Help with differing opinions on my warm loft

[Stu68]

Our build (4 ears old) is of reasonable airtightness (approx 3.2 in the airtightness test). Our contractor left us in the lurch before completing our 3500sqft home, leaving us to complete it ourselves using 3rd parties. I am trying to improve the overall insulation performance and am being given differing views what's best.

We have GSHP, and MVHR. The contractor advised me that we have a "warm roof" which I understand, and consists of Foil space blanket, over the rafters forming an airtight loft space. My issue is:

 

Currently we have,  170mm rock wool insulation between the ceiling joists in the loft space which has for the majority then been boarded with chipboard flooring. There is no added insulation between the roof joists (i can see the underside of the space blanket). All the MVHR ducting is exposed in this "warm " loft space and I was told it doesn't need any further duct insulation as it sits in  aware loft?

MVHR aside, the main thing is surely the Rockwool in the ceiling joists are counterproductive and is prevent heat rising into the 'warm" loft? I am looking for something assuring of if it is best for me to remove the rock wool from the ceiling joists (which means taking up the boards as well), and putting that insulation (or new insulation) between the roof rafters (180mm of joist depth spare).

Some builders have said " waste of time". just add new rock wool type insulation between the row joists to enhance the space blankets effectiveness, others are telling me exactly the opposite, that my loft won't be as warm as it should be with the ceiling rock wool in place, and my MVHR is suffering as a result. Lastly yet another contractor told us that its fine as it is and we should just insulate the MVHR ducting.

The temp in the loft space in the winter is typically a few 1-2 degrees lower than the rooms below. 

 

Is there a definitive correct answer here as to how I should proceed ?

 

I have included a pic of the house and its mainly the long single story section I am referring to.

[Ferdinand]

To me that sounds like a semi-cold loft.

 

But can  you educate me about the thickness and u-value of the Space Blanket layer.

 

I think a key qu is what you intend to do with your loft in the future - will it be used for anything eg bedrooms, Scalextric, model railway layout of the entire London Underground? Though if you have 350 sqm then arguably that should be enough for anyone this side of Imelda's shoe cupboard.

Cheers

 

Ferdinand

Edited December 2, 2019 by Ferdinand

[SteamyTea]

What I was thinking.

It may be designed as a warm roof i.e. reasonably airtight, but if the insulation is poor, then it will not be too warm.

Generally, by controlling heat losses, you can then decide if you need secondary heating in an area.

@joe90did a warm roof and it works well. If he wanted to live in it he may need some ventilation, but as a storage area it is fine.

I think his MVHR pipes are uninsulated, but they are short.

My feeling is to always insulate everything that is warmer or colder.

Edited December 2, 2019 by SteamyTea

[Mr Punter]

Morning @Stu68 and welcome.

 

That looks like a lovely house in a fine location.

 

I don't see any point in removing the ceiling level rockwool.  It may help attenuate any noise from outside and the MVHR unit.  The 2 degree temp difference between loft and adjacent heated space will make very little difference to the MVHR heat losses / efficiency and is nowhere near enough to be a condensation risk on the ducting.

 

Unless there is an issue I would leave all as is and enjoy the place.

[joe90]

1 hour ago, SteamyTea said:

What I was thinking.

It may be designed as a warm roof i.e. reasonably airtight, but if the insulation is poor, then it will not be too warm.

Generally, by controlling heat losses, you can then decide if you need secondary heating in an area.

@joe90did a warm roof and it works well. If he wanted to live in it he may need some ventilation, but as a storage area it is fine.

I think his MVHR pipes are uninsulated, but they are short.

My feeling is to always insulate everything that is warmer or colder.

 

Yes my warm loft is only insulated at rafter lever, 400mm rockwall above glued OSB and as @SteamyTea says it works well. I am surprised you only have 1 or 2 degrees temp difference, not sure what your space blanket actually is but multi foil insulation is considered to be snake oil by some but hey if it works. The only MVHR pipes I insulated were the pipes to outside as condensation was forming on the incoming (cold air) pipe in a warm loft. My only comment is if you want your house/loft warmer why not put more rockwall between joists, it’s already fairly airtight so easy peasy.

 

P.S. lovely location.

Edited December 2, 2019 by joe90

[Temp]

The term warm roof is ambiguous. Some people use it to mean a warm loft others that the insulation is above the rafters so the rafters are warm. Personally I would encourage people to use the following terms..

 

Cold loft - insulation is at joist level

Warm loft - insulation is at rafter level

 

Cold roof - rafters are on the cold side of the insulation (eg the insulation is between or under the rafters or at joist level.

