rockwool -v- fibreglass in ceiling

creedp

Hi

Just a quick query. I have 225mm joists and will be filling these with either rockwool or fibreglass. Any opinions on which is better? The u-value of each is similar so what are the advantages of either? Also I plan to put a composite board under the joists. The original spec is for a 38mm composite board. I was considering opting for a 50mm board but builder considers it unnecessary as the 225mm of rockwool which can also be supplemented by another 150mm across the joists in the attic crawl space. However insisde the crawl space the attic space will be insulated, plasterboarded and skimmed so obviously I won't be able to add the 150mm across the joist in this area. Would it be worthwhile to opt for a 50mm composite board because of this?

Thanks

clint_eastman

I'd just like to add to this and see if there are any responses, I don't mean to hijack.
In addition to the rockwool vs fibre glass issue, what are peoples opinions on blown insulation? For instance, has anyone got a square meter rate for these 3 options to say 300mm depth (between and over joists) with their expected u-values?

soldsold

I'd just rollout extra insulation in the attic and put insulation boards on the joists before putting down the flooring in the crawlspace. The little you would lose in the crawlspace would be offset by the gain in the main attic and it should be a lot cheaper and you save on ceiling height.

Clint Eastman the 8 inch rolls of fibreglass are about 14 to 15 euro per roll, and pumped attic insulation should be 12 to 12 euro per m2 for about ten inch depth. It's a days work to fit the rolled out insulation so in my case I went with this. 8 inches in the joists tightly fitted and 8 inches on top. U value should be the same whichever you choose for the same depth but the roll out stuff needs to be fitted carefully and fluffed up in the joists

creedp

soldsold wrote: »

I'd just rollout extra insulation in the attic and put insulation boards on the joists before putting down the flooring in the crawlspace. The little you would lose in the crawlspace would be offset by the gain in the main attic and it should be a lot cheaper and you save on ceiling height.

Soldsold. I'm a bit slow here so can I just confirm are you advising that in the main area of the attic I put an insulation board over the joist before I floor the attic to compensate for the fact that I can't rollout insulation across the joists? Is this because of the significant cold bridge created by the joists when they are not covered by insulation? Is this even though I could go with a 50mm composite board under the joist? If I didn't place the insulation board over the joists would this result in significant heat loss into the main attic or would it help in keeping the main attic from getting too cold

Inrelation to the crawl space is it OK to roll out another 200mm across the joists even though they are not full filled - 225 joists with 200mm laid between them?

Thanks for you help

soldsold

Sorry mix up in my reading of your question, I thought you wanted to use the crawl space and not the other way around. If you are using the attic and will have the roof over the attic well insulated then any heat loss from downstairs into the attic is still staying within the heated zone in the house so not a big issue. I'd use the composite boards or an insulated service cavity in the sloping attic ceilings - ie the perimeter of the house

If the attic is for occasional use only, then composite boards make sense as they get rid of the cold bridging through the joists. It's what I would do in this case and I'd use the thicker boards.

creedp

Thanks Soldsold for clearing that up for me. I'm a bit easier of mind now!

One other thing I would like to comment on is the recommendation that a rigid insulation board and silicone or other appropriate sealant shouls be used to close the gap between the joists at the outside of the knee walls of an insulated attic space. I'm assuming this is to stop cold air getting in between the underside of the attic floor and the top of the ceiling insulation thereby reducing the value of the insulation plus drawing heat from the attic space.

Is there any risk that this action could result in condensation/dampness in this sealed space as there would be no airflow over the top of the insulation? Presumably if you have an airtight memberane on the ceiling there is very little change of moisture getting into the space in the first place. Would an alternative approach be to tightly pack rockwool into these spaces thereby inhibiting the flow of cold air into this space but not entirely sealing it?