Does insulating ceiling where there is a warm roof above reduce heating demand? Any downsides & what is MVHR impact?

[Willus]

We have a single story barn style new build which will have a warm roof across all of the property. Communal part of the building will be vaulted up to the warm roof but the more private areas such as bedrooms will be normal 2.4 height with a plasterboard ceiling and then a loft space with trusses.   Can I reduce the heating requirement of the building by insulating above the ceiling to the bedrooms so that we are not heating right up to the warm roof in that area?  It would make the loft space unheated / less heated but do I risk creating a moisture issue in the loft space?   My thinking was that if I could get the insulation layer right the loft space would still be heated from below but just to a lower temp than if we just have plasterboard but no so cold that we risk condensation.   We will be ventilating the rooms below by MVHR so hopefully will keep humidity at sensible levels so not too much moisture is moving into the loft space.     I did try so see if there was already a topic / thread covering this but I could only see ones on flat warm roofs and impact of insulating under them.   Any thought or resources you could point me at.    Thanks 

[JohnMo]

Our house was originally planned for a mix of vaulted and flat in bedrooms, kitchen/diner and wet rooms. But we changed to all rooms are vaulted, way easier to make airtight, no vertical bits to insulate, construction is cheaper and easier to do.

 

Various layer of insulation you have to be careful you don't get moisture ladden air in and it ends up with no where to go.

 

All vaulted makes very little difference to heat loss, goes up a little, but the feeling of space trumps that every day. Assuming you are using a low temperature heating system. If you are using a high temperature heating system, you really shouldn't be anyway.

 

Airtightness details are very easy all vaulted.

 

Any loft space treat the same way as all other vaulted areas, the airtight layer is the roof line.

 

You really need to do the following 

1. Determine your air tightness and vapour tight layer, this should be continuous around the building.

2. Decide where your insulation layer is. Get a cross sector building - take a pencil can you draw a continuous line through insulation and only insulation, around the building without lifting the pencil? If not you need to add insulation to bridge gaps.

 

The MVHR is building ventilation and has no bearing on your insulation layers. Condensation within the insulation is factor the temperature gradient within the insulation and control of moisture or lack of by a vapour protection layer, normally coupled with airtightness layer.

[Willus]

@JohnMo - we did think about having it completely vaulted but I lost that discussion.  I did not push too hard as I am scared of heights and did not fancy that much decorating at height the internal apex is about 3.8m.   HeatPunk seemed to suggest that if we fully insulated above the ceilings that it would change the heating demand by about 12% although if we insulated to that level it definitely would cause issues with temperature in the loft space.   I think I will probably just limit the activity in the loft to a level of sound proofing and a bit of flooring for storage but no more than that.

On the external barrier - the insulation is continuous between the warm roof and the ICF, which also goes down to below ground to act as the perimeter barrier for the slab.  As our roof overhangs the walls by about 1m we do have a bit of finessing to do around the point at which the trusses land on the ICF walls to make sure the detailing is correct for the air tightness and vapour layer.

 

Thanks for the feedback 

[ProDave]

You have just what we have, some rooms vaulted, others with various ceiling heights and loft space above them.

 

While we did not attempt to insulate the ceilings, just plasterboard, plaster and paint, I can confirm the "loft" temperature remains just that of the roof.  So insulating the ceiling to "reduce heat loss" must imply you would expect the loft space to be cooler.  I very much doubt that will be the case unless your overall insulation and air tightness spec is poor.

[Willus]

@ProDave I was thinking was that if only x% of the heat transferred into the loft space it would have a lower temperature with the warm roof then acting as the main barrier to the outside.  That was just based on 'gut thinking' which I was not sure was realistic hence the question to the group on the impact.   Whilst I guess that the temperatures would equalise over time between the rooms and the loft when initially heating the rooms my assumption was that it would allow the rooms to heat faster.   That said given we are going for much lower flow rates and a slow heating approach so I need to stop thinking about things in the way I would in a less energy efficient / air tight house.   Have you found any issues with noise transference between the rooms via the loft if you only have plasterboard, plaster & paint on the ceilings?    Thanks 

[cjsparkey]

Our build is similar mix of vaulted ceiling in the living areas and 2.4m ceilings in the bedrooms, bathrooms, utility etc. with loft space over all under warm roof.

We chose to put insulation in the ceilings really just for extra sound proofing, the loft spaces are pretty much the same temperature as the rest of the house.

 

[Willus]

@cjsparkey - thanks, that makes sense