Warm roof - rafters are on the warm side of the insulation (eg the insulation is mainly above, but possibly some between, the rafters).

 

The above also effects how/if the loft is ventilated. 

 

Are there any drawings available to show what was intended? Building Control might have a set?

 

 

 

 

 

[scottishjohn]

51 minutes ago, joe90 said:

not sure what your space blanket actually is but multi foil insulation is considered to be snake oil by some but hey if it works.

I have it below my suspended ground floor to seal off the joists and stop heat loss from my retro fit  UFH above

when i do my yearly check to make sure its all taped up still  i open up a litle corner to get my fingers in and it is very much warmer than the space below floor where i am sitting 

It does work .but is it as good as solid pir?--Idoubt it ,providing the pir is well taped and sealed

but for some places its a good easy solution 

Edited December 2, 2019 by scottishjohn

[scottishjohn]

1 hour ago, Stu68 said:

The contractor advised me that we have a "warm roof" which I understand, and consists of Foil space blanket, over the rafters forming an airtight loft space. My issue is:

as already said a picture of roof build up  will clarify what you have 

As i understand it

rafters then blanket,then battons ,then slates will make it a cold roof

-as your insulation is on the underside of your roof ,so keeping loft warm  and the actual roof would  be cold ? the cold + damp area area is above your rafters?

I am guessing from description that your roof is not sheeted with OSB on rafters --just space blanket draped across them and taped joints  and acting as a water barrier as well ?

confusing  isn,t it 

 a rough drawing  of the make up of roof will make it all clear

Edited December 2, 2019 by scottishjohn

[Ed Davies]

It'd be interesting to know what the temperature of the rafters is. It seems to me that if the loft is only a few degrees cooler than the house then there must be massive thermal bypass of the insulation between the ceiling joists (assuming it's actually there - has that been verified or it just a matter of assumption which can't be checked because of the chipboard?). Most likely that's down to air leakage through the ceiling in which case it seems likely there's warm moist house air in direct contact with chilly rafters which seems like a poor idea.

 

It'd be worth checking the temperatures of the tops of the rafters with an IR thermometer.

 

[ADLIan]

+1 on comments above. Not a very good roof design and the space blanket (multi-foil?) will be doing very little to add to the overall thermal performance.

U-value is probably 0.25 W/m²K or even worse.

[Stu68]

Firstly, can I just thank you all for replying, I really do appreciate it and will take on board any advice given. After you replies I have tried to answer some of the points made as best as i can  and taken some more measurements. The photos I hope will explain also.

The roof space is purely used for storage as its 30m long I suppose a air rifle range would be the only option!

To add the the mix, i think i was little optimistic in temp differences between rooms and loft space. Its icy outside and the interior rooms are 20deg C, the loft space is currently 15 Deg.C.

You will see that in places (Hall) there is a  vaulted ceiling which only differ inso much that there is rockwell 100mm, between the roof rafters in that section of ceiling.

 

The space blanket is Actis Triso Super 10+

 Was sold this idea on the basis that is was so say the mutts nuts and the builder had extensive good experiences/ results when using it in the same way as my property. Didn't help that my building inspector seemed to agree!

 

I am happy to consider putting rigid celotex between the roof rafters or mineral wool if you guys think it will make a worthwhile improvement too. I was thinking that the ceiling joist wool, was actually trapping heat in the rooms downstairs and preventing the roof space to warm up and become constant temperature. Is this likely to be the case or is the main reason for the temp difference heat loss through the roof? I suppose I am still unsure if my roof is a warm or cold or a hybrid, in which case how best to approach things. 

Many thanks for thing the time to reply.

Regards

stuart

Hope these pics help show you what's what.

 

Edited December 2, 2019 by Stu68
More info

[A_L]

58 minutes ago, Stu68 said:

I was thinking that the ceiling joist wool, was actually trapping heat in the rooms downstairs and preventing the roof space to warm up and become constant temperature. Is this likely to be the case or is the main reason for the temp difference heat loss through the roof?

 

The ceiling joist wool is resisting the passage of heat to the roof space and the heat that does pass through is being lost to the outside. Both effects together are responsible for the room/roof space temperature differential. With the ceiling insulation in place the loft space will never be at a constant temperature. If your measurements are accurate and we can assume that the outside is at 0°C then we can say that the rafter insulation is providing 3/4 of the thermal resistance of the whole structure and the ceiling insulation 1/4. Given a little time to come to an equilibrium the roof space will tend to be the room temperature minus 25% of the difference between room temperature and the outside temperature. This assumes no significant solar heating.

