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Ed and Fran

Aug 24, 2006, 3:08 PM

Post #1 of 9 (5675 views)

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Insulating a concrete roof?

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We have a fairly standard 10cm (4") concrete slab roof (slight slope). Because it is exposed all day to the sun, it gets pretty toasty upstairs. The a/c is hard pressed to keep up. I wonder if there is anything worthwhile that I can do to retrofit insulation to this existing roof? I have 3 possible thoughts, and would appreciate either comments on these ideas, or any other techniques that you know about.

1. Painting the roof with one of those combination waterproofing/thermal insulation paints, probably in white. Easiest, but I wonder how much help this type of product actually provides. (Note: don't need waterproofing, but I've usually seen this type of product as the combination.)

2. Installing spanish tiles (either the real ones, or those new panel type products) over the existing roof. If I leave the spaces under the curved tiles open, I wondered if the combination of shade and air movement under the tiles would help knock down the temperature in the concrete slab.

3. Put a thin layer (maybe 2") of styrofoam insulation on the roof and pour maybe 2" of lightweight concrete on top to hold it in place.

I suppose I could also consider installing a suspended ceiling inside to help insulate from the radiant heat from the slab, but that wouldn't be my preferred option if I can avoid it.

If I were starting from scratch, I'd install one of those roofs that use the large styrofoam blocks on the formwork to produce a honeycomb slab. But we're not going to rip the roof off and do that. By the way, that style of roof slab has its own name down here, but I'm not sure of what it is. I believe it's something like "losa reticular". Anyone know for sure?

Thanks

Ed

Rolly / Moderator

Aug 24, 2006, 4:25 PM

Post #2 of 9 (5667 views)

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Re: [Ed and Fran] Insulating a concrete roof?

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#3 is your best choice. But I would go with 4" foam blocks.

My roof, like many of my neighbors' has 4" of dirt with a light concrete cap. I think Styrofoam would be better, and I would have used that if it had been available here at the time I built my house (not to be confused with my other building projects which came later).

The concrete cap needs to be waterproofed so you don't get a buildup of water (added weight) in the foam.

The reduction in heat transmission through coffered ceilings using foam panels is not great; it helps, but not a whole lot, because there is still a lot of concrete to conduct the heat. I've built three and have been a bit underwhelmed.

Rolly Pirate

jreboll

Aug 24, 2006, 7:19 PM

Post #3 of 9 (5653 views)

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Re: [Ed and Fran] Insulating a concrete roof?

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Our home and most houses around here (Jiquilpan, Mich)have a layer of valdoza(sp?) on top of the concrete roof. These are thin bricks which serve as insulation and to water-proof the roof.

patricio_lintz

Aug 24, 2006, 8:08 PM

Post #4 of 9 (5647 views)

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Re: [jreboll] Insulating a concrete roof?

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In Jalisco, the tiles are known as Azotea. They are very brittle and do not stand up well to foot traffic. Usually, they are coated with a mixture of cemento blanco, sellador y agua. Color pigment can be added if you do not want white.

morgaine7

Aug 24, 2006, 11:26 PM

Post #5 of 9 (5632 views)

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Re: [Ed and Fran] Insulating a concrete roof?

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My roof, same sun scenario, is being totally replaced (not retrofitted). The architect/engineer described two options at roughly the same cost:

A) Foam panels as in your last paragraph and as Rolly describes (shown very clearly on his website). Roof thickness for my place was specified as 20 cm including foam, and the honeycomb method was referred to as "nervada" or "nervadura", though that could be Baja Sur-speak.

B) Thermal insulation sprayed over the new (thinner, 10 cm I think) reinforced slab, not a paint but a thick synthetic substance. Can't find the paper w/ details and specs just now. This method is relatively new and also can be modified for use on exterior sun-facing walls ... ends up looking something like textured rustic stucco when painted.

They recommended Plan B as more efficient for the roof of my little one-story Mexican brick house (for large expanses, they use the foam panels for obvious weight reasons). If the spray-on stuff is available in your area and contractors are familiar with the method, it might be a possibility for your existing roof; otherwise I'd go with Rolly's recommendation.

Kate

caldwelld

Aug 25, 2006, 9:37 AM

Post #6 of 9 (5609 views)

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Re: [Ed and Fran] Insulating a concrete roof?

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Before doing anything else I would just paint it white with some relatively cheap acrylic water based paint. If that doesn't do enough for you I would then turn to the other options following Rolly's advice.
dondon

morgaine7

Aug 26, 2006, 10:35 PM

Post #7 of 9 (5561 views)

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Re: [morgaine7] Insulating a concrete roof?

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FYI, found my contractor's quote: "cimbrado, armado y colado de losa de concreto de un espesar de 10cm incluye aplicación de poliuretano espreado con acabado color blanco". As explained in context and shown to me: 10 cm (~4") reinforced concrete slab including application of spray polyurethane foam (SPF) coated with a white reflective finish.

More about SPF in this article and others on the roofingcontractor.com site, which I've found to be a useful resource for understanding things I haven't much of a clue about:
http://www.roofingcontractor.com/...CM100000f932a8c0____

Bottom line is that SPF works on existing roofs (including concrete) presuming those applying it have the proper equipment and training. My Mexican contractors do a lot of commercial and government work, which would explain their experience with the method.

Kate

Judy in Ags

Aug 27, 2006, 7:47 PM

Post #8 of 9 (5528 views)

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Re: [jreboll] Insulating a concrete roof?

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They are called cuÑas here (Opps, got a cap.)

MariaLund

Sep 11, 2006, 6:47 PM

Post #9 of 9 (5460 views)

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Re: [Ed and Fran] Insulating a concrete roof?

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An ecofriendly Nordic solution is somewhat similar to your #3 option, but instead of lightweight concrete on top of the dirt, a blanket oc cacti and succulents is planted in the dirt on the roof. I wonder, however, how it would fare in a tropical rain - probably not well, pity. They do look great, those "natural" roofs.
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