Airtightness Query

creedp

I've got a quote for airtightness SIGA membranes/tapes for a new build and while the tapes for the window/doors opes are relatively OK, the cost of the ceiling membrane is very expensive. My builder thinks Im mad as he says he can get down to 3 ach without using any membranes/tapes!

Just a quick question to you all, is the SIGA ceiling membrane a good option or are there other more reasonable membranes or alternative options to achieve ceiling airthigtness? Also is is sufficient to staple the membrane to the rafers or should I use doublesided tape for this job?

Thanks

creedp

Sorry I should have said joists and not rafters as I have an attic overhead which will also be insulated, slabbed and skimmed down to the knee wall, i.e. the crawl space will just have rockwoll between the joists. Incidentally, would it be necesary to use a SIGA or equivalent membrane in the attic as it will not be used for accommodation purposes or would 500 guage polythene/OSB be sufficient?

Thanks again

L driver

Creedp,
These membranes are a bit on the expensive side but I saw a post on boards recently, by beyondpassive I think, and using osb instead was a little dearer perm2 on capital costs. Intello and Isover are other brands.
Regarding the attic space I'm a fan of airtighting from wallplate up the rafters across the collar ties and down the other side. I think stapling to the rafters is easier and you can always go over the staples with a piece of tape.

creedp

Thanks L Driver. I would like to airtighten from wallplate also but i have a dormer roor and its very messy to get to the wall plate especially around the dormer windows and also my builder went ahead and put in the studs for the knee walls - he must have had a quiet day and wanted to keep lads occupied. Also I have block dividing walls upstairs and for some inexplicable reason they have been build right up into the rafters, i.e. up to the slate. This would make it even more difficult to apply the membrane. Again is a polythene membrane unsuitable for this work?

Btw I've been telling the builder that bulding the dividing walls into the rafters is a big cold bridge problem and asked him to knock the top off the wall but he's quite reluctant to do so - should I insist? Irrespective of the airtightness issue I would like to be able to put insulation on top of these walls to stop then drawing coldness from the roof down into the bedrooms! Sorry for going off main topic but would appreciate all views.

MicktheMan

creedp wrote: »

Sorry I should have said joists and not rafters as I have an attic overhead which will also be insulated, slabbed and skimmed down to the knee wall, i.e. the crawl space will just have rockwoll between the joists. Incidentally, would it be necesary to use a SIGA or equivalent membrane in the attic as it will not be used for accommodation purposes or would 500 guage polythene/OSB be sufficient?

Thanks again

Why are you insulating the ceiling (i.e, the attic floor) and also the rafters & kneewall?

MicktheMan

creedp wrote: »

Btw I've been telling the builder that bulding the dividing walls into the rafters is a big cold bridge problem and asked him to knock the top off the wall but he's quite reluctant to do so - should I insist? Irrespective of the airtightness issue I would like to be able to put insulation on top of these walls to stop then drawing coldness from the roof down into the bedrooms! Sorry for going off main topic but would appreciate all views.

Ask the builder why he did this. It does not make any sense and you are right that he is building-in unnecessary thermal bridges into the structure. They are difficult enough to design out in the first place! Have him bring the walls down to below joist level.

creedp

MicktheMan wrote: »

Why are you insulating the ceiling (i.e, the attic floor) and also the rafters & kneewall?

I'm insulating and airtightening the ceiling of the 1st floor as I'm not going to use the attic for accommodation and I don't want the 1st floor heat to be leaging up into the attic space creating a greater heating demand. Originally I wasn't planning to develop the attic space other than to floor it out for storage but when I saw how spacious it was I thought it would be wasteful not to insulate the space inside the knee walls and slab it so it could be used for more than just storage. I will also make provisions for radiators in the space although I don't intend to heat it on an ongoing basis.

With this approach are you suggesting that it would not be necessary to undertake airtighteness work in the attic but to simply insulate/slab/skim the space? In order to reduce costs I was thinking of using polythene on the warm side of the insulation in order to increase airtightness but maybe this is not necessary. Instead maybe it would be sufficient if I insulated with insulation boards, taped the joins of the boards and then slab and skim? My one concern is that I will have a stairs from 1st floor level to attic and unless I insulate and undertake airtightness work on this landing, i.e. seal the the attic landing from the main attic space, then the membrane of the ceilings will be less effective as heat/air will escape up the attic landing and into the attic.