[joe90]

12 hours ago, Stu68 said:

I am trying to improve the overall insulation performance and am being given differing views what's best.

 

This from your original post. It will be difficult to add insulation to the ceiling as this will mean raising the flooring in your storage loft. Insulation does  not stop heat loss, it only slows it down so the ceiling insulation is doing no harm. I would add insulation between rafters, either cellotex or rock wool depending on budget or target U value. If using rockwall you will need something to keep it in place. If you use rigid foam it can be fixed in place with expanding foam. The only thing I do not know (and I am sure someone will explain this for you) is a condensation risk from additional insulation.

[NSS]

If, as appears to be the case, the overlaps of the Actis blankets are not taped, the first thing I'd do is tape them as this will significantly improve your airtightness. 

 

We have a warm roof with Actis Hybris between the rafters and then Actis H-Control over that and the rafters (ie on the inside of the rafters not the outside as yours is) and it works very well. However, we spent a lot of time taping the joins/overlaps and achieved 1.6 on the air test.

 

That said, we also have 200mm of rockwool between the ceiling joists, and at this time of year the loft is usually a couple of degrees cooler than the rest of the house as there is obviously no heating in there. Our MVHR trunking runs in the loft space and is not insulated.

[ProDave]

What about ventilation?

 

In a true warm roof, you don't ventilate it.  A cold roof needs ventilation.  What have they done for this half way house?

 

From experience, Something like Frametherm 35 will push in between the rafters and stay there.  I had a test piece in place like that for over 6 months before finally covering it with OSB and it did not budge.  A LOT easier to cut to width so it is a snug fit than any solid insulation.  To be sure it stays, once in place fix some thin battens across to hold it in place.

[ADLIan]

The Actis claim of being equivalent to over 200mm of mineral wool was demolished many years ago. If I remember correctly it was back in 2006 when new guidance in support of the Building Regs was issued.

[Ed Davies]

10 hours ago, NSS said:

If, as appears to be the case, the overlaps of the Actis blankets are not taped, the first thing I'd do is tape them as this will significantly improve your airtightness.

 

It's hard to tell but it seems to me like that might increase the condensation risk in the loft considerably.

 

Personally, I wouldn't mix the insulation layers like this; either have all the insulation immediately on top of the ceiling (cold loft) with good ventilation of the loft to the outside or all at rafter level (warm loft, warm or cold roof) with the airtightness layer there as well. With this mixed insulation it's almost impossible to get the ventilation of the loft space right to avoid condensation. If the current level of airtightness seems to be working it'd probably be better not to mess with it without rethinking the insulation as well.

[Stu68]

On 02/12/2019 at 22:46, NSS said:

If, as appears to be the case, the overlaps of the Actis blankets are not taped, the first thing I'd do is tape them as this will significantly improve your airtightness. 

 

We have a warm roof with Actis Hybris between the rafters and then Actis H-Control over that and the rafters (ie on the inside of the rafters not the outside as yours is) and it works very well. However, we spent a lot of time taping the joins/overlaps and achieved 1.6 on the air test.

 

That said, we also have 200mm of rockwool between the ceiling joists, and at this time of year the loft is usually a couple of degrees cooler than the rest of the house as there is obviously no heating in there. Our MVHR trunking runs in the loft space and is not insulated.

The Actis is taped on all joins but on the outside (ie. the slates side) however I can easily tape the inside  as well.

Thanks for the reply, its good to see others using the product!

 

[Stu68]

Thanks for your replies, Im pretty sure that the loft is designed to be a warm roof as it is airtight inso much that the Actis is taped on all joins on the outside overlaps and there is no ventilation to the outside. Infact when we had the airtightness test done we had a result of 8.4, which was higher than we were expecting. The chap did a test on the attic space (using the fan on the loft hatch) and he said we weren't losing a lot through the roof and he said it was performing well, so I think the loft was/is intended to be a warm one. (we tracked down the leaks to elsewhere downstairs and then returned a result of 3), which were happy with considering the construction method.

 My main thought was, could i add extra  insulation between the rafters (leaving a 25mm gap between Actis and insulation) and not create condensation?

 

With my attic is 15 deg and the rooms 20, is there  much benefit in me insulating the MVHR ducting that runs in the loft or trying to make the loft space warmer?

 

Many thanks.Stuart

Edited December 3, 2019 by Stu68