GC in whom I had faith. Failing that I'd be finding a structural engineer with residential construction experience. The typical roofing contractor may not know nearly as much about roof structlure as how to cover it with tarred felt and shingles.

Any given roof construction will have a load handling capacity that can be given as part static load (weight of shingles, snow load and such) and lilve load (roofers running around up there doing there thing.) Some builders cut it pretty close so don't automatically assume that you can add significant weight with no problems.

The weight of the insulated ventilated nail deck is essentially the weight of the OSB (typically 7/16, 1/2 inch or similar) as the foam sheets are very light and the spacers are also foam.

If you r roof can't handle the load then there are 3 choices that immediately come to mind: 1. don't do it, 2. strengthen the roof and proceed with insulating it, 3. insulate the underside, i.e. your current ceiling with glued up rigid foam.

Option 3 would not require any structural modification as the foam is so light. You would want to cover the foam with something aesthetically pleasing. You can glue up bead board or other sheet goods with an appearance you like.

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[SLOBuds]

Thanks again Pat. I would not go the route of packing insulation on the inside because we enjoy the open beamed look. So I will try to find an engineer to give me an assessment of whether the current ceiling is strong enough for exterior materials.

That would essentially be the weight of x sheets of plywood, plus whatever shingles I decide to go with. There are shingles on the cabin now, so plywood would be my delta.

I saw some VERY thick insulated nail deck - 12" thick, of course with very high R Value. It was interesting looking at the various products on the market.

I always enjoy reading your posts. I'm going to shoot a separate question about walls that would be good to hear your thoughts on as well.

Martin

[Pat]

Martin, In general the priorities for insulation are: 1. ceiling/roof, 2. walls, and 3. floor. This is prioritized as regards payback. Of course if warm feet are a big deal and heating costs are not paramount, you would insulate a floor ahead of the walls.

Infiltration can be a significant factor. If you don't stop excess infiltration then U-values of windows and R-values of ceilings, walls and floors can't "make it right."

Ceilings/roofs are less frequently the site of significant fenestration as compared to walls. If you install enough windows you can cancel out a lot of the R-value gains found in insulating the walls. There is a point after which additional R-value in walls does little to reduce heat transfer. Think of a bucket with a hole in it for carying ice water. It doesn't matter much how well you insulate the bucket if the hole leaks fast enough the bucket is inneffective. I know I mixed my metaphors but I think I made the point.

Windows in walls are a lot like that. If you put in enough windows the heat transfer characteristic of the combined wall/window system becomes muuch like that of the window. You don't have enough money to buy windows with a U-value that would allow their extensive use without seriously degrading the overall window/wall R-value.

Note: Window perforrmance figure of merit of interest is U-value which happens to be the inverse of R-value, i.e. an R-value of 10 is a U-value of 0.10 but don't blame me as this was all decided before I got involved. I guess it keeps some of the civilians in awe of the practitioners,a secret handshake priesthood thing.

Anyway it doesn't pay to get too carried away with the R-value in the walls if you put enough windows in to leak so much heat that the wall R-value is an insignificant contribution to the calculations.

Yet another analogy: You are required to have two pipes running between two volumes. One pipe (#1) is large in crossection but does things you think you want done so it has to be there. How small do you have to make the other pipe (#2) in order to minimize the total flow capacity? OK, #1 is your window heat transfer capability and #2 is your wall heat transfer capability.

If you pay enough, you can buy an expensive pipe to use for #2 and it will have a very very small diameter and not pass much... B U T so what since pipe #1 has a large diameter and passes a lot.

This is the tradeoff quandary that architects have to deal with as they are tossed on the horns of this dilemma. This is why builders desirous of having an Energy Star label on their homes do not exceed guidelines for percentage of floorspace to have in fenestration.

You can only improve the windows so much without getting into seriously expensive technologies far beyond commercial off the shelf stuff stocked by Anderson, Pella, and the like. Also for what it is worth, Anderson Windows and Pella Windows (their good stuff not the second tier big box store stuff) virtually NEVER give the best bang for the buck. They are over hyped highly advertised product that may be of good quality but no better than other brands where you pay for windows and not so much TV and print advertisements.

Even if you do not have a crawl space you can insulate the floor. You put down rigid foam with a thin layer of OSB tongue and grove and raise the thresholds comensurate to the change in elevatioin and they trim or replace the doors. Then carpet or other floor covering. Some folks put down the foam insullation and then a layer of gypcrete or light weight type of cement to bury hydronic tubing for radiant floors. You don't have to put down the insulation if your calculations show that the mass of the slab gives more gains than the leakage of heat from it. But this is another story...

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[SLOBuds]

Well well well - I picked the correct 'surface' for my first post: the ceiling.

I have 4 windows in the small cabin that I can convert to double-pane. 3 others were already traded out, and the difference between the old aluminum/singles compared to the new vinyl/doubles is amazing.

So I think that I will change out the 4 and see what happens. Along with the roof insulation that I mentioned above.

The floor is slab. I have tile over slab, which we like. We would LIKE them to be radiant, but they aren't - so be it. Nice when it's hot though.

The 2 other things I will most likely do with this cabin is a whole house fan (one of these ultra quiet types - located up close to the peak of the cathedral ceiling) and possibly some trellis' outside on the south and west side.

Thanks,
Martin

[Pat]

Martin, You can put the fan on the roof and just have a grill inside. Not as ugly, not as noisy, and works fine. My range hood exhaust fan is on the roof. Works fine and isn't as noisy.

If you are conceerned with aesthetics, build a cupola or phony chimney to enclose the fan.

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]