I've got a flat roof ( wooden, sealed with tar and then roofing iron on top of that ) with a 30 cm gap between the ceiling. Not really enough for someone to crawl in unless skinny.

There's no insulation there at present. The lowest the temperature gets is 0 Centigrade.

I've read on various sites that you need a gap between the roof and any insulation, about 30 cm. But that would mean I can't insulate.

Is the purpose of the gap to prevent condensation of the insulation material against the roof? Is that a concern when I have a wooden roof?

Could I use blown wool? That is said to both absorb and release moisture.

I don't have any down lighting ( which can be a hazard if covered with insulation ). Recommended R value for this area for the ceiling is 2.9 metric ( US R of 16.5 )

video of roof space

3 Answers 3


The principle behind a flat roof is the same as with a sloped roof: keep moisture out of the uninsulated space of the roof. Most roof systems require venting. Venting is necessary to remove any moisture that comes from the inside of the home into the insulated space and causes condensation. Condensation causes mold and other moisture problems. As long as there is enough room for air to move unimpeded throughout the space and in and out through the vents, you should be fine. If you do not have the space for proper air movement, you can consider packing the entire roof with insulation. The idea with this method would be to prevent air movement in the entire space, preventing vapor from entering in the first place. This method is used for vaulted ceilings and is fine if done right. You should do a super job of sealing the interior ceilings to prevent vapor transfer to that space.

As far as recessed lighting is concerned, there are "cans" that are made for use in insulated spaces. They have a temperature sensor that turns the fixture off if it gets too hot.

  • I'm in Zone 2 dbh.govt.nz/quick-energy-guide and it says R value of 2.9 for roof insulation. Jul 16, 2014 at 4:17
  • 2
    This site buildingscience.com/documents/insights/bsi-043-dont-be-dense seems to suggest that dense packing of unventilated flat roof cavities does not work unless you first remove the ceilings to install an air impermeable membrane. Jul 16, 2014 at 4:26
  • I have a small unused solar hot air collector. I wonder if I could use that to pump hot air into the ceiling space to keep it under positive pressure, and dry, after I insulate the ceilings. Jul 17, 2014 at 4:30
  • I had loose natural wool blown into the space today covering all the spaces between the rafters and ceiling, so leaving lots of room still for air circulation. Jul 31, 2014 at 5:57

If (and it's a big if) you can find a company in the business that won't have to travel too far, this might be a good candidate for a spray foam (usually polyurethane) roof, applied on top of the existing roof. More commonly seen on commercial buildings, but a flat roof is a flat roof...

If you cannot find a company who can get to you and do it for reasonable cost, then the more typical "sheets of rigid foam" (expanded poly-styrene - XPS) on top of the deck, under the final waterproofing membrane "built-up roof" insulation method when re-roofing is probably your best bet. 3-4 inches of foam will do the job.

The spray on version has a higher materials costs but far less labor (and it can go on over the iron.)


The page http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling is a compendium of several other pages covers the subject pretty well, though it is targeted at cathedral ceilings, all of it applies equally well to a moderately sloped or level ceiling.

Keeping moisture out is the prime consideration for unvented rafters. To do it right, you'll have to either remove the ceiling or tear off the roof (and rebuild it afterwards).

For those of us in temperate areas, 2.9 seems pathetic—almost a Why bother? Indeed, three layers of 3/4 inch plywood achieve R2.82, so adding almost any roofing material over that would meet the requirement.

Edit: Since R2.9 metric is R16.5 non-metric, this is indeed a worthy level of insulation and should be done right.

  • Sorry, R 2.9 metric is equivalent to R 16.5 US Jul 16, 2014 at 6:47
  • @GrahamChiu: Whoa! I thought R values were dimensionless, and therefore universal. Sorry.
    – wallyk
    Jul 16, 2014 at 16:08
  • 1
    @wallyk: Nope, R value is a ratio but it's not unitless: it's the ratio of how much energy gets through a certain amount of area based on the temperature difference, in the US the units are ft2-°F-hr/BTU (and units for U value are the inverse).
    – Hank
    Jul 17, 2014 at 2:57
  • I have a fairly high stud, of 1.7m, so I guess creating a floating ceiling sandwiched with some insulation might be feasible, but not sure how expensive that might be. Jul 17, 2014 at 4:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.