Any advice would be very welcomed as I didn't budget for developing the attic and the bills are mounting

creedp

MicktheMan wrote: »

Ask the builder why he did this. It does not make any sense and you are right that he is building-in unnecessary thermal bridges into the structure. They are difficult enough to design out in the first place! Have him bring the walls down to below joist level.

Thanks MicktheMan. I will pursue this with the builder. For the most part, its easy enough the get at the top blocks and knock them back to rafter height except in one location where there is a double rafter installed, because of a velux, snugly on either side of the wall. How he managed to be so precice in his measurements I can't imagine!

Thanks

MicktheMan

creedp wrote: »

I'm insulating and airtightening the ceiling of the 1st floor as I'm not going to use the attic for accommodation and I don't want the 1st floor heat to be leaging up into the attic space creating a greater heating demand. Originally I wasn't planning to develop the attic space other than to floor it out for storage but when I saw how spacious it was I thought it would be wasteful not to insulate the space inside the knee walls and slab it so it could be used for more than just storage. I will also make provisions for radiators in the space although I don't intend to heat it on an ongoing basis.

With this approach are you suggesting that it would not be necessary to undertake airtighteness work in the attic but to simply insulate/slab/skim the space? In order to reduce costs I was thinking of using polythene on the warm side of the insulation in order to increase airtightness but maybe this is not necessary. Instead maybe it would be sufficient if I insulated with insulation boards, taped the joins of the boards and then slab and skim? My one concern is that I will have a stairs from 1st floor level to attic and unless I insulate and undertake airtightness work on this landing, i.e. seal the the attic landing from the main attic space, then the membrane of the ceilings will be less effective as heat/air will escape up the attic landing and into the attic.

Any advice would be very welcomed as I didn't budget for developing the attic and the bills are mounting

You are contradicting yourself. You are either going to use the attic as habitual space or not.

If you make the attic airtight but have the insulation on the ceiling, then you run a high risk of condensation in the attic as the airtight layer is on the cold side of your ceiling insulation.

There should only be one continuous insulation/airtight line. Get your plans and draw two continuous lines (use different colours); 1st to mark your insulation and 2nd your airtight boundarys. You should not double back on either, otherwise it is a waste of effort & money. You should finish back where you started. Do this for all sections including the landing area. Remember, insulation should be between the warm interior and the cold exterior. Your airtightness line should be inside (on the warm side) the insulation line preventing the outside air from entering the building. If this does not make sense to you then get a professional to advise.

MicktheMan

creedp wrote: »

Thanks MicktheMan. I will pursue this with the builder. For the most part, its easy enough the get at the top blocks and knock them back to rafter height except in one location where there is a double rafter installed, because of a velux, snugly on either side of the wall. How he managed to be so precice in his measurements I can't imagine!

Thanks

If you include the attic in your thermal envelope then these walls will not be an issue. Otherwise I would advise to remove them.

Btw, the ph institute recommend insulating 1 meter above the insulation line in order to prevent thermal bridging (i.e. if the walls protrude 2m above the ceiling, then insulation needs to be applied to the first meter). Mind you they would be talking about 300mm insulation thickness, depending on the thermal conductivity.

creedp

MicktheMan wrote: »

You are contradicting yourself. You are either going to use the attic as habitual space or not.

If you make the attic airtight but have the insulation on the ceiling, then you run a high risk of condensation in the attic as the airtight layer is on the cold side of your ceiling insulation.

There should only be one continuous insulation/airtight line. Get your plans and draw two continuous lines (use different colours); 1st to mark your insulation and 2nd your airtight boundarys. You should not double back on either, otherwise it is a waste of effort & money. You should finish back where you started. Do this for all sections including the landing area. Remember, insulation should be between the warm interior and the cold exterior. Your airtightness line should be inside (on the warm side) the insulation line preventing the outside air from entering the building. If this does not make sense to you then get a professional to advise.

Thank MicktheMan, its all getting a bit confusing for me and you're right I should get some professional advice on this. Can I maybe elaborate a little on one point you made above. I understand that the airtightness layer should be on the warm side of the insulation and that you should only have one airtightness layer. So my options are to install insulation and the airtightness layer at ceiling level, including the attic landing, to minimise heat loss to the unheated attic. Im assuming though that I could still go ahead and put insulation in the attic so as to make it a more comfortable place for storage and accommodation of HRV Unit and SHW cylinder, etc. I don't want the added expense of continually heating this space and so don't need it to be as well insulated as the main living area.

