Minimising Overheating on Room-in-Roof Bungalow

[Nethermoor]

I'm building a bungalow with rooms in the roof and I want to get the right materials specified at this stage (planning granted, next step BC) to minimise overheating.  I want to avoid the need for AC if possible but don't want to be installing materials with a payback of more than 10 years really (may have moved on by then).  Going for 'above regs' spec but not passive.

 

My parents house is also a RIR bungalow and for 2-4 weeks of the summer it is not pleasant to sleep upstairs unless there is a portable AC running.  Obviously the basic spec of my build will be a lot better than theirs anyway, so I'm not sure how many extra measures are worth taking.

 

Been reading other posts on overheating on the forums so have an idea of the different options, but no real clue what will be my weakest points in terms of overheating and what to prioritise...

 

• Juliette balcony on the south elevation upstairs (master bedroom), 2x 3m french doors downstairs.  External blinds for these?  Solar control glass?
• 4x large Velux roof windows - solar control glass, external blinds?  Seen the Solstro ones which look good.
• Large roof area - foil membrane to reflect heat?  Insulation with high decrement delay (architect is specifying Gutex/Pavaflex wood fibre but saw @ProDave post that Frametherm was similar decrement delay and a lot easier on the budget)?  Guessing PIR would be a bad option.  Rafters are 222mm deep so a far amount of space to play with.
• Accept it will overheat and plan for AC from the start?  (bearing in mind I don't want PV due to long payback time)  I will have MVHR but have seen from others that comfort cooling on this won't really make enough of an impact.

 

Plans attached to add context.

 

Be very glad of advice - don't want to spend a load extra on wood fibre roof insulation if it won't make a noticeable difference.

 

19-003-20-M Proposed Floor Plans-[A1].pdf 19-003-21-H Proposed Elevations-[A1].pdf

[Jeremy Harris]

Best approach is to try and keep the heat out as best you can, rather than try and deal with it once it's got in and warmed things up, so the idea of fitting external blinds on the Velux windows is a very good one, if the budget can stretch to them, as those windows are going to be the main source of solar gain. 

 

High decrement delay insulation definitely works very well, we have a South facing roof elevation, with the bedrooms in the roof space, and are in a sheltered spot that tends to be warmer than the average for the area, and yet very little heat seems to penetrate through the roof insulation.  I've tried to measure it, but haven't been able to see a significant increase in the temperature of the internal skin at all.  We have 400mm of pumped cellulose, giving a U value of about 0.1 W/m².K.  PIR has the advantage of giving a better U value, or the same U value for less thickness, but it has a pretty rapid decrement delay, so prolonged sunshine during the day will get through it.  Whether this can be mitigated by the combination of a reflective membrane plus very good ventilation under the roof covering I'm not sure.  The latter makes sense anyway, as it would mimic the effect of something like a safari roof, as fitted to some Landrovers (and that works very well).

 

Given that A/C units aren't very expensive, I think I'd at least build in provision to fit one.  I'd not be without ours, it's running right now and is keeping the bedroom at a nice and comfortable 19°C, whereas downstairs it's sitting at a bit over 23°C at the moment, with the floor cooling and MVHR cooling running . . .

[ProDave]

The No 1 thing I would suggest is ditch the plan to use attic trusses.  They are a devil of a problem to insulate properly without leaks, and in any event you have cupboards going into the eaves.

 

Instead, build the roof as a cut roof supported from a ridge beam.  You have enough internal walls that with a bit of re jigging you can get intermediate support for a ridge beam end to end of the main roof.

 

That will enable you to well insulate and seal the entire roof structure with much easier detail to get right than attic trusses, and it means your eaves storage cupboards will be part of the heated, insulated, sealed envelope of the house so much more usable.

 

Then you want to insulate the roof well with a low decrement delay material.  Since this will be a warm roof now, over board the outside of the roof structure with something like wood fibre board. and insulate between the rafters with something like Frametherm 35 or blown celulose.  I have 100mm wood fibre sarking and 200mm frametherm 35 and it performs very well indeed.

[BristolBill]

5 minutes ago, Jeremy Harris said:

Best approach is to try and keep the heat out as best you can, rather than try and deal with it once it's got in and warmed things up, so the idea of fitting external blinds on the Velux windows is a very good one, if the budget can stretch to them, as those windows are going to be the main source of solar gain.

 

I cannot comment on the other aspects without getting firmly out of my depth but fully agree with Jeremy here.  Awning blinds just added to three velux in an extension are making the world of difference.

[Gone West]

We have a triple glazed roof window in our bathroom and I specified a g value of 0.3 for the glazing. We don't have any problems with overheating.

[Nethermoor]

1 hour ago, ProDave said:

The No 1 thing I would suggest is ditch the plan to use attic trusses.  They are a devil of a problem to insulate properly without leaks, and in any event you have cupboards going into the eaves.

@ProDave  Oh dear...just paid my deposit on these to be able to get the full truss design for the structural engineer ? 

