ICF construction - south facing but no direct sunlight in the winter

[Andy H]

I am planning on building a single storey building 12m by 6m by 3.25m (approx 0.7m underground) at the bottom of my garden in London. Whilst the building will be South facing it will receive very little direct sunlight in the winter as the sun will be obscured by the House and other houses on the Street. The North, East and West facing walls receieve minimal light due to neighbouring properties or fencing and will therefore not have any windows.

 

Whilst I woud want a very low U value for the winter months my concern is that could lead to overheating in the Summer. Am I right to be concerned or can this be dealt with by the MVHR unit, install awnings that can be rolled out in the summer to shade the windows and opening the windows over night. In escence should I still be aiming for a low U value.

 

Connected to this is the question as to whether there would be any benefit in removing the inner eps of the ICF to allow for the concrete wall to help with temperature regulation. As an aside I quite like the idea of have a concrete wall as the wall finish.

 

Thanks Andy

[George]

I would go for a sealed/polished concrete floor (or tiled) and leave the ICF blocks alone. It would create a lot of waste and the finish on the EPS would be.... uneven to say the least. Plus you'd have issues with dusting, fitting furniture in etc.

 

As I understand it, the MVHR won't remove heat as such, but if you place the intake on the north face the summer bypass mode will help.

 

A low u-value will help prevent overheating if you prevent a warm air getting in and solar gain. (Worst comes to worst you could fit an air conditioning system running on PV) So a brise soleil or awnings are probably your best bet - personally I'd go for a fixed brise soleil if it suits the building, because you know you will need to shade and you're not preventing any winter sun by the sound of it.

 

Also, openable rooflights will allow for a lot of ventilation.

Edited January 27, 2021 by George

[Bitpipe]

16 hours ago, Andy H said:

Whilst I woud want a very low U value for the winter months my concern is that could lead to overheating in the Summer. Am I right to be concerned or can this be dealt with by the MVHR unit, install awnings that can be rolled out in the summer to shade the windows and opening the windows over night. In escence should I still be aiming for a low U value.

 

A low U value, combined with good airtightness, is useful all year round. It will keep your interior at a consistent temperature in summer and winter. Being partially underground will also increase your temp stability as ground temp is pretty consistent year round. Decrement delay (how long your wall build up takes to transfer heat from outside to inside) is also key.

 

I'd recommend  a passive style slab - i.e. insulate the slab off the ground using 2-300mm high density EPS and if you extend that insulation just beyond your slab, your ICF exterior insulation should be able to line up and (assuming you design your roof appropriately ) give you a nice insulated & airtight envelope. We have a basement / timber frame house with this approach.

 

You can then incorporate low temp UFH directly into the slab and if used with an ASHP it can cool the slab in summer. ICF inherently lends itself to good airtightness so you just need some attention to detailing to achieve a decent result all round. 

 

Solar gain is the usual reason for overheating so if you minimise this with adjustable blinds, overhangs etc then that will make a big difference - note that spring and autumn can be prone to overheating as the sun is lower in the sky.

 

MVHR is there to provide adequate ventilation and minimise heat loss in doing so, it's not very effective at moving hot or cold air at the required volumes to make a major difference. If summer overheating remains a concern, make provision for a split air con (i.e. put the ducting in) and then put one in later if needed.

 

16 hours ago, Andy H said:

Connected to this is the question as to whether there would be any benefit in removing the inner eps of the ICF to allow for the concrete wall to help with temperature regulation. As an aside I quite like the idea of have a concrete wall as the wall finish.

 

No, as well as being time consuming and messy, it will have the opposite effect.

 

As above, enabling 'stack' ventilation (fancy term for roof window + ground level ventilation) in summer will cool your house at night and then you keep it closed during day to keep the nice cool air in.