Other option is to include the attic in the insulation and airtighness envelope but to also have rockwool insulation in the ceiling (but not airtightness layer) to minimise heat loss to the relatively cooler attic. I thought having insulation inside your airtightness layer was not very problematic where the area above this insulation is not a cold uninsulated area, e.g in some cases people put an insulated plaster board on the ground floor ceiling to reduce heat loss to hollowcore. Is this a problem also? The guy who installed the MHRV unit sais he always recommends this as it reduces heat loss to the cavity.

Final point on this issue I promise! I read some threads elsewhere which suggested that a professional heating/insulaiton consultant advised that insteadof using a membrane, attache foilbacked rigid insulation board to the underside of rafters/joists and tape the joints and tape to wall and this is your airtightness layer. Is this an alternative if you are going with rockwool or refterloc between the joists rahter than the superior softboard/cellulose/OSB approach?

Sorry for the ongoing questions but again I would appreciate any comments on above.

MicktheMan

creedp wrote: »

Can I maybe elaborate a little on one point you made above. I understand that the airtightness layer should be on the warm side of the insulation and that you should only have one airtightness layer. So my options are to install insulation and the airtightness layer at ceiling level, including the attic landing, to minimise heat loss to the unheated attic. Im assuming though that I could still go ahead and put insulation in the attic so as to make it a more comfortable place for storage and accommodation of HRV Unit and SHW cylinder, etc. I don't want the added expense of continually heating this space and so don't need it to be as well insulated as the main living area.

What you're describing above is a cold roof system where the attic needs to be well vented to the outside to prevent condensation and therefore it won't be much warmer than outside whether the rafters are insulated or not. If HRV unit is going in this space then make sure all ducting etc are properly insulated.

creedp wrote: »

Other option is to include the attic in the insulation and airtighness envelope but to also have rockwool insulation in the ceiling (but not airtightness layer) to minimise heat loss to the relatively cooler attic. I thought having insulation inside your airtightness layer was not very problematic where the area above this insulation is not a cold uninsulated area, e.g in some cases people put an insulated plaster board on the ground floor ceiling to reduce heat loss to hollowcore. Is this a problem also? The guy who installed the MHRV unit sais he always recommends this as it reduces heat loss to the cavity.

No problem with this approach. If different parts of the house are going to be thermally seperated from each other, then this also needs to be reflected in the heating design and control.

creedp wrote: »

Final point on this issue I promise! I read some threads elsewhere which suggested that a professional heating/insulaiton consultant advised that insteadof using a membrane, attache foilbacked rigid insulation board to the underside of rafters/joists and tape the joints and tape to wall and this is your airtightness layer. Is this an alternative if you are going with rockwool or refterloc between the joists rahter than the superior softboard/cellulose/OSB approach?

There is no single best way to insulate and make a house airtight. All houses and uses are different and lot of different items need to be taken into account. Some of these are ease of installation, system components certification, system (component) guarantee/lifespan, etc etc.

creedp

MicktheMan wrote: »

What you're describing above is a cold roof system where the attic needs to be well vented to the outside to prevent condensation and therefore it won't be much warmer than outside whether the rafters are insulated or not. If HRV unit is going in this space then make sure all ducting etc are properly insulated.

At the expense of completely annoying you does the above mean that I can't insulate and slab/skim the knee walls and up the sloped rafter to the collar leaving the 50mm gap for ventilation as this would cause condensation in the insulated attic space unless I allowed for significant ventilation from this space, e.g. install multiple wall vents in the knee walls? I was advised that as long as the rafters are ventilated from soffit to ridge, as they will continue to be, there wouldn't be a condensation problem as the cold roof structure is outside the insulated attic. Where wil the condensation be generated from as the main house will be seperated from this space using a airtight layer at 1st floor ceiling level.

BTW, the MHRV guy also installed a fresh air supply vent into this space in case I wanted to supply fresh air to the space - not sure if that adds anything to the discussion!

Sorry MicktheMAn I can understand it if the above questions are simply too frustrating to respond to. I am arranging to meet an airtightness 'consultant' who is an agent for SIGA products to get a better understanding of what is required but would appreciate any additional independent advice on this issue.

Thanks very much for all advice to date, it has certaintly made me give this issue more serious consideration.