 

The plan was to keep the eaves and apex void within the insulated area as red line below.  What bits are hard to insulate properly compared to a cut roof?...may as well get prepared!

 

[Nethermoor]

Thanks for the replies, would anyone think the south elevation glazing is likely to be a major problem (3 sets of french doors)?

 

If not then ideal because it looks like external blinds for these would be quite expensive...

 

Is solar glass on this glazing worth it or best to focus on Velux windows and roof insulation?

[Jeremy Harris]

 

 

6 minutes ago, Nethermoor said:

@ProDave  Oh dear...just paid my deposit on these to be able to get the full truss design for the structural engineer ? 

 

The plan was to keep the eaves and apex void within the insulated area as red line below.  What bits are hard to insulate properly compared to a cut roof?...may as well get prepared!

 

That's a shame, as we have a roof similar to the one @ProDave has described, with a ridge beam and rafters hung from it.  Our's has 22mm OSB sarking outside, and just a VCL inside to hold the insulation in place, and the design gives the same sort of space as yours, but is very much easier to both insulate well and make airtight:

 

[ProDave]

I have yet to find anyone that has done a trussed roof and insulated it well.

 

You will be cutting a lot of insulation, and it is vital that at every junction of every truss you get the detail absolutely perfect.

 

The danger, and what happens so so often, is you get gaps in the insulation, allowing cold roof air to get between the insulation and the plasterboard.  Then you get the situation I encounter very often that on a windy day, you take an upstairs socket off the wall and a blast of cold air comes out of the socket box.  that is letting cold air in in the winter, and warm air in the summer.

 

I would say if you are doing it yourself and are prepared to spend the time to get the detail right then carry on.  If you are employing a builder then I almost guarantee the detail will not be right.

[Nethermoor]

Hmmm.  I will be insulating the roof myself.  Design is here https://sketchfab.com/3d-models/5608i-088a0c8b982c485d893e8bf55933449f

 

Which bits do I need to watch out for?  Had assumed it would be quite simple fitting Frametherm between the rafters...?

[Jeremy Harris]

6 minutes ago, Nethermoor said:

Hmmm.  I will be insulating the roof myself.  Design is here https://sketchfab.com/3d-models/5608i-088a0c8b982c485d893e8bf55933449f

 

Which bits do I need to watch out for?  Had assumed it would be quite simple fitting Frametherm between the rafters...?

 

 

The tricky bit is sealing it up and getting it airtight.  With a ridge beam hung rafter arrangement you have a completely free internal face that makes fitting and sealing the VCL very easy.  With attic trusses the VCL has to be cut and taped very carefully around dozens of timber projections through the membrane, creating a lot of work to get right, and using up a lot of airtightness tape.

 

Our roof just has a layer of VCL stretched from one end of the house to the other inside, fastened to the inner face of the rafters with 50 x 50 battens, with the only taping being the joins between the runs of VCL and to the membrane coming up from the walls.  The plasterboard vaulted ceilings are just fastened to the 50 x 50 battens, with the void behind being used for wiring etc.

[Ferdinand]

2 hours ago, Nethermoor said:

 

If not then ideal because it looks like external blinds for these would be quite expensive...

 

Is solar glass on this glazing worth it or best to focus on Velux windows and roof insulation?

 

I'm in an upwards extended bungalow.

 

I can't comment on solar glazing, but I'm about to put it in my South Facing bay windows, where my office and the spare room run up to 35C too easily - I am also planning one of those through the wall air conditioning units a la Jeremy. I would also like a veranda / breakfast terrace, but it will all wait (except for the glazing) until I have bought family out of the other half of the house.

 

Quote

Thanks for the replies, would anyone think the south elevation glazing is likely to be a major problem (3 sets of french doors)?

 

I think this is likely to be issue,  such that it should at least be provisioned for,  if not implemented.

 

Possibilities include one or more of:

 

(To manage the interior)

 

- A warm / cool unit as suggested.

- An underfloor heating application with the capability to reverse and cool it instead (or distribute the heat more evenly).

-  Through or stack ventilation. This means either windows on windward / leeward side that you can open together, or a roof window that can be opened or partially opened at the same time as something on the cool,  shaded side of the house. You want it such that you can do it safely overnight, or ideally whilst you can go out in the day (windows securely ajar, for example). This makes a significant difference to my place. I would suggest making sure that one of your rooflights is no the central landing, and I would consider making it openable into "vent" position from a downstairs switch - mine is easy to reach but I have to go upstairs to open it slightly.

 

(To keep the heat out)

 

- A pergola

- A brieze soleil

- Window blinds

- An anti-solar layer in the glass, as you suggest (which is probably better than a surface film).

- Something else similar.

 

F

 

[SteamyTea]

4 hours ago, Jeremy Harris said:

we have a South facing roof elevation

Your PV is going to take 15 to 18% of the energy out of the sun as well.

 

[Nethermoor]

Thanks for the responses, will certainly help in my